Les Séminaires du LOMA ont lieu le mardi à 14h dans le centre de documentation du 2ème étage (salle 215).
Contact : Julien Burgin (Bureau 107)
| Date | Titre | Abstract | présenté par | lieu |
| Mardi 27/11/2018 à 14h | The transport of deformable objects by a turbulent flow is common in environmental sciences which are interested, for instance, in the dynamics of plankton in the ocean, and in industry, such as the papermaking or textile industries. However most of the fundamental studies focus on rigid particles and the amplitude of the deformations and their influence on the transport of the fibers remain to be explored. My presentation is focused on the characterization of the deformations of flexible fibres in homogeneous isotropic turbulence. Two main questions will be tackled here: when does a fiber start to be deformed by the flow? And what is the amplitude of these deformations? Answering these questions will allow to draw an analogy with wormlike chain polymer. However, the spatiotemporal correlations of turbulence are responsible of interesting features such as a straightening of long fibers which become statistically less distorted by turbulence as their length increases.. |
Gautier Verhille Université Aix-Marseille, CNRS, IRPHE |
Salle des séminaires LOMA (A4N) | |
| Mardi 16/10/2018 à 14h | Polaritonic chemistry: Manipulating molecules through QED effects in a cavity |
 Traditional nanophotonics is concerned with engineering material systems to control light on a nanometer scale. However, over the last years, it has become clear that the reverse is also possible, i.e., to engineer light modes so as to modify material properties and dynamics. Remarkably, this can be achieved even when no actual light is present in the system by engineering the electromagnetic vacuum and its fluctuations, based on quantum electrodynamics (QED). The principal tool to achieve such modifications is so-called strong light-matter coupling, which occurs when the coherent energy exchange between a (confined) light mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixed light-matter character, so-called polaritons. Organic molecules present a particularly favorable type of emitter to achieve this regime even at room temperature due to their large dipole moments and stability. I will discuss how polariton formation leads to changes in the excited-state character and energy levels (i.e., potential energy surfaces), and how this can be used to modify static and dynamic properties, such as energy transport, photochemical reactions, and even thermally driven ground-state chemical reactions. |
Johannes Feist Universidad Autonoma de Madrid |
Salle des séminaires LOMA (A4N) |
| Mardi 09/10/2018 à 14h | Model Experiments of Active Matter : at the interface between living organisms and theoretical models |
The ubiquity of collective motions observed at all scales in biological systems has driven a surge of scientific activity. Within physics, important theoretical progress was achieved by studying microscopic point-particles models and their continuous descriptions. Among the landmark results are the possibility of a true long-range polar ordered collective motion as well as of a Motility Induced Phase Separation (MIPS). The robustness of these observations against the numerous factors integrated out in the above effective models is a matter of crucial importance.
This is where human-designed model experimental systems have a key role to play. Janus colloids, swimming droplets or walking grains are amazing experimental realization of self propelled particles. They are far more simple than their biological inspiration, and already contain important realistic factors, such as hydrodynamics effects and pairwise force interactions, which, at least in principle, can be controlled. In the present talk, I will illustrate that matter in the case of two remarkable experimental systems, namely rolling colloids [1] and walking grains [2,3].[1] Bricard, A., Caussin, J.-B., Desreumaux, N., Dauchot, O. & Bartolo, D. Emergence of macroscopic directed motion in populations of motile colloids. Nature 503, 95–98 (2013). [2] Deseigne, J., Dauchot, O. & Chaté, H. Collective Motion of Vibrated Polar Disks. Phys. Rev. Lett. 105, (2010). [3] Briand, G. & Dauchot, O. Crystallization of Self-Propelled Hard Discs. Phys. Rev. Lett. 117, 098004–5 (2016). |
Olivier Dauchot ESPCI, Laboratoire Gulliver, Paris |
Salle des séminaires LOMA (A4N) |
| Mardi 02/10/2018 à 14h |
Modeling fluid flows in channels is a general problem in science and engineering. For ideal liquids, the situation is simple: there is no dissipation due to fluid movement. For real liquids, some energy is lost. Navier, in his pioneering work on fluid mechanics identified two possible sources of dissipation: bulk dissipation, associated to the viscosity and the friction of the last layer of liquid molecules sliding on the solid surface. For surface dissipation, a classical assumption of fluid dynamics is that a liquid element adjacent to the surface is equal to the velocity of the surface, i.e. a non-slip boundary condition, which leads to no surface dissipation. This is not the only possibility. Navier, postulating the existence of a slip velocity at the surface, introduced the possibility of surface dissipation. He proposed a linear relation between the shear stress at the solid-liquid interface and the slip velocity: , where k is the interfacial friction coefficient. Indeed, it is also possible to define the slip length b as the distance from the solid surface where the fluid velocity profile extrapolates linearly to zero. Figure 1: Definition of the slip length. During this presentation, I will briefly review what we know on the boundary condition for simple Newtonian liquids and show that polymers, due to their entanglements present a unique tool to study and understand the Navier condition. Based on a setup using the photobleaching of fluorescent polymers [1], I will present our last results on the slip of polymer melts [2,3] and polymer solutions [4]. |
Frédéric Restagno CNRS, Université Paris-Sud, (Orsay) |
Salle des séminaires LOMA (A4N) | |
| Jeudi 27/09/2018 15h |
Formation and propagation of a semi vortex-ring connected to a free surface |
The vortex rings are torus-shaped vortices existing in many different situations in fluid mechanics. They are involved in the locomotion of numerous animals like birds and fish. The understanding of their formation process is essential to optimize the propulsion in engineering. They also play a major role for blood pumping in left ventricle and are considered as an index of cardiac health in the human heart. As proof of the phenomenon diversity, the presence of charged vortex ring with a quantized circulation has also been identified in superfluid helium.
The first part of this talk will give a short overview about interest and properties of the vortex rings. In a second part, I will present an experimental study on a semi vortex ring connected to a free surface, notably involved in the locomotion of water striders. This U-shaped object is here generated by the circular motion of a flat circular disc in water. Digital particle image velocimetry provides velocity fields and vortex properties. The formation process highlights not only the existence of a classical rolling up at the rear of the disk, but also a shedding phenomenon on the leading edge of the disc. The created small vortices affect final properties of the semi vortex ring. |
Alexandre Vilquin LOMA, CNRS, Université de Bordeaux |
Salle des séminaires LOMA (A4N) |
| Mardi 18/09/2018 à 14h |
Dyamics of prey prehension by chameleons through viscous adhesion |
Among predators using an adhesive tongue to feed, chameleons are able to capture large preys by projecting the tongue at high acceleration. Once in contact with a prey, the tongue retracts with very large accelerations to bring it to the mouth. A strong adhesion between the tongue tip and the prey is therefore required during the retraction phase to overcome the inertial forces and ensure a successful capture. To investigate the mechanism responsible for this strong bond, the viscosity of the mucus produced at the chameleon’s tongue pad is measured, using the viscous drag exerted on rolling beads by a thin layer of mucus. We show that the viscosity of this secretion is about 400 times larger than that of human saliva. We incorporate this viscosity into a dynamical model for viscous adhesion, which describes the motion of the compliant tongue and the prey during the retraction phase. The evolution of the maximum prey size with respect to the chameleon body length is then derived, and compared with in vivo observations for various chameleon species. Our study shows that the size of the captured prey is not limited by viscous adhesion, owing to the high mucus viscosity and large contact area between the prey and the tongue.
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Prof. Pascal Damman Laboratoire Interfaces & Fluides Complexes, Université de Mons |
Salle des séminaires LOMA (A4N) |
| Mardi 11/09/2018 à 14h |
Nonreciprocal Interactions and Devices via Reservoir Engineering |
An interaction process between two quantum systems is in general reciprocal. This means that forward and backward process are inherently |
Dr. Anja Metelmann Freie Universität (Berlin ) |
Salle des séminaires LOMA (A4N) |
| Mercredi 05/09/2018 à 13h30 |
Light is a powerful tool to control complex systems at micro and nanoscale, as it provides flexible and precise (submicron) spatiotemporal modulation. The first part of this talk focuses on the light-guided self-assembly of colloidal building blocks. In Nature, energy input is needed to develop advanced features, e.g. self-healing or self-regulation. I will show how we can extend this principle to the artificial world by the self-assembly of dissipative components that respond to light cues [1]. Following sequential light-patterns, they autonomously assemble into robust self-spinning structures, or microgears. The gears act as contactless ‘teeth’, synchronizing their motion, and constitute the fundamental components of synchronized micro-machineries that auto-regulate and whose dynamics is tuned by the spins of their internal components. The study demonstrate the potential of non-equilibrium interactions to program self-assembly and control dynamical colloidal architectures beyond static, equilibrium assemblies. In a second part I will show how luminescent nanoparticles can be exploited to probe complex soft matter systems. The local refractive index, in particular, can give critical physical and structural information but can be hard to characterize in strongly heterogeneous media. [1] A. Aubret, M. Youssef, S. Sacanna and J. Palacci, “Targeted assembly and synchronization of self-spinning microgears” Nature Physics, 2018 |
Antoine Aubret Universitty of California San Diego (USA) |
Salle des séminaires LOMA (A4N) | |
| Mardi 4/09/2018 à 14h |
Friction with self assembled monolayers: hydrodynamic slip and activated solid/solid friction |
The friction between a solid and a liquid was historically assumed to be so large that fluid molecules in contact with a solid boundary were stuck. Thus, the classical no-slip boundary condition was applied. With the advent of micro- and nano-fluidics in the last decades, however, this boundary condition has been observed to fail in many instances. Particularly, as will be discussed here, a slip boundary condition is observed when unentangled polystyrene (PS) dewets from a hydrophobic, alkylsilane self-assembled monolayer (SAM). This boundary condition can furthermore be tuned over an order of magnitude by subtly changing the SAM [1], indicating that the friction between solid and liquid is highly sensitive to atomic level details. We have also investigated the friction between these same SAMs and a solid metal tip of nanoscopic dimensions. Remarkably, this apparent solid/solid friction is velocity dependent and exhibits a crossover from a linear to logarithmic scaling in the velocity [2]. The results can be described in terms of a distribution of nanocontacts with very weak stiffness compared to the expected one for metal/solid contacts. The work presented was completed in collaboration with the authors of refs. [1, 2]. [1] J.D. McGraw, M. Klos, A. Bridet, H. Hähl, M. Paulus, J.M. Castillo, M. Horsch, K. Jacobs, J. Chem. Phys. 146 203326 (2017).
[2] J.D. McGraw, A. Niguès, A. Chennevière, A. Siria, Scaling crossover of the velocity dependence for solid/solid friction at the nanoscale, 17 6335 (2017).
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Joshua D. McGraw |
Salle des séminaires LOMA (A4N) |
| Mardi 17/07/2018 à 14h |
Slip over nano-structured surfaces & nano gaseous films: Insight from molecular simulations |
In this talk, I will first present non-equilibrium molecular dynamics (NEMD) simulations to characterize the liquid flow over different nano-structured surfaces. I will discuss the role of liquid-gas coverage in determining surface slip properties. In the second part of my talk, I will present the slip gas-cushion model (slip-GCM) that we recently developed to estimate the apparent slip of liquid flowing over a gas film, which manifest rarefied gas effects. Our slip-GCM enables to calculate an effective hydrodynamic slip length given the gas film thickness, Knudsen number and bulk fluid viscosity. I will present results showing a good agreement between our slip-GCM and NEMD simulations of shear-driven liquid flow over a gas nanofilm covering a solid surface.
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Srinivasa Ramisetti, University of Leeds (UK) | Salle des séminaires LOMA (A4N) |
| Mardi 3/07/2018 à 14h |
Light is a fascinating phenomenon. It has been since the beginning of the XXth century described as a gas of non-interacting particles: photons. Confined between two mirrors in a cavity, photons acquire an effective mass and, due to imperfect mirrors a finite lifetime. Con- sequently, this gas exchanges constantly with its environment through dissipation. Based on methods borrowed from both open quantum system and statistical physics, the thermodynamics of these exchanges can be resolved to reveal its true stochastic nature as I will present in a first part [1]. This allows, behind others, to the definition of fluctuation theorem [2]. Moreover, me- diated light-matter interaction, effective photon interaction can be engineered. In this context, cavity light behaves as a fluid and surprisingly as a quantum fluid [3]. In a second part, I will explore this hydrodynamical analogy illustrating how the underlying driven-dissipative nature of quantum fluids of light modifies their properties as well as the topological excitations that may form in them [4].
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Simon Pigeon (Laboratoire Kastler Brossel, Paris) |
Salle des séminaires LOMA (A4N) | |
| Mardi 26/06/2018 à 14h | Granular friction: from building the pyramids to the anatomy of individual contacts at the nanoscale |
I will discuss the rheology and mechanical properties of wet granular materials, and show why the behavior can be very subtle. Once one understands the mechanical properties, I will show that one can use this knowledge to construct the perfect sandcastle, or to understand why the ancient Egyptians wetted the desert sand with water before sliding heavy stones over it (Figure).
I will then go on to show some new results on friction at the microscopic scale, between 2 grains. Amonton’s famous friction law states that the friction force is proportional to the normal force since both are proportional to the area of contact. However for spherical grains, the contact area is not proportional to the normal force, as shown by Hertz long ago. We use a new fluorescence technique that allows us to probe the real area of contact between 2 rough surfaces. In our case, we conclude that important deviations from Amonton’s law are observed. |
Daniel Bonn (Institute of Physics, University of Amsterdam) |
Salle des séminaires LOMA (A4N) |
| Mardi 19/06/2018 à 14h | Topological phase detection in Rashba nanowires |
We study the detection of the topological phase transition occurring in Rashba/Majorana nanowires with proximity-induced superconductivity using a quantum dot. The lowest in energy bulk states of such a nanowire have a spin polarization parallel or antiparallel to the applied magnetic field in the topological or trivial phase. We show that this property can be probed by the quantum dot created at the end of the nanowire by external gates. By tuning one of the two spin-split levels of the quantum dot to be in resonance with nanowire bulk states, one can detect the spin polarization of the lowest band via transport measurement. Thus, this allows us to determine the topological phase of the Rashba nanowire independently of the presence of the zero-energy Majorana bound states. Our work opens an entirely new path for experimental approaches to detect topological superconductivity. Our results demonstrate the feasibility of such measurements in realistic setups and thus provide a strong incentive for experimental implementations for many labs worldwide. Such experiments would be highly welcome by the community as they can provide independent support of topological phases, and in this way help to resolve the controversy surrounding the interpretation of zero-bias peaks as Majorana bound states.
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Denis Chevallier (Université de Bâle, Suisse) |
Salle des séminaires LOMA (A4N) |
| Mercredi 23/05/2018 à 14h | Soft Materials at surfaces and interfaces: Elastocapillarity |
The physics of soft materials is distinct from hard matter as the weaker intermolecular bonds can result in a large response to external stresses. In recent years, there has been a significant interest in understanding the interaction between a liquid’s surface tension and a solid’s elasticity: elastocapillarity. In particular, liquids can generate significant deformations of highly compliant materials. These elastocapillary interactions are highly relevant in a wide variety of systems including capillary origami and folding, soft tissues, wetting of fibers and hair, and micro-patterning of soft surfaces. In this talk I will summarize our recent work on the capillary interactions of liquid droplets with compliant elastic structures.
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Kari Dalnoki-Veress (McMaster University, Hamilton, ON, Canada) | Salle des séminaires LOMA (A4N) |
| Mardi 22/05/2018 à 14h | Avalanches in dead and living systems: From solids to society |
The behaviour of many systems can be described in terms of a (typically large) number of elementary units and their interactions. Examples range from solids to the human body and society. A small change in the environment of such systems may lead to avalanche-like responses of the system associated with sudden changes of state in a large number of elementary units. In the last decades, network models have become essential abstractions of such systems. They are applications of graph theory in which a set of vertices (or nodes) represent elementary units and a set of edges (or links) gives the interaction between them. In this talk, I will present prototype network models for avalanches in solid-solid phase transitions, capillary condensation in porous media and avalanche-like phenomena in social contagion.
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Francisco Perez Reche (University of Aberdeen) | Salle des séminaires LOMA (A4N) |
| Mardi 15/05/2018 à 14h | Nano-optomechanics with suspended nanowires |
We will present our recent developments in the realisation of ultrasensitive vectorial force field sensors based on suspended silicon carbide nanowires, at room [1] and dilution temperatures and analyse the impact of non-conservative (rotational) force fields on their dynamics: leading to eigenmode orthogonality breaking, noise reduction and violation of the fluctuation dissipation relation [2].
[1] L. Mercier de Lépinay et al, Nature Nanotech. 12, 156 (2017) [2] L. Mercier de Lépinay et al, Nature Comm. 9, 1401(2018) |
Olivier Arcizet (Institut Neel, Grenoble) | Salle des séminaires LOMA (A4N) |
| Vendredi 04/05/2018 à 11h | What have we learned after two decades of studying confined polymer glasses? |
One of the most challenging problems in condensed matter physics is the nature of the glass and the glass transition. Despite decades of experimental and theoretical research, a detailed description remains elusive. I will describe general features of glass forming materials, the questions brought up, and the ideas that have been put forward to understand them. I will then discuss the development of an experimental research plan involving polymer glasses developed to shed light on some of these questions. Such experiments include size dependent effects on glass formation, the nature of the free surface of glasses and how that affects glass formation, and how polymeric glasses may differ from small molecule glasses.
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James Forrest (University of Waterloo) | Salle des séminaires LOMA (A4N) |
| Mardi 24/04/2018 à 14h | Soft lubrication Lift forces at a bascular wall mimic |
Interactions between circulating cells and blood vessel walls are central to many physiological processes such as the early stages of the immune or inflammatory response, gas exchanges with tissues, or vascular remodeling. Among these interactions, hydrodynamic forces play a key role, as they control the radial migration of the circulating cells towards or away from the vascular walls. A point that has been largely overlooked so far is the contribution of the wall deformability to such hydrodynamic forces, and in particular the role of the endothelial glycocalyx, a compliant meshwork of polysaccharides lining the lumen of blood vessels. In this context, we have developed an experimental setup allowing for the study of microparticles flowing past a surface bearing a macromolecular layer mimicking the nature, thickness and elastic properties of the endothelial surface layer. Combining parallel plate flow assays and 3D particle tracking based on interference microscopy, we show that:
(i) non-deformable spherical microbeads traveling close to the macromolecular layer are repelled and lift away from the surface under strong enough (yet physiological) shear rates, (ii) the bead/surface distance increases with increasing shear rate and/or layer compliance, (iii) our experimental results can be quantitatively described in the theoretical framework of elastohydrodynamics accounting for the effect of substrate deformations. This work underlines the important mechanical role that the soft endothelial glycocalyx is likely to play in regulating cell/wall interactions in blood flow. |
Lionel Bureau (Université de Grenoble Alpes) | Salle des séminaires LOMA (A4N) |
| Mardi 03/04/2018 à 14h | Model Experiments of Active Matter : at the interface between living organisms and theoretical models |
The ubiquity of collective motions observed at all scales in biological systems has driven a surge of scientific activity. Within physics, important theoretical progress was achieved by studying microscopic point-particles models and their continuous descriptions. Among the landmark results are the possibility of a true long-range polar ordered collective motion as well as of a Motility Induced Phase Separation (MIPS). The robustness of these observations against the numerous factors integrated out in the above effective models is a matter of crucial importance.
This is where human-designed model experimental systems have a key role to play. Janus colloids, swimming droplets or walking grains are amazing experimental realization of self propelled particles. They are far more simple than their biological inspiration, and already contain important realistic factors, such as hydrodynamics effects and pairwise force interactions, which, at least in principle, can be controlled. In the present talk, I will illustrate that matter in the case of two remarkable experimental systems, namely rolling colloids [1] and walking grains [2,3].[1] Bricard, A., Caussin, J.-B., Desreumaux, N., Dauchot, O. & Bartolo, D. Emergence of macroscopic directed motion in populations of motile colloids. Nature 503, 95–98 (2013). [2] Deseigne, J., Dauchot, O. & Chaté, H. Collective Motion of Vibrated Polar Disks. Phys. Rev. Lett. 105, (2010). [3] Briand, G. & Dauchot, O. Crystallization of Self-Propelled Hard Discs. Phys. Rev. Lett. 117, 098004–5 (2016). |
Olivier Dauchot (ESPCI, Laboratoire Gulliver) | Salle des séminaires LOMA (A4N) |
| Mardi 27/03/2018 à 14h | Moving at the air-water interface |
It is generally believed that in order to generate waves, a small object (like an insect) moving at the air-water surface must exceed the minimum wave speed (about 23 centimeters per second). We show that this result is only valid for a rectilinear uniform motion, an assumption often overlooked in the literature. In the case of a steady circular motion (a situation of particular importance for the study of whirligig beetles), we demonstrate that no such velocity threshold exists and that even at small velocities a finite wave drag is experienced by the object. This wave drag originates from the emission of a spiral-like wave pattern. The results presented should be important for a better understanding of the propulsion of water-walking insects. For example, it would be very interesting to know if whirligig beetles can take advantage of such spirals for echolocation purposes.
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Elie Raphael (ESPCI, Laboratoire Gulliver, Paris) | Salle des séminaires LOMA (A4N) |
| Mardi 20/03/2018 à 14h |
Large and quantized non-linear responses in topological metals |
In this talk I will discuss two of our recent results concerning nonlinear responses of topological metals. The first is the prediction of a quantized circular photogalvanic effect, the part of the photocurrent which changes sign when the light’s polarization flips. We find it is quantized in units of a large universal constant e^3/h^2 times the Weyl monopole charge in mirror free Weyl semimetals (e.g. SrSi2), three-dimensional Rashba materials (e.g. doped Te) and multifold fermion materials with negligable spin orbit coupling. The second is the measurement and modeling of the second harmonic generation in TaAs, the frequency doubling of an incident pulse upon reflection or transmission. We find it experimentally to be extremely large and anisotropic compared to materials in the same symmetry class such as GaAs. We show that these features are well captured phenomenologically in terms of a model of coupled ferroelectric chains, which suggests that TaAs is close to saturating a photocurrent bound that we derive.
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Adolfo Grushin (Institut Néel) | Salle des séminaires LOMA (A4N) |
| Mardi 13/03/2018 à 14h |
Nanoscale Capillary Freezing of ionic loquids confined between metallic interfaces |
Ionic liquids are composed of equal amount of positive and negative ions, without any solvent. They recently received considerable attention as a new class of materials with fundamental importance for energy storage and active lubrication. Their unique properties result from the competition of strong electrostatic interactions with properly designed molecular structure to avoid crystallization at room temperature. They are however unusual liquids, which challenge fundamentally the classical frameworks of electrolytes. In this talk, we show how we can use quartz tuning fork based AFM nanorheological measurements to explore the properties of ionic liquids in nanometric confinement. We unveil a dramatic change of the ionic liquid towards a solid-like phase below a threshold thickness, pointing to capillary freezing in confinement. This threshold thickness is found to be intimately related to the metallic nature of the confining materials, with more metallic surfaces facilitating capillary freezing. We interpret this behavior in terms of the shift of the freezing transition, taking into account the influence of the electronic screening on ionic liquid wetting of the confining surfaces.
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Jean Comtet (Laboratoire de Physique Statistique, ENS) | Salle des séminaires LOMA (A4N) |
| Mardi 06/03/2018 à 14h |
Rolling and ageing in T-ramp soft adhesion of microparticle |
Immediately before adsorption to a horizontal substrate, sinking soft colloids can undergo a complex sequence of landing, jumping, crawling and rolling events. Using video tracking we studied the soft adhesion of micron-size colloids coated by polymer PNIPAM which is temperature sensitive. It allows for sticking of colloids to the surface when the temperature is above Tc = 32 ◦C. In order to capture the very final events before immobilization of colloids the T-ramp protocol was designed : the particles suspension is injected in the cell at room temperature, the temperature is increased at 10◦ C/min up to 38◦ C>Tc , and kept constant until the end of the acquisition. Attraction between beads and the flat plate is thereby triggered by crossing the critical temperature Tc = 32◦C. 3D beads motion is observed by slightly defocus microscopy in parallel illumination decorating bead image with interference rings observed with video camera. Particles are tracked in real time using a PICOTWIST apparatus and PICOUEYE software. Analysis of the tracking records indicates the Brownian rolling is the most relevant phenomenon before the immobilization. The fundamental differences between Brownian rolling and Brownian jumping are discussed.
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Drazen Zanchi (ENS) | Salle des séminaires LOMA (A4N) |
| Mardi 27/02/2018 à 14h |
Thermoplasmonic imaging: Free space propagation and confinement of plasmons in metallic structures |
Recent advances in nano-photonics lead to extreme light confinement (ELC) and light manipulation. This progress has spawned a variety of new important technological possibilities for the efficient delivery, control and manipulation of optical radiation on the nanoscale. Although the physical principles of ELC with plasmons i.e. nano-focusing has been clearly demonstrated in several studies, further fundamental studies are needed to optimise these processes and control losses in plasmonic devices for viable technological applications.This talk will introduce the coupling of the ELC with the electron/hole and lattice dynamics in metals. In one of our recent works, we have demonstrated the capability to image and film plasmon propagation in a metallic film. We probed the hot electrons heated by the plasmon dissipation via a Time Domain Thermoreflectance (TDTR). The figure describes an appropriated designed plasmonic device where plasmons can be concentrated in specific locations with a precision of about 10nm in a 100nm thick gold layer. TDTR offer a unique opportunity to reveal and study energy transport processes induced by extreme light confinement in nanometric devices that have not been explored so far in low-dimensional systems. We have measured and characterised the hot carriers generated in the hot spot and exploited the mechanism of plasmon absorption in metals for the generation of hot carriers at femtosecond time scale, and this energy conversion was measured with femtosecond pump-probe technique. Femtosecond plasmon pulses will be launched and probed over hundreds of femtoseconds through the permittivity variations induced by the hot-carriers.
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Stefan Dilhaire (LOMA, Univ BX) | Salle des séminaires LOMA (A4N) |
| Mardi 12/02/2018 à 14h |
Field-embedded particles driven by active flips |
Systems of independent active particles embedded into a fluctuating environment are relevant to many areas of soft-matter science. We use a minimal model of noninteracting spin-carrying Brownian particles in a Gaussian field and show that activity-driven spin dynamics leads to patterned order. We find that the competition between mediated interactions and active noise alone can yield such diverse behaviors as phase transitions and microphase separation, from lamellar up to hexagonal ordering of clusters of opposite magnetization. These rest on complex multibody interactions. We find regimes of stationary patterns, but also dynamical regimes of relentless birth and growth of lumps of magnetization opposite to the surrounding one. Our approach combines Monte-Carlo simulations with analytical methods based on dynamical density functional approaches.
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Jean-Baptiste Fournier (Université Paris Diderot) | Salle des séminaires LOMA (A4N) |
| Mardi 06/02/2018 à 14h |
Interfacial nanobububbles and their impact on substrates |
During wetting of a hydrophobic surface with an aqueous solution, spherical-cap-shaped interfacial bubbles with heights between 5 nm and 100 nm and diameters between 50 nm and 800 nm can be nucleated. They are normally referred to as surface or interfacial nanobubbles (NBs). NBs have great potential in numerous applications, such as drug delivery, ultrasonic tumor imaging enhancement, mineral flotation and separation, nanostructured surface fabrication.
This talk focuses on the experimental investigation of interfacial NBs using atomic force microscopes. It starts with the morphological characterization of NBs. After that, the nucleation of NBs on nanostructured surfaces will be presented. The results show that the nanobubble nucleation can be tuned by surface nanostructures, regarding size, location, or even morphology. Our study also shows that the NBs have large impact on sample substrates. They can generate nanostructures on polystyrene (PS) films, with tunable size and shape of nanostructures. The mechanism of nanobubbles induced nanoindents is revealed. |
Yuliang Wang (Beihang Université, China) | Salle des séminaires LOMA (A4N) |
| Mardi 16/01/2018 à 14h |
Self-propulsion of autophoretic particles | Designing artificial micro swimmers has so far mostly followed bio-inspired design, reproducing the helical flagellum of bacteria or the beating of sperm cells. Those require however an external magnetic forcing to move, and as such are not truly self-propelled. Fuel-based or phoretic swimmers represent a promising alternative approach. Exploiting short-range interactions and physicochemical exchanges with their environment, they create a net slip forcing at their boundary in response to a tracer gradient. In this presentation, I will propose an overview of our recent work on the fundamental principles driving the individual locomotion of such active colloids in low-Re flows, focusing specifically on the role of geometry and the different symmetry-breaking mechanisms. | Sébastien Michelin (LadHyX, Ecole Polytechnique, Palaiseau, France) | Salle des séminaires LOMA (A4N) |
| Mardi 05/12/2017 à 14h | Using the Leidenfrost effect and hot hydrogels to make better bouncy balls | The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well-understood for liquids and stiff sublimable solids, nothing is known about the effect with materials whose stiffness lies between these extremes. In this talk, I will introduce a new phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces we see that, rather than hovering, they harvest energy from their interaction with the surface and energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover high-frequency microscopic gap dynamics at the sphere-substrate interface. These otherwise-hidden agitations constitute work cycles that siphon energy from the vapour and sustain the bouncing. The findings unleash a widely applicable strategy for injecting mechanical energy into soft materials, with potential relevance to fields ranging from soft robotics and metamaterials to microfluidics and active matter. | Scott Waitukaitis (Universiteit Leiden/AMOLF, Amsterdam, The Netherlands) | Salle des séminaires LOMA (A4N) |
| Mardi 28/11/2017 à 14h | A phase diagram for the organization off cell colonies |
The regular distribution of neuronal cells, self-healing epithelial monolayers or 3D tumor formation are examples of active-organization of cells in tissues. While cell-substrate and cell-cell adhesions have a widely acknowledged implication in these macroscopic organizations, the role of the cell-specific contact repulsion called contact inhibition of locomotion (CIL) is less clear. In this work, we include these fundamental cellular functions in large scale 2D simulations of cell-like particles. We built a phase diagram which describes within the same unifying framework several of the known multicellular tissues. We provide theoretical scaling laws for transitions such as epithelial-mesenchymal transitions or the dewetting of a 2D epithelium into a 3D agglomerate, which signpost the biophysical pathways available to control these transitions. At last, I will present preliminary results of lens-free microscopy, a method that offers a unique mean to test the organization of large cell colonies.
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Romaric Vincent (CEA, Grenoble) |
Salle des séminaires LOMA (A4N) |
| Mardi 07/11/2017 à 14h | Confinement–induced dynamical regimes |
Recent studies have shown that the presence of boundaries can strongly affect the dynamics of physical systems. For example, negative mobility and fluid recirculation occurs when an electrolyte is driven in a varying-section channels, rectification occurs active particles such as molecular motors or active swimmers moving in inhomogeneous environments and off–diagonal terms in the mobility tensor appears for binary mixtures of hard sphere confined between corrugated plates. Clearly the interplay between the confined systems and the confining walls is maximized when the typical length scales of the confined system match with the size of the confining walls. In this contribution I will discuss the general mechanisms at the basis of the interplay between the confined system and the confinement. In particular, by means of a few example I will show how analytical insight into the possible regimes can be attained.
Firstly, I will discuss how the electrostatic interaction between a tracer particle and the channel walls affect particle dynamics. In such a scenario, novel dynamical regimes such as negative mobility and asymmetric passage times can arise due to the interplay between the electrostatic interactions and the local entropic drive induced by the varying–section channel. Secondly I will show how the insight gained by such studies can be transferred to the problem of polymer translocation across varying–section channels. In particular, I will show that, under suitable approximation, it is possible to reduce the problem of polymer translocation across varying–section channels to that of a single point–like particle under an effective potential that can be derived from the equilibrium local free energy of the polymer. Interestingly, the model predicts a non-monotonous dependence on the translocation time of the polymer across the pore appears. By comparing with Brownian dynamics simulations I will discuss the quantitative reliability of the point–like approximation as well as its regime of validity. Finally, when the interactions between the confined systems and the confinement are non-conservative, as it happens for hydrodynamic interactions, the interplay between the confined system, say an active swimmer, and the confinement is more involved. I will, briefly discuss how such a scenario can be described on the basis of the experienced gained with the conservative forces. |
Paolo Malgaretti (Max Planck Institute for Intelligent Systems, Stuttgart, Germany) | Salle des séminaires LOMA (A4N) |
| Lundi 16/10/2017 à 14h | Electrostatic interaction between chemically identical colloids | The interaction between two chemically identical colloidal particles dispersed in a bulk electrolyte solution as well as trapped at the interface between two immiscible electrolyte solutions will be discussed. For the former case, the surfaces of the particles are charge-regulated. It is shown that, contrary to common assumptions, such surfaces are not necessarily equally charged and the resulting interaction can be attractive instead of repulsive [1]. For the latter case, the colloidal particles carry fixed surface charges, which can be different on both sides of the liquid-liquid interface. Within the framework of nonlinear Poisson-Boltzmann (PB) theory it turns out that, under certain circumstances, an attractive interaction between them is still possible [2], which, however, cannot be captured by using the linearized PB theory [3].[1] A. Majee, M. Bier, and R. Podgornik, submitted. [2] A. Majee, M. Bier, and S. Dietrich, J. Chem. Phys. 145, 064707 (2016). [3] A. Majee, M. Bier, and S. Dietrich, J. Chem. Phys. 140, 164906 (2014). |
Arghya Majee (Max Planck Institute/Universität Stuttgart, Germany) | Salle des séminaires LOMA (A4N) |
| Mardi 19/09/2017 à 14h | Hydrodynamique des solutions de poly-électrolytes confinées : friction interfaciale et longueur de glissement complexe | Les fluides complexes sont ainsi nommés à cause de leurs propriétés rhéologiques. Mais la façon dont ils s’écoulent, en particulier aux petites échelle comme dans les applications bio-médicales ou impliquant des milieux poreux (génie pétrolier), dépend également de leurs propriétés interfaciales. Nous avons étudié des solutions de poly-électrolytes plus ou moins confinées avec un appareil de mesure de forces de s urfaces dynamique, permettant de sonder leurs propriétés visco-élastiques dans une gamme d’épaisseur allant du nanomètre à la dizaine de micromètres. Sur cet exemple nous démontrons que la notion habituelle de longueur de glissement utilisée pour décrire l’hydrodynamique interfaciale des liquides simples, ne convient pas pour les liquides visco-élastiques. Pour ceux-ci, la description appropriée est la loi de Navier originale. Celle-ci permet de décrire très précisemment la force hydrodynamique complexe sur plusieurs décades spatiales et temporelle, avec un coefficient de friction liquide/solide simple, indépendant de la fréquence, et lié à la structure interfaciale de la solution, indépendamment de la complexité de sa visco-élasticité de volume. |
Elisabeth Charlaix (Université Grenoble Alpes) | Salle des séminaires LOMA (A4N) |
| Vendredi 8/09/2017 à 14h | Landau – Zener interferometry in multilevel systems | We propose a universal approach to Landau-Zener (LZ) problem in a multilevel system. The problem is formulated in terms of generators of SU(N) algebra and maps the Hamiltonian onto the effective anisotropic pseudospin (N-1)/2 model. The vector Bloch equation for the density matrix describing the temporal evolution of the multilevel crossing problem is derived and solved analytically for two generic cases: i) three-level crossing problem representing a minimal model for a LZ interferometer and ii) four-level crossing problem corresponding to a minimal model of coupled interferometers. It is shown that the analytic solution of the Bloch equation is in excellent quantitative agreement with the numerical solution of the Schroedinger equation for the 3- and 4- level crossing problems. The solution demonstrates oscillation patterns which radically differ from the standard patterns for the two-level Landau- Zener problem: « beats », when the dwell time in the interferometer is smaller compared to a tunnel time and « steps » in the opposite limit. The possibilities of the experimental realization of LZ interferometers in the system of in two-well traps in optical lattices for ultra-cold gases are discussed. | Mikhail N. Kiselev, ICTP, Trieste, Italy | Salle des séminaires LOMA (A4N) |
| Mardi 4/07/2017 à 14h | Signatures of Novel Spin Liquids in Kagome-Like Lattices | Frustrated Magnets provide a playground for many-body phenomena, where competing magnetic short-range interactions allow for novel magnetic ground states. They offer the possibility to explore unconventional types of order, and to achieve spin liquids, which defy conventional thermodynamic ordering mechanisms down to the lowest temperatures. While conventional magnetic order can easily be seen in the magnetic susceptibility and spin-spin correlation functions, absence of order is very hard to characterise [1].
In this talk I will explain, based on two frustrated models with kagome-like structure made of triangular units, what kind of exotic thermodynamic and dynamical signatures appear in classical spin liquids. [1] Leon Balents, Nature 464, 11 (2010) |
Rico Pohle
(OIST, Japon) |
Salle des séminaires LOMA (A4N) |
| Mardi 27/06/2017 à 14h | Rides géantes sur la comète 67P/Churyumov-Gerasimenko engendrées par un vent thermique | The recent approach of comet 67P/Churyumov-Gerasimenko by the spacecraft Rosetta has revealed the presence of astonishing dune-like patterns. How can the radial outgassing, caused by heating when passing close to the sun, produce a vapor flow along the surface of the comet dense enough to transport grains? We show that the vapor flow emitted by the comet around its perihelion spreads laterally in thermal winds, due to the strong pressure difference between zones illuminated by sunlight and those in shadow. Drawing on the physical mechanisms at work for the formation of dunes on Earth and planetary bodies, we can explain the emergence of these bedforms in such extreme cometary conditions at the observed crest-to-crest size, about 10 m. Although generated by a rarefied atmosphere, they are in fact analogous to ripples emerging on granular beds submitted to viscous shear flows.
La sonde Rosetta, en orbite autour de la comète 67P/Churyumov-Gerasimenko pendant près de 2 ans, a révélé d’étonnants motifs sédimentaires ressemblant à des dunes. En effet, comment le dégazage dû au passage de la comète près du soleil, qui produit un flot de vapeur essentiellement radial, peut-il également produire un écoulement le long de la surface, assez dense pour transporter des grains? Nous montrons que ce dégazage se répand latéralement sous la forme d’un vent thermique, en réponse à la forte différence de pression entre la face au soleil et la face à l’ombre. À partir des mécanismes de formation des dunes, déclinés ici dans les conditions cométaires, nous arrivons à expliquer l’apparition de ces motifs à la taille observée: environ 10 m de crête à crête. Bien que générés par une atmosphère raréfiée, ils sont en fait analogues à des rides sur un lit granulaire cisaillé par un écoulement visqueux.
Ref: P. Jia, B. Andreotti and P. Claudin, Proc. Natl. Acad. Sci. USA 114, pp 2509-2514 (2017). |
Philippe Claudin (ESPCI, Paris) | Salle des séminaires LOMA (A4N) |
| Mardi 20/06/2017 à 14h | Questions of wetting | This talk presents two recent studies in which wetting and wettability play key roles:1) How do gas hydrates spread over a substrate ? Clathrate gas hydrates are ice-like solids in which a gas– N2, methane, CO2 or even larger– is encapsulated in molecular-sized pockets in a frozen aqueous matrix. Stable at high pressures (~100bar) and low temperatures (<~10°C), hydrates were long the bane of the petroleum industry, since they may block pipelines. But recently, they have been viewed either as a fabulous source of natural gas, locked in deposits on the ocean floors or under the permafrost, or a frightful hazard, should their accidental or intentional dissociation destabilize ocean sediments, not to mention the contribution to the greenhouse effect. Other applications include gas purification, refrigeration, and water desalination. The importance of the interaction of the hydrate with a substrate– the container, or a sediment– is recognized, but so far was relatively little examined, and hardly at all from the point of view of mechanisms at microscopic scales. We have applied optical microscopy at ~1µm resolution to elucidate the growth processes. Combining wide field and confocal measurements with phase contrast, fluorescent tracer nano-particles, and a rigidochrome fluorescent dye to highlight interfaces, we show the importance of wetting, specifically a water precursor film and a breath figure (‘dew’), both nm’s thin, in the spreading of gas hydrates over a substrate.References: M-L. Martínez de Baños et al, Cryst. Growth Des. 16 (2016) 4360-4373. N. Hobeika et al, Langmuir 33 (2017) 5179-5187.2) Fluorescent ‘marigolds’ are prettier than ‘coffee-stains’. A drying droplet containing suspended matter may leave a variety of patterns on the substrate, the most unwelcome being the familiar ‘coffee-stain’. This part will report, and we hope elicit discussion of preliminary measurements of a spectacular ‘marigold’ stain. |
Ross Brown (UPPA, Pau) | Salle des séminaires LOMA (A4N) |
| Mardi 13/06/2017 à 14h | La rhéologie des suspensions non-Browniennes : une histoire de contact | Les écoulements lents de suspensions de particules solides non-browniennes sont omniprésents dans les problématiques industrielles (polymères chargés, béton frais, moulage des propergols) et les phénomènes naturels (écoulements sédimentaires marins, glissements de terrain). Même lorsque le fluide suspendant est simple, ces suspensions exhibent une rhéologie complexe, et ce d’autant plus que la fraction de solide est élevée. Les modélisations en rhéologie des suspensions se sont longtemps focalisées sur les interactions hydrodynamiques entre particules. Cependant, la question des interactions directes entre particules, en lien avec la microstructure induite par le cisaillement, s’est posée de façon croissante, jusqu’à devenir centrale actuellement.
Dans cet exposé, j’aborderai quelques aspects récents de la physique des écoulements de suspensions non-browniennes en m’appuyant sur des expériences et des simulations. Je montrerai comment des mesures directes de la microstructure induite par un écoulement confirment l’existence de contacts directs entre particules via les rugosités de surface. Je discuterai ensuite, à travers des résultats de simulation numérique, de l’influence des forces de contact et en particulier de la friction sur le comportement rhéologique des suspensions. Enfin, je terminerai en présentant quelques expériences qui permettent d’estimer la contribution des forces de contact à la viscosité totale d’une suspension. |
Elisabeth Lemaire (Université de Nice) | Salle des séminaires LOMA (A4N) |
| Mardi 06/06/2017 à 14h | Instabilités Interfaciales : 2 illustrations | – Détergence Comment peut-on extraire de l’huile d’un milieu confiné? Nous étudions une expérience de détergence dans laquelle de l’huile piégée dans un coin formé par deux plaques est chassée par une solution de tensio-actif, mouillant préférentiellement les plaques. La rencontre des deux liquides conduit à une déstabilisation de l’interface, des doigts réguliers apparaissent progressivement et des gouttes finissent par se détacher. Ces gouttes fuient la partie confinée de la cellule et l’huile peut facilement être collectée.- Étalements fleuris de Marangoni Déposons une goutte d’un mélange d’eau et d’alcool à la surface d’un bain d’huile. Si la goutte est suffisamment alcoolisée, elle s’étale et éclate spontanément en une myriade de gouttelettes. La taille des gouttelettes éjectées dépend fortement de la concentration initiale d’alcool. Tout comme dans l’effet « larmes de vin », l’évaporation plus rapide de l’alcool induit des gradients de tension de surface (effet Marangoni) qui eux-mêmes engendrent un écoulement. Le couplage complexe entre hydrodynamique interfaciale, mouillage et évaporation peut être appréhendé par des lois d’échelle analytiques. Nous tenterons d’illustrer ces deux phénomènes par des démonstrations en direct. |
José Bico & Ludovic Keiser (PMMH, ESPCI, Paris) | Salle des séminaires LOMA (A4N) |
| Mardi 30/05/2017 à 14h | Electrostriction and giant permittivity of polymer nanocomposites | The inclusion of conductive particles into insulating polymer matrices allows the synthesis of nanocomposites with tunable dielectric properties. In particular, giant permittivity is achieved when the conductive inclusions form near percolated networks. The permittivity of such nanocomposites strongly varies when the soft polymer matrix is deformed, giving rise to large electrostriction coefficients as needed in variable capacitors of energy harvesting devices. Near percolated networks can easily be obtained with particles of anisotropic shape, such as carbon nanotubes, metal nanowires or graphene platelets. Because of their large aspect ratio, the particles exhibit a large excluded volume and a resultant low percolation threshold compared to spherical or quasi-spherical particles. The control of the spatial organization of the particles in the matrix is critical for the control of the material dielectric properties. We will present recent approaches to control the ordering of carbon nanotubes using emulsion templates to obtain enhanced dielectric permittivity and electrostriction coefficients [1]. We will also discuss differences or rod like particles from graphene platelets with giant anisotropy. The percolation behavior of graphene platelets has been recently predicted to be far more complicated than generally anticipated by excluded volume concepts [2]. Here, by characterizing the percolation transition in a liquid crystalline graphene based elastomer composite, we confirm experimentally that graphene flakes self-assemble into nematic liquid crystals (LCs) at concentrations below the percolation threshold [3]. We find that the competition of percolation and LC transition provides a new route towards high-permittivity materials. Near-percolated liquid crystalline graphene based composites display a giant permittivity along with a low loss tangent. The near percolated nanocomposites exhibit large permittivity variations in response to small strain deformations, giving rise to a giant electrostriction coefficients of about M= -5×10-14 m2/V2 at 100 Hz. The present materials are promising for uses in variable capacitors of energy harvesters. Their implementation in actual electronic devices is currently investigated. [1] Luna, A., Yuan, J., Neri, W., Zakri, C., Poulin, P. and Colin, A., (2015) Giant Permittivity Polymer Nanocomposites Obtained by Curing a Direct Emulsion. Langmuir 31: 12231-12239. [2] Mathew, M.; Schilling, T.; Oettel, M., Phys. Rev. E 2012, 85 (6), 061407. [3] Graphene Liquid Crystal Retarded Percolation for High Permittivity Materials , J. Yuan, A. Luna, W. Neri, C. Zakri, T. Schilling, A. Colin, P. Poulin , Nat. Comm. (2015) 6, 8700. |
Philippe Poulin (CRPP, Bordeaux) | Salle des séminaires LOMA (A4N) |
| Mardi 09/05/2017 à 14h | Le monde des brevets | We all know that inventions can be protected thanks to a patent. But what exactly is a patent? What types of inventions can be protected by a patent? And, also how can a patent be obtained ?
My presentation will focus on an introduction of the patent field to help determining how research results can be protected or not by the intellectual property laws. |
Marion Fauré (Cabinet Plasseraud, Paris) | Salle des séminaires LOMA (A4N) |
| Mardi 02/05/2017 à 14h | Mécanisme de biogénèse de gouttelettes lipitiques dans les cellules | Les cellules eucaryotes contiennent un grand nombre de compartiments, ou « organelles », délimités par une membrane fluide faite de lipides et protéines. Il s’agit d’une organisation dynamique dans laquelle des structures apparaissent et disparaissent continuellement par déformation, fission et fusion de membrane. Ces processus sont en général effectués par des machineries protéiques consommant de l’énergie.Au cours de ce séminaire, je discuterai d’un modèle théorique de genèse d’un type particulier d’organelle appelée les gouttelettes lipidiques dont le rôle est de stocker et délivrer sur demande l’excès de gras ingéré par une cellule. Je montrerai que la fabrication d’une nouvelle gouttelette lipidique pourrait résulter d’une instabilité dans la membrane de l’organelle mère, instabilité contrôlée par les tensions interfaciales en jeu et les propriétés élastiques de la membrane. | Lionel Foret (ENS, Paris) | Salle des séminaires LOMA (A4N) |
| Mardi 25/04/2017 à 14h | What can we learn from Friedel oscillations in 2d semimetals? | During this seminar, we will revisit the natural problem of elastic scattering through a localised impurity in (quastwo-dimensional (semmetals. Such an impurity is responsible for quantum interferences known as Friedel oscillations, i.e., long-range oscillations in the electronic density whose algebraic decay with the distance to the impurity was shown to be universal in every dimension [1]. However, the observation of Friedel oscillations in graphene via scanning tunnelling microscopy (STM) actually taught us that their algebraic decay can be different from the one in a two-dimensional electron gas and is, in this sense, non-universal [2]. This non-universality has been related to the impossibility for the graphene electrons to be backscattered.On the one hand, we shall see how such non-universal behaviours of Friedel oscillations may be used in STM experiments to characterise a two-dimensional topological Lifshitz transition, namely the Dirac cone merging transition [3].On the other hand, we will discuss Friedel oscillations in a quasi-two-dimensional semimetal, that is, rhombohedral N-layer graphene, one of the two most stable staking of graphene layers experimentally isolated. We will show that, even when Friedel oscillations exhibits conventional algebraic decay, there exists additional observable oscillations from which we can reconstruct the whole low-energy Bloch band structure of the material. This suggests that it would additionally be possible to image the 𝜋-quantised Berry phases that topologically protect the existence of nodal points in the semimetallic spectrum from STM experiments [4]. |
Clément Dutreix,
Laboratoire de Physique, ENS de Lyon |
Salle des séminaires 2e étage Bât A4N |
| Mercredi 08/03/2017 à 14h | Searching for spin liquids in pyrochlore magnets |
Spin liquids are often thought of as interacting magnetic systems which refuse to break a symmetry at any temperature. There is, however, much more to the physics of spin liquids than that- they exhibit novel and subtle properties such as algebraic correlations, topological excitations and emergent gauge fields.
Amongst the best studied examples of spin liquid physics are the spin ice materials Ho2Ti2O7 and Dy2Ti2O7, frustrated magnets on the pyrochlore lattice. Here, the spin liquid state is of a classical nature, being stabilised by its entropy. The topological excitations in spin ice take the form of magnetic monopoles and the correlations are described by an “electromagnetic » gauge field. These discoveries in the spin ices have led to an intense research interest in other pyrochlore materials, with the goal of finding a quantum variant of the spin ices in which the spin liquid is dominated by quantum fluctuations and the T=0 ground state is a highly entangled superposition of many spin ice configurations. Experimental studies of these candidate “quantum spin ices” have revealed many unexpected properties, however, which don’t fall easily into the spin ice picture. In this talk I will review progress in this area and present a theory of these frustrated pyrochlore materials, starting from a symmetry-based treatment of their anisotropic exchange interactions. This leads to an understanding of how competing phases govern the properties of many pyrochlore magnets, and to a prediction of completely new spin liquid phases falling outside the spin-ice paradigm. |
Owen Benton (RIKEN, Tokyo, Japon) | Salle des séminaires LOMA (A4N) |
| Mardi 07/03/2017 à 14h | Les voiliers modernes vus par un physicien |
Les voiliers de courses modernes sont en train de révolutionner leur mode de navigation. En s’élevant de plus en plus au-dessus de l’eau, perchés sur leurs « foils », ils ont battu de nombreux records notamment cet hiver.
En partant des principes physiques du fonctionnement d’un voilier je présenterai les causes et les conséquences de ces changements (par exemple que ces voiliers peuvent naviguer plus vite que le vent) et nous discuterons de ce qui peut encore limiter la vitesse de ces engins à voile.
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Marc Rabaud (Univ. Paris Sud) | Salle des séminaires LOMA (A4N) |
| Mardi 28/02/2017 à 14h | How predictable is evolution? | The relative importance of determinism and contingency in biological evolution is the subject of a long-standing debate. In the words of Stephen J. Gould, if we could replay the tape of life on earth, would the outcome at all resemble the present biosphere? Recent work in the more modest arena of experimental evolution with microbes has begun to address some aspects of this question. In the colloquium I will introduce different notions of predictability that are relevant in this context, and describe factors that affect the stochasticity of the adaptive process in ways that can be quantified by mathematical models. In particular, the roles of the distribution of fitness effects, the constraints due to the structure of the space of genotypes, and the size of the adapting population will be discussed in some detail. | Joachim Krug (Univ. Cologne) |
Salle des séminaires LOMA (A4N) |
| Jeudi 23/02/2017 à 14h | Kolmogorovian turbulence in transitional pipe flows | As everyone knows who has gradually opened a water faucet, pipe flows are initially laminar but become turbulent at high flow velocities. At intermediate velocities there is a transitional regime in which plugs of laminar flow alternate along the pipe axis with “flashes” of fluctuating, non-laminar flow. While it is known that flashes can fission, decay, and crowd out the intervening laminar plugs, the nature of “flash flow” (the flow inside flashes) remains poorly understood. In this talk, we show experimentally that flash flow satisfies the Blasius law of fluid friction, which is diagnostic of turbulence; thus, flash flow is likely to be but turbulent flow. To verify this possibility, we show that the statistics of flash flow are in keeping with Kolmogorov’s theory of the statistical structure of turbulence (so that, for example, at high wavenumbers, the turbulent-energy spectra of flash flow collapse onto the turbulent-energy spectra of turbulent flow, consistent with small-scale universality), with the implication that transitional pipe flows are segregated mixtures of a laminar phase and a turbulent phase. Our findings lend support to the widely-held, but hitherto unsubstantianted, notion that transitional pipe flows signal a non-equilibrium phase transition to turbulence. This research was carried out with Rory Cerbus, Chien-Chia Liu, and Pinaki Chakraborty, Okinawa Institute of Science and Technology Graduate University. | Gustavo Gioia
(OIST, Japan) |
Salle des séminaires LOMA (A4N) |
| Mardi 21/02/2017 à 14h | Multi-terminal Josephson junctions a topological matter |
We show that n-terminal Josephson junctions with conventional superconductors may provide a straightforward realization of tunable topological materials in n−1 dimensions, the independent superconducting phases playing the role of quasi-momenta. In particular, we find zero-energy Weyl points in the Andreev bound state spectrum of 3 and 4-terminal junctions. The topological properties of the junction may be probed experimentally by measuring the transconductance between two voltage-biased leads, which we predict to be quantized
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Manuel Houzet (INAC/PHELIQS CEA/Université de Grenoble | Salle des séminaires LOMA (A4N) |
| Mardi 17/01/2017 à 14h | Drude weight fluctuations in many-body localized systems | Many-body localized systems are supposed to be perfectly insulating for temperatures below a critical temperature Tc. In this talk, we will discuss some of the transport properties of systems showing a transition to a many-body localized phase. In particular, we numerically investigate the distribution of Drude weights D of many-body states in disordered one-dimensional interacting electron systems across the transition. Drude weights are proportional to the spectral curvatures induced by magnetic fluxes in mesoscopic rings. They offer a method to relate the transition to the many-body localized phase to transport properties. In the delocalized regime, we find that the Drude weight distribution at a fixed disorder configuration agrees well with the random-matrix-theory prediction P(D)∝ (γ^2+D^2)^(-3/2), although the distribution width γ strongly fluctuates between disorder realizations. A crossover is observed towards a distribution with different large-D asymptotics deep in the many-body localized phase, which remarkably is not reproduced by the expected Cauchy distribution. We show that the average distribution width <γ>, rescaled by LΔ, Δ being the average level spacing in the middle of the spectrum and L the systems size, is an efficient probe of the many-body localization transition, as it increases/vanishes exponentially in the delocalized/localized phase. | Michel Filippone (Université de Genève) | Salle des séminaires LOMA (A4N) |
| Mardi 13/12/2016 à 14h | Non-Markovian quantum thermodynamics: second law and fluctuation theorems | Fluctuation theorems show us that the second law of thermodynamics is only valid on average, with fluctuations violating it on short time-scales. I will give a gentle introduction to fluctuation theorems; explaining why they are interesting, and deriving one for a simple quantum system.I will then discuss our recent work which brings together Keldysh theory and quantum thermodynamics, by showing that a real-time diagramatic technique can provide a quantum equivalent of stochastic thermodynamics for non-Markovian quantum machines (heat engines, refrigerators, etc). We identify symmetries between quantum trajectories and their time-reverses on the Keldysh contour. These lead to quantum fluctuations theorems the same as the well-known classical ones (Jarzynski and Crooks equalities, non-equilibrium partition identity, etc), but which hold whether the system’s dynamics are Markovian or not. Our proof applies to systems with Kondo effects or other strong correlations, and to systems in superposition states or with time-dependent driving. [Preprint: R.S. Whitney arXiv:1611.00670] |
Dr. Robert S Whitney (Grenoble, France) | Salle des séminaires LOMA (A4N) |
| Lundi 12/12/2016 à 10h | Quantum non-linear dynamics of photons and Cooper-pairs in a superconducting circuit | Embedding a Josephson junction within a superconducting microwave cavity provides a novel way of exploring strongly non-linear quantum dynamics. Energy from a flow of Cooper pairs through the Josephson junction (driven by an applied voltage) pumps the cavity generating a large population of photons which in turn strongly affect the charge dynamics. This results in interesting non-equilibrium steady-states in the cavity which can have a range of non-classical properties. In this talk I will outline a simple theoretical model inspired by recent experimental work on Josephson junction-cavity systems and describe the coupled dynamics of the cavity photons and Cooper pairs. | Andrew Armour (University of Nottingham) | Salle des séminaires LOMA (A4N) |
| Mardi 06/12/2016 à 14h | On light slower and faster than light | We shall present, in a very-easy-to-understand language, three curiosities associated with light propagation and light scattering as follows : (1) It has been recently experimentally demonstrated that transverse spatially structured photons travel in free space slower than the speed of light [1]. We shall show that this experimental fact receives a very simple explanation in the language of Maxwell electromagnetism in the framework of the description of an arbitrary shaped beam in terms of an expansion over a (usually) continuous set of plane waves, providing an angular spectrum decomposition. The argument relies on the fact that almost all plane waves in the spectrum are tilted and therefore propagate along the direction of propagation of the beam with a velocity component lower than the speed of light. (2) Incidentally, the same decomposition provides an easy way to understand the failure of the optical theorem in the case of arbitrary shaped beam illumination of particles which has been analytically demonstrated a few years ago with fairly sophisticated mathematical instruments (both for vectorial [2] and scalar beams [3]). The argument relies on the fact that each plane wave has its own forward direction and that the forward direction of almost all plane waves is different from the forward direction of the beam. (3) opposite to item 1 dealing with light propagating slower than light in vacuum, we shall demonstrate that intensity spots inside a spherical particle illuminated by a pulsed laser may travel faster than light [4], and shall speculate on circumstances which would allow to produce Cerenkov radiation. The argument relies on the fact that, in nonlinear media, high energy intensity spots would possibly produce quasi-particles.
References |
Gérard Gouesbet (Université de Normandie, Rouen) | Salle des séminaires LOMA (A4N) |
| Mardi 22/11/2016 à 14h |
Computational Nonlinear Optics | After a short general introduction, I will focus on optics in so-called nonlocal nonlinear media. It is nowadays well appreciated that nonlocality has a dramatic impact on the dynamics in nonlinear systems. Nonlocality is usually associated with transport processes or inter-molecular long-range interactions. After a brief introduction to nonlocal nonlinear systems, with special emphasize on optics, I will first talk about wave collapse in systems with attractive spatially nonlocal nonlinearities. I will present rigorous collapse criteria for systems with singular nonlocal nonlinear interaction, similar to those known for the famous local nonlinear Schroedinger equation. In the second part of my talk I will switch to self-organization of light in optical media with competing nonlinearities. Self-organization constitutes one of the most fascinating phenomena appearing in nonlinear systems. During the process, strong interactions among the system components lead to the formation of spatial structures and long-range ordering. We recently demonstrated that the nonlocality of competing focusing and defocusing nonlinearities gives rise to self-organization and stationary states with stable hexagonal intensity patterns. Signatures of this long-range ordering turn out to be observable in the propagation of light in optical waveguides and even in free space. |
Stefan Skupin (CELIA, Bordeaux) | Salle des séminaires LOMA (A4N) |
| Mardi 08/11/2016 à 14h |
Glass Transition at Interfaces | The simple geometry of a polymer film on a substrate with a step at the free surface -a stepped film- is unfavourable due to the excess interface induced by the step, thus allowing for a fine rheological nanoprobe. After describing the experimental technique and the theoretical tools [1], we demonstrate how they enable to directly probe and understand the surface evolution of thin polymer films below the glass-transition temperature Tg [2]. While above Tg the entire volume between the substrate and the free surface participates to the flow, below Tg only a near-surface region responds to the excess interfacial energy. In the latter case, the developed thin-film theory for flow limited to the free-surface region is in excellent agreement with experimental data. Furthermore, a microscopic theory based on random cooperative strings of molecular rearrangements provides support to those observations and allows to explain the Tg reductions observed in thinner films [3]. Strikingly, the system transitions from whole-film flow to surface-localised flow over a narrow temperature region near the bulk glass-transition temperature. The measurements and models presented provide a quantitative measure of surface mobility. Therefore, this study may contribute to feed further the ongoing debate around glass transition, at interfaces, and in confinement.  |
Thomas Salez (ESPCI, Laboratoire Gulliver) | Salle des séminaires LOMA (A4N) |
| Mardi 18/10/2016 à 14h |
Conical refraction: fundamentals and applications | In 1832 Hamilton predicted that a collimated light beam propagating through a biaxial crystal parallel to one of its optical axes refracts as a slanted cone within the crystal and emerges as a hollow light cylinder, this optical effect being named as conical refraction (CR). In this seminar, I will explain the CR phenomenon following different approaches that allow understanding light propagation in biaxial crystals, including the case of multiple crystals in cascade. We will also see the description of the singular properties of the CR beams, presenting some examples such as optical bottle beams and beams carrying orbital angular momentum. All these features will serve to introduce some of the most appealing applications of CR in the fields of optical trapping, free-space optical communications, polarization metrology, super-resolution imaging, two-photon polymerization, and lasers.
Légende : Illustration of conical refraction of an ideally collimated beam propagating through a biaxial crystal
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Alex Turpin (Spain) | Salle des séminaires LOMA (A4N) |
| Mercredi 12/10/2016 à 14h | Nanometric imaging of ultrashort plasmonic fields in single metallic nanoobjects | Surface plasmons are elementary optical excitations resulting from the collective oscillation of the conduction electrons at metal surfaces. They dictate the optical properties of metallic nanoobjects. Surface plasmons allow confining optical energy at nanometer spatial scales, far beyond the diffraction limit. When the optical energy is provided by few femtosecond laser pulses, this strong confinement in space goes along with a strong confinement in time. While such strong spatio-temporal confinement opens up exciting perspectives, measuring the plasmonic fields induced by few-femtosecond laser pulses at the relevant spatial scales is extremely challenging to achieve. We recently combined PhotoEmission Electron Microscopy (PEEM) with a few-femtosecond laser system to image ultrashort plasmonic fields in metallic nanoobjects with nanometric spatial resolution. After introducing PEEM as a plasmonic imaging tool, I will explain how, by using an interferometer, we could demonstrate that these ultrashort plasmonic fields show different time evolution between single nanoobjects as well as within single nanoobjects. I will then discuss how and why the time evolution of the plasmonic fields varies locally. In particular, I will highlight the main parameters governing the time evolution, namely, the laser excitation used in the setup and the shape of the nanoobject. |
Arthur Losquin (Lunds Universitet) | Salle des séminaires LOMA (A4N) |
| Mardi 11/10/2016 à 14h |
Cooperativity and laning of driven travers | Several tracers driven in a dense bath tend to follow each other to increase their mobility, thereby forming lanes. This phenomenon is encountered in suspensions of charged colloids as well as in pedestrian traffic. The nature of this so-called « laning transition » has been debated: is it a crossover or a phase transition? In order to quantify the order in the system, we focus on the correlations between the tracers themselves and between the tracers and the bath. Our analytical approach is based on the linearization of the stochastic equations for the density fields. The correlations are found to be anisotropic and long ranged. Brownian dynamics simulations confirm our results and show that the shape that we predict holds far beyond the validity range of our computation, suggesting that it is universal. Finally, I discuss the model dependence of this behavior. | Vincent Démery (Paris) | Salle des séminaires LOMA (A4N) |
| Lundi 10/10/2016 à 11h |
Helicity and duality symmetry in light-matter interactions | My research has been revolving around the use of electromagnetic helicity and electromagnetic duality in light matter interactions. I use this pair in the general context of exploiting the symmetries of the scattering operators of the objects of interest. In my talk, I will give an overview of the framework and then speak about some results that I have obtained using it. For example, insights in optical activity, generalized zero back-scattering conditions, and a robust definition of electromagnetic chirality. I will then speak about future work, where I wish to use symmetry derived guidelines for the design of nanostructures with particular light-matter interaction properties. | Ivan Fernandez-Corbaton | Salle des séminaires LOMA (A4N) |
| Mardi 28/06/2016 à 14h |
Une dualité onde-particule à échelle macroscopiques : les effets d’une « mémoire de chemin » | Une gouttelette rebondissant sur un bain liquide vibrant peut devenir auto-propulsée par son couplage aux ondes de surface qu’elle génère. Il en résulte la formation d’une entité composite associant une particule et une onde. A travers plusieurs expériences nous nous sommes intéressés à une même question: comment une goutte localisée et insécable peut-elle avoir une dynamique commune avec une onde spatialement étendue? Il y a entre ces deux composantes du système un échange itératif d’information. C’est la goutte qui génère l’onde et c’est cette dernière qui détermine où la goutte va aller. Il ne s’agit pas d’une écho-localisation classique car l’onde associée est stationnaire. A chaque rebond sur le bain vibrant la goutte crée une onde de Faraday localisée dont on peut régler à volonté le temps de vie Tau. Si ce Tau est long le champ d’onde global résulte de la superposition des ondes successivement émises. Il contient donc dans sa structure d’interférence une mémoire de la trajectoire antérieure. De façon surprenante, en présence de cette “mémoire de chemin“, plusieurs comportements de dualité apparaissent, conduisant dans ce système, en dépit de sa nature classique, à des formes de quantification. On discutera plus particulièrement des orbites possibles d’une goutte soumise à une force centrale. | Yves Couder (Paris) |
Salle des séminaires 3ème étage |
| Jeudi 09/06/2016 à 14h |
Bistability and Kondo effect in molecular quantum dots: numerics vs analytical results | Electrically contacted molecules might potentially become the basic building blocks of future nanoelectronic circuitry. The paradigm for description of their transport properties is a single fermionic level coupled to a local harmonic mode and external metallic electrode(s). This model is usually referred to as Holstein-Anderson or molecular quantum dot model. Within the framework of a very simple adiabatic approximation it can be shown to possess a regime of bistability, in which the current-voltage characteristics of the system turns out to develop a hysteresis. However, the precise parameter window, in which this kind of behaviour is possible is very difficult to identify. This is the central issue of the presentation, in which the problem is going to be discussed using recent results of numerical simulations with the help of conventional and diagrammatic Monte Carlo techniques as well as multilayer multiconfiguration time-dependent Hartree method. It will be shown, that the numerical results can be understood by an explicit mapping of the low energy effective theory to that of a well-understood single channel Kondo model.
References : |
Andreas Komnik (Allemagne) |
Salle des séminaires 3ème étage |
| Mardi 24/05/2016 |
Infrared surface plasmon and waveguide modes spectroscopy for live cell biosensing | Over the past few years we study live cells using a unique infrared surface plasmon spectroscopy method. We measure infrared reflection spectrum in the vicinity of the critical angle using highrefractive index prism coated with semitransparent gold film. This optical configuration, that also known as Kretschmann geometry, allows excitation of surface plasmon polaritons which traveling through the cells cultured on the gold film. By tracking the resonant wavelength and attenuation of the surface plasmon we characterize the morphology of cell cultures and resolve different phases in cell organization.
In addition to surface plasmon resonance in infrared range, we discovered a novel bio-optical phenomenon of waveguide modes propagating inside living cells. We found that such waveguide modes can be observed in intact epithelial cell monolayers and thus can provide complimentary information about cell connectivity and cell layer thickness.
Combined together surface plasmon and waveguide modes enable precise structural analysis of cell populations in real-time and in label-free manner. This has great importance for understanding the collective behavior of cells in the context of tissues and organs.Reférences : 1. Yashunsky, V. et al. Real-Time Sensing of Enteropathogenic E. coli-Induced Effects on Epithelial Host Cell Height, Cell-Substrate Interactions, and Endocytic Processes by Infrared Surface Plasmon Spectroscopy . PloS ONE 8, e78431 (2013). 2. Yashunsky, V. et al. Surface plasmon-based infrared spectroscopy for cell biosensing. Journal of Biomedical Optics 17, 081409-1-081409-8 (2012). 3. Yashunsky, V. et al. Real-Time Sensing of Cell Morphology by Infrared Waveguide Spectroscopy. PloS ONE 7, e48454 (2012). 4. Yashunsky, V., Lirtsman, V., Golosovsky, M., Davidov, D. & Aroeti, B. Real-time monitoring of epithelial cellcell and cell-substrate interactions by infrared surface plasmon spectroscopy. Biophysical Journal 99, 4028-4036 (2010). 5. Yashunsky, V. et al. Real-time monitoring of transferrin-induced endocytic vesicle formation by mid-infrared surface plasmon resonance. Biophysical Journal 97, 1003-1012 (2009). |
Victor Yashunsky (Paris) |
Salle des séminaires 3ème étage |
| Mardi 26/04/2016 |
Approche multispectroscopique pour l’étude de l’interaction laser-solides |
L’irradiation d’un solide par une impulsion femtoseconde modifie la structure du matériau de façon transitoire ou permanente. Le fait que l’énergie lumineuse soit déposée dans le système électronique en un temps plus court que le temps d’équilibration des températures électronique et ionique conduit a une situation ou le système électronique est chaud (typiquement quelques eV) alors que les atomes sont encore froids. La physique de cet état hors-équilibre thermodynamique (parfois appelé « warm dense matter ») est à mi-chemin entre la physique du solide et celle des plasmas, ce qui la rend pour le moment très complexe à modéliser. L’interaction conduit, suivant le type de matériaux (métaux, semi-conducteurs etc.), à des processus spécifiques tels que l’affaiblissement des liaisons (« bond softening ») conduisant à la fusion « non-thermique », ou la création d’états liquides transitoires qui prennent place aux échelles femtoseconde/picoseconde. Le programme actuellement développé au sein du groupe PETRUX au CELIA a pour but de caractériser et de comprendre la dynamique de ces processus. Expérimentalement, cette compréhension nécessite de pouvoir mesurer les différents paramètres macroscopiques (températures, pression, constante diélectrique etc.) et microscopiques (ordre atomique, structure électronique etc.). Il est donc nécessaire d’appliquer une approche combinant plusieurs techniques expérimentales et différentes sources de lumière ultra-courtes (XFEL, source synchrotron, lasers optiques). En présentant les résultats de deux expériences (absorption X près des seuils (XANES) [1,2] et interférométrie fréquentielle (FDI) [3,4]) nous essaierons de montrer l’apport de cette approche « multi-spectroscopique ». Les résultats expérimentaux sont comparés à des simulations obtenues par méthode ab-initio ou de dynamique moléculaire classique. Ces résultats, outre leur intérêt purement fondamental, permettent de mieux comprendre des problèmes dans des champs aussi variés que la planétologie ou le stockage de données. Reférences : 1. F. Dorchies, et al..Phys. Rev. B 92, 144201 (2015) 2. J. Gaudin, et al. Sci. Report B 4, 4724 (2014) 3. A. Levy, et al. Phys. Plasmas 22, 030703 (2015) 4. C. Fourment, et al.. Phys. Rev. B 89, 161110(R) (2014) |
Jérôme Gaudin
(CELIA, Université de Bordeaux) |
Salle des séminaires 3ème étage |
| Jeudi 31/03/2016 |
Bad Metal Behavior and Mott Quantum Criticality |
According to early ideas of Mott and Anderson, the interaction-driven metalinsulator transition – the Mott transition – remains a sharp T=0 phase transition even in absence of any spin or charge ordering. Should this phase transition be regarded as a quantum critical point? To address this question, here we examine the phase diagram and transport properties of the maximally frustrated half-filled Hubbard model, in the framework of dynamical mean-field theory (DMFT). We identify a “quantum Widom line” (QWL) which defines the center of the corresponding quantum critical region associated with Mott metal-insulator transition in this model. The evolution of resistivity with temperature is then evaluated along trajectories following (parallel to) the QWL, displaying remarkable scaling behavior characteristic of quantum criticality. Precisely this kind of behavior was found in very recent experiments on organic Mott systems [1,2]. In the case of the doping-driven Mott transition, we show that the mysterious “Bad Metal” behavior (T-linear resistivity around the Mott-Ioffe-Regel limit) coincides with the Quantum Critical region of the Mott transition.
References :
[1] Quantum criticality of Mott transition in organic materials, Tetsuya Furukawa, Kazuya Miyagawa, Hiromi Taniguchi, Reizo Kato & Kazushi Kanoda, Nature Physics, 9 Feb. 2015; doi:10.1038/nphys3235. [2]Seealso:http://condensedconcepts.blogspot.com/2015/03/quantum-criticality-near-mott.html |
Vladimir Dobrosavljevic (Florida State University, Tallahassee, Florida, USA)
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Salle des séminaires 3ème étage |
| Mercredi 23/03/2016 |
Spin transport and giant thermomagnetic effects in ferromagnetic/superconductor structures | Recent studies of non-equilibrium spin states in superconductors revealed a rich physics originating from the interplay between magnetism and superconductivity. Several experiments [1,2] observed a giant spin accumulation available in superconducting wires generated by the current injected from normal electrodes. The striking phenomenon of an extra-large spin relaxation length and time in superconductors with strong Zeeman splitting has been reported. In this talk I will suggest a theoretical explanation of these experiments. It will be shown that a non-equilibrium spin polarization can be generated from heat injection due to the peculiar thermomagnetic effect in superconducting wires [3]. Various possibilities of generating long-living non-equilibrium spin states and manipulating spin-polarized currents in superconductors will be discussed. I will demonstrate that superconductivity can either strongly enhance or suppress the coherent spin rotation [4], depending on the type of spin relaxation mechanism being dominated either by spin-orbit coupling or spin-flip scattering at impurities.
References :[1] C.H.L. Quay, D. Chevallier, C. Bena, M. Aprili, Nature Phys. 9, 84 (2013). |
Mihail Silaev (KTH Royal institute of technology, Stockholm, Sweden) |
Salle des séminaires 3ème étage |
| Mardi 22/03/2016 |
Propriétés optiques linéaires et non-linéaires de nanofils de silicium | Les nanostructures de silicium présentent des propriétés optiques originales grâce à l’apparition de résonances optiques dans la gamme UV-NIR dépendant de leur taille et forme. Ces résonances peuvent être utilisées pour augmenter et contrôler l’interaction lumière-matière. Les efficacités de diffusion, d’absorption et le champ électromagnétique local peuvent être amplifiés et ajustés spectralement et spatialement, propriétés intéressantes pour le photovoltaïque ou les spectroscopies exaltées. Les effets non-linéaires peuvent être amplifiés par ces nanostructures, d’autant qu’ils sont intrinsèquement faibles. Ainsi, la génération de seconde harmonique (SHG), permise dans Si uniquement en cas de rupture de symétrie (surfaces), peut être augmentée par la présence de résonances et un rapport surface-sur-volume élevé dans les nanostructures. Je présenterai les propriétés linéaires et non linéaires de nanofils individuels de silicium obtenus par croissance CVD ou par lithographie électronique sur SOI. Je montrerai que les résonances de Mie sont fonction du diamètre du nanofil et de la polarisation de la lumière incidente, et que ces modes peuvent être utilisés pour modifier la photoluminescence de nanocristaux de Si placés dans le champ proche du nanofil. Ensuite, je m’intéresserai à la génération de seconde harmonique qui est fortement augmentée si une résonance de Mie correspondant à l’excitation ou à l’émission est supportée par le nanofil. A l’opposé, aucune SHG n’est détectée en l’absence de résonance. Je discuterai de l’origine de la SHG par les nanofils de Si et de phénomènes comme la rotation de polarisation non linéaire en fonction de la taille du nanofil. |
Vincent Paillard (Univ. Paul Sabatier, Toulouse) |
Salle des séminaires 3ème étage |
| Mardi 15/03/2016 |
Thermoelectric diffusion in ferrofluids: Possible application of complex fluids in waste-heat recovery | In simplest terms, thermopower describes the material’s property where an electric voltage ∆V, is induced by the application of a temperature difference ΔT across its body: ∆V=-Se ∆T, where Se is the Seebeck coefficient. The great majority of today’s thermoelectric (TE) research is focused on improving the efficiency of solid-state TE materials via nanostructuring; which presents a substantial production cost. A cheaper and simpler alternative to convert low temperature waste heat into electrical energy can be found in “liquid thermocells” using complex fluids such as ferrofluids. In ferrofluids, Se is coupled to the movement of charged ionic species (macro-ions, nanoparticles, etc.) and thus it is closely related to the “Soret effect,” i.e., the thermal diffusion of nanoparticles. At SPEC/CEA-Saclay, we are investigating the thermopower of ionically stabilized ferrofluids made of maghemite nanoparticles dispersed in various non-magnetic liquid media (e.g., dimethyl sulfoxide, water and ionic liquids) under a temperature gradient. The thermopower is measured as a function of nanoparticle concentration and the results are compared to the corresponding Soret coefficients determined via Forced Rayleigh scattering technique (by PHENIX/UPMC). We interpret the connection between the thermoelectric and thermodiffusion effects in terms of a common parameter, the Eastman entropy of transfer of nanoparticles. The values of the Eastman entropy of transfer obtained from both experiments are a few orders of magnitude larger than typical electrolyte ions and are in a quantitative agreement, lending support to the existing theoretical models. Simultaneously, our findings open a new research and technological path for ferrofluids, and other complex fluids in thermoelectric device applications. With time permitting, I will also discuss other examples of thermoelectric complex fluids currently studied in our group. |
Sawako Nakamae (CEA Saclay, IRAMIS) |
Salle des séminaires 3ème étage |
| Mardi 08/03/2016 |
Etudes expérimentales de l’instabilité de Rayleigh-Taylor dans des plasmas denses et chauds | La mise en service des lasers de puissance de nouvelle génération comme le NIF (National Ignition Facility) ou le LMJ ouvre de nouvelles perspectives pour des expériences d’hydrodynamique non-linéaire dans des plasmas denses et chauds [1]. On détaillera en particulier dans cet exposé la physique de l’Instabilité de Rayleigh-Taylor (IRT) se développant au front d’ablation. Le contrôle de l’IRT ablative est actuellement l’une des principales difficultés rencontrées dans les expériences de Fusion par Confinement Inertiel (FCI), tant en attaque indirecte [2] que directe [3].
Dans le cadre d’expériences académiques Discovery Science sur le NIF, nous avons pu étudier le stade fortement non-linéaire de l’IRT ablative en attaque indirecte [4]. En tirant parti des durées et distances d’accélération plus grandes permises par le surplus d’énergie laser, nous avons pu accéder pour la première fois en attaque indirecte à un régime de compétition / mélange de bulles au front d’ablation [5] en partant d’un motif 2D multimode de perturbations initiales (cf figure 1). La sensibilité aux conditions initiales et au mécanisme de stabilisation de l’IRT a été quantifiée au cours d’une campagne de 10 expériences s’étalant de mars 2013 à août 2015. Reférences : |
Alexis Casner (CEA LE Barp) |
Salle des séminaires 3ème étage |
| Mardi 01/03/2016 |
« Wavelet-based multifractal analysis of dynamic IR thermograms and X-ray mammograms to assist in early breast cancer diagnosis » | Breast cancer is the most common type of cancer among women and despite recent advances in the medical field, there are still some inherent limitations in the currently used screening techniques. The radiological interpretation of X-ray mammograms often leads to overdiagnosis and, as a consequence, to unnecessary traumatic and painful biopsies. First we use the 1D Wavelet Transform Modulus Maxima (WTMM) method to reveal changes in skin temperature dynamics of women breasts with and without malignant tumor. We show that the statistics of temperature temporal fluctuations about the cardiogenic and vasomotor perfusion oscillations do not change across time-scales for cancerous breasts as the signature of homogeneous monofractal fluctuations. This contrasts with the continuous change of temperature fluctuation statistics observed for healthy breasts as the hallmark of complex multifractal scaling. When using the 2D WTMM method to analyze the roughness fluctuations of X-ray mammograms, we reveal some drastic loss of roughness spatial correlations that likely results from some deep architectural change in the microenvironment of a breast tumor. This local breast disorganisation may deeply affect heat transfer and related thermomechanics in the breast tissue and in turn explain the loss of multifractal complexity of temperature temporal fluctuations previously observed in mammary glands with malignant tumor. These promising findings could lead to the future use of combined wavelet-based multifractal processing of dynamic IR thermograms and X-ray mammograms to help identifying women with high risk of breast cancer prior to more traumatic examinations. Besides potential clinical impact, these results shed a new light on physiological changes that may precede anatomical alterations in breast cancer development.
References : |
Alain Arneodo (CNRS, LOMA, Bordeaux) |
Salle des séminaires 3ème étage |
| Mardi 16/02/2016 |
Quantum Dots for Bio-energetics and Enhanced Light Harvesting; Exploration of Energy Transfer from Semiconductor Nanocrystas to Photosynthetic Biological complexes | Review will be presented of our recent results on development of hybrid material build from semiconductor nanocrystals and photoactive bio-complexes: – membrane protein bacteriorhodopsin and photosynthetic reaction centers purified from bacteria Rhodobacter spheroides. We have demonstrated that nanocrystals specifically immobilized on the surface of the photo-active bio-systems are able to play the role of a built-in light energy convertor by harvesting light which would not be absorbed efficiently by the bio-system alone. Semiconductor nanocrystals were further demonstrated to be able to transfer the harvested energy via highly efficient FRET to this complex biological system. In nonlinear optical regime the integration of nanocrystals with backteriorhodopsin results in strong wavelength-dependent enhancement of nonlinear refraction index. We have also demonstrated a first proof-of-the-principle evidence that the bacteriorhodopsin is able to utilize the transferred by nanocrystals additional energy to improve the efficiency of its biological function. This new hybrid materials with exceptional linear and nonlinear optical properties might have numerous photonic and optoelectronic applications employing photochromic, energy harvesting, transfer and conversion properties.
References : |
Yuri Rakovich (CFM, San Sebastian, Espagne) |
Salle des séminaires 3ème |
| Mardi 26/01/2016 |
Effet Marangoni de tensioactifs solubles | La présence d’un gradient thermique ou d’un gradient de concentration de surface en molécules tensioactives le long d’une interface entre deux liquides conduit à l’existence d’un gradient de tension interfaciale et à l’établissement d’un écoulement dans le volume de chaque liquide : c’est l’effet Marangoni, thermique dans le premier cas et solutal dans le second. Durant ce séminaire, je présenterai les travaux auxquels j’ai participé ces dernières années concernant l’effet Marangoni solutal [1, 2, 3]. Bien qu’important pour de nombreuses applications (enduisage, stabilité des films de savon,etc), l’effet Marangoni solutal a été étudié essentiellement dans le cas où les molécules tensioactives s’étalent à la surface libre d’une couche liquide mince dans laquelle elles sont insolubles. Cependant, la solubilité des tensioactifs doit affecter l’écoulement de Marangoni obtenu puisque les molécules peuvent non seulement se déplacer le long de l’interface, mais aussi s’en désorber pour diffuser dans le volume. Je montrerai comment nous avons relié les propriétés de l’écoulement de Marangoni à aux propriétés des tensioactifs (solubilité, présence d’impuretés, géométrie du système, etc), et les développements que nous menons autour d’une méthode rapide de caractérisation des tensioactifs. Je terminerai en présentant une étude en cours concernant la structure de l’écoulement de surface, et en particulier l’origine des paires de tourbillons bidimensionnelles observées à l’extérieur de la zone Marangoni.Références [1] M. Roché, Z. Li, I. M. Griffiths, A. Saint-Jalmes, and H. A. Stone. Phys. Fluids, 25, 091108 (2013). [2] M.Roché, Z. Li, I. M. Griffiths, S. Le Roux, I. Cantat, A. Saint-Jalmes, and H. A. Stone. Phys. Rev. Lett., 112, 208302 (2014). [3] S. Le Roux, M. Roché, I. Cantat, and A. Saint-Jalmes. accepté à Phys. Rev. E. |
Matthieu Roché (Univ. Paris Diderot) |
Salle des séminaires 3ème |
| Mardi 12/01/2016 |
Quantitative Phase Imaging in optical (super-resolution) microscopy for biology studies | Quantitative phase imaging (QPI) is a powerful optical method to enhance the knowledge about a micro/nanoscopic samples. It relies on the capability to measure not only the intensity of the light but also its phase. This provides several types of information depending on the illumination scheme or imaging mode.
QPI was initially developed to image microscopic structures inside semi-transparent samples (ex. nucleus), without labelling/staining and using coherent transmitted light -such as a laser. I will show recent breakthrough using our QPI technique based on quadriwave lateral shearing interferometry [1] coupled with incoherent halogen trans-illumination. It allows label-free cytoskeleton and organelle trafficking imaging at up to 50 Hz and for any duration [2]. I will then present for the capability to retrieve the 3D spatial distribution of particles (such as gold nanobeads) with a sub-nanometric localization by the simultaneous use of intensity and phase imaging. I will show the use of this method to stabilize the drift of a super-resolution microscope. [1] Bon, Maucort, Wattellier, and Monneret, « Quadriwave lateral shearing interferometry for quantitative phase microscopy of living cells, » Opt. Express 17, 13080-13094 (2009) |
Pierre Bon
(LP2N, CNRS, IOGS) |
Salle des séminaires 3ème |
| Mardi 08/12/2015 |
The active motion of Janus colloids at the surface of water | At the single-particle level, the main difference between active colloids and passive ones is the time scale over which the motion crosses from ballistic to diffusive regime. In both cases, friction coefficients or equivalently diffusion coefficients determine this time scale. For instance, the motion of a passive colloid of 1µm radius is diffusive when observed over lag times longer than a microsecond, once the direction of its momentum has been randomized by collisions with solvent molecules. At the macroscopic scale these collisions are accounted for by the translational friction coefficient. For an active colloid the effective diffusive behavior observed over lag times larger than few seconds results from the randomization of the direction of self-propulsion by rotational diffusion. This talk deals with an experimental investigation of the motion of isolated active Janus colloids trapped at air-water interface. Spherical catalytic Janus colloids have been prepared through the deposition of platinum metal at the surface of silica particles. Immersion depth of the Janus colloid as well as their orientation with respect to the water surface, has been characterized and interpreted in terms of the nonuniform wetting properties of the Janus particles. The motion of the active Janus colloids in the presence of various concentration of hydrogen peroxide H2O2 as fuel was characterized by video microscopy and the trajectories analyzed through the mean square displacement and the velocity autocorrelation function. The types of trajectories, directional and circular ones that we observed in our experiments, revealed the effective force and torque induced by the catalytic decomposition of H2O2. At the water surface, active colloids perform more persistent directional motions as compared to the motions performed in the bulk. This has been interpreted as due to the loss of degrees of freedom resulting from the confinement at interface and also to the partial wetting conditions that possibly bring new contributions to the rotational friction at interface.
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Antonio Stocco (Univ. Montpellier) |
Salle des séminaires 3ème |
| Mardi 01/12/2015 |
Thermal microscopy in plasmonics using wavefront sensing. Applications in physics, chemistry and cell biology |
The use of gold nanoparticle as nanosources of heat is now at the basis of a well-established area of research, termed thermoplasmonics. |
(CNRS, Institut Fresnel, Marseille) |
Salle des séminaires 3ème |
| Mardi 10/11/2015 |
Ecoulements induits par l’absorption d’une onde laser dans un système fluide à une et deux couches | Le dépôt de particules, dont un exemple est donné sur la figure a), engendré par l’évaporation d’une goutte chauffée par l’absorption d’une onde laser continue est un phénomène complexe . En effet, cette goutte est un système composé de deux fluides : un liquide et de l’air. Lorsque la goutte est chauffée, les particules sont mises en mouvement par des écoulements qui résultent de la compétition entre deux effets. D’une part, la masse volumique du liquide est modifiée par le gradient de température (flèches rouges sur le schéma b)). Le gradient de masse volumique ainsi généré est l’un des moteurs de l’écoulement : ce sont les effets thermogravitaires (flèche bleues sur le schéma b)). D’autre part, le gradient de température génère un gradient de tension de surface le long de l’interface (flèches vertes sur le schéma b)). Cela induit des contraintes visqueuses qui sont le second moteur de l’écoulement : ce sont les effets thermocapillaires. Afin de bien comprendre la compétition entre ces deux effets, nous avons utilisé une approche expérimentale et numérique afin d’étudier ces deux effets séparément. |
David Rivière (LOMA, Univ. Bordeaux) |
Salle des séminaires 3ème |
| Mardi 03/11/2015 |
Gravitation Analogue en Hydrodynamique |
Ce séminaire sera l’occasion d’introduire un courant de recherche à l’interface entre la cosmologie et la physique de la matière condensée. La gravitation dite analogue a pour objet la simulation de processus astrophysiques avec des expériences de Physique classique ou quantique . Nous présenterons nos travaux en hydrodynamique classique et plus particulièrement sur l’interaction houle-courant. Ainsi, l’analogue d’un trou noir, d’une fontaine blanche et d’un trou de ver seront discutés.
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Germain Rousseaux
(PPrime, CNRS, Univ. Poitiers) |
Salle des séminaires 3ème |
| Mardi 20/10/ 2015 |
Dual Terahertz frequency comb | THz frequency comb, composed of a series of frequency spike regularly spaced by a repetition frequency, can be used as a precise and accurate frequency marker for broadband THz spectroscopy because it can be phase-locked to a microwave frequency standard by laser control. The mode-resolved THz comb spectrum is obtained by dual comb technique and applied for low-pressure gas spectroscopy. | Takeshi Yasui
(Univ. Tokushima) |
Salle des séminaires 3ème |
| Mardi 13/10/ 2015 |
Success After University: Taking the right steps towards a career in science |
What will you do after your degree? How will you achieve your ideal science career? This interactive presentation aims to help make a smooth transition from student to professional and includes tips on networking, career paths, CV preparation, and how to market yourself to potential employers. Professional development is crucial for everyone – especially non-permanents – from Intern to Masters, from PhD to Postdoc, and everyone must actively prepare for their future career. |
Bryan Kuropatwa
(LOMA, Univ. Bordeaux) |
Salle des séminaires 3ème |
| Mardi 22/09/2015 |
Drops in an electric field: instabilities, bifurcations and phase field modelling | The influence of electric fields on the shape and evolution of drops is an important problem in the context of technological applications such as electrowetting. The term electrowetting is commonly used for some techniques to change the shape and wetting behaviour of liquid droplets by the application of electric fields and charges. First, we describe the presence of symmetry breaking bifurcations and their physical role in the development of instabilities. We also discuss the possible formation of singularities at the liquid surface. We then develop and analyze a model for electrowetting that combines the Navier-Stokes system for fluid flow, a phase-field model of Cahn-Hilliard type for the movement of the interface, a charge transport equation, and the potential equation of electrostatics. A critical role in the deduction of suitable couplings between phase field and other physical fields is played by variational principles involving suitable energies. | Marco Fontelos
(ICMAT, Madrid) |
Salle des séminaires 3ème |
