Thomas BAROIS

Surface clustering in circular arenas

The cockroach robots are forming a gas phase at low densities. Even if their elongated shape should suggest an alignment in the bulk, this is not observed because two particles colliding will bounce back with non-parallel trajectories after the collision. For moderate densities, typically 10% in surface fraction, a surface cluster is observed with a spontaneous ordering of the robots perpendicularly to the walls.

Traffic jams with robots

The robots are circulating in a circular track mimicking a lane reduction. In half of the track, the particles are in single file motion and on the other half, the track is large enough to allow the overtaking. For the lowest densities, the traffic is free. At high densities, the traffic is jammed due to a clogging of the particle at the bottleneck. For intermediate densities, the traffic is intermittent and spontaneously transits between free and jammed states.

A slowing down of the particle is operated by a modification of the track surface. An improvement of the traffic near congestion is reported according to the “slower is faster” effect.

Spontaneous sorting of circling active particles

The cockroach robots can be modified to move in circular trajectories either clockwise or anti-clockwise. Two types of motions are observed : a first mode without boundary interaction and the particle moving in quasi circular trajectories and a second mode with boundary interaction and the particles captured and guided by the walls. The wall guiding is a one way motion that depends on the particle’s polarity.

The polar wall current is at the origin of a spontaneous sorting mechanism within a population with an equal number of clockwise and anti-clockwise elements.

Superstructure

When active granular particles are placed inside a soft circular membrane, a superstructure is obtained that presents a rich dynamics arising from the coupling of the individual active elements and the deformation of the enclosing membrane. This superstructure can transit between different locomotion modes depending on the presence of particle clusters inside the membrane. This notably includes the ability to deform to go through apertures that narrower than the resting circular membrane shape.

Rotation of a chiral particle in levitation

The experiments are performed with an immerded water jet. A chiral particle is constructred by attaching a helical tail to a sphere. The chiral particle is buoyant and the jet is pointing downward (inverted levitation). The spinning velocity is proportional to the tail twist and an optimal length for the tail is found to maximize the rotation for a given twist.

This work was featured on the cover of Physics of Fluids in December 2019.

Levitation of a sphere in a double trap

Two vertical turbulent jets are used to levitate a ball. With symmetric flow rates, the particle is trapped on a fork structure, reminiscent of a pitchfork bifurcation. For asymmetric flow rates, the jet of higher flow rate may capture the ball after a cycle of flow rate.

This works show that it is possible to decide above which trap the sphere is capture only by acting on the flow rates without moving the nozzles.

Transition paths of individual particles falling in viscosity-stratified fluid

A water mixture is prepared in a columnar tank so that the viscosity is continuously varied from a high a value at the free surface to a low value at the bottom of the tank. An individual particle falling in the stratified fluid overgoes a path transition from a vertical falling regime to a oblique settling regime in the lowest part of the tank. The consequences of fluid entrainment is discussed as the origin of a delay for the establishment of the oblique regime.

Columnar structures for dilute suspensions of particle falling in the oblique regime

A set of particles is released (glass beads, 2 mm) in a fluid at rest with a viscosity such a single particle would fall in the oblique regime. For a relativelly low particle concentration (volumic fraction 10^-3 to 10^-4), the particles organize in columnar structures. The origin of this collective behaviour is attributed to the particle-particle wake interactions whose statistical occurence is enhanced because of the non vertical exploration of the particles in the non-vertical regime.

An effective reconfiguration experiment with a rigid sphere trapped in a jet

If someone wants to let think that science is somehow magic, levitation is a good option. One can use for instance the turbulent jet of a hair dryer oriented upward to capture and manipulate a ping-pong ball. The fluid forces in  a turbulent jet has some remarkable properties which makes that moving a sphere on the jet axis is somehow equivalent to change its size without moving it. This principle is used to build an analogy between the problem of a rigid sphere free to find an equilibrium position in a turbulent jet and the problem of an effective deformable sphere free to find its shape in a stationnary flow.

Reconfiguration in the constant drag regime

For a plant in the wind, the deformability makes that the relation between drag force F and velocity v is not the usual F_R ~ v² that would be measured for a rigid body (R). Actually, forces for soft objets (S) are found to obey a similar scaling relation where F_S ~ v^2-E where the exponant 2 is a reduced by a so called Vogel exponent E, that is typically of order 1 for soft elastic structure (plates, rod, leaves, branches, …). The outreach of this work was to show that it is possible to design a deformable structure for which the Vogel exponent equals 2 which means that the drag on the structure is independent of the flow velocity.

Shape of curved strips

A thin elastic plate like a sheet of paper can be easily bent by its own weight. To avoid this situation, newspaper readers have developped a well-known trick (figure, imaget 1) that is to impose a transverse curvature to the newspaper sheet they are reading. The same trick is also observed in plants (figure, imaget 1) with leaves shaped as long strips (yucca, maize, …).  In this work, a thin elastic strip has been pressed onto a curved frame. The curvature imposed by the clamping is smoothly decreasing and after a given distance, the ribbon is flat. In terms of mechanical properties, it means that the sheet is rigid with respect to bending in the curved region and soft and easily bent by gravity in the flat terminal portion.

Crumpling instability of self-contacting sheets

We consider a rectangular piece of paper and we form a curved structure by contacting the two points at the mid-length of the strip. If this simple procedure is applied for a square paper strip, the obtained structure is smoothly curved. If the procedure is applied to an elongated strip, ie a strip with the length significantly larger than the width, a stress focusing instablitity occurs and the paper sheet is permanentaly damaged at the focusing region of the unstable modes.

Frequency-driving of mechanical resonators

Silicon carbide nanowires are used as mechanical resonators. The vibration modes are observed by the harmonic forcing for some specific frequencies. The forcing is coming from an oscillating electric field applied between the nanowire and a counter-electrode next to it. The mechanical damping has been specifically investigated and it was shown that the electric forcing scheme may introduce additionnal mechanical damping.

Self-oscillation

A self-oscillator may be presented as a device that generates an oscillating output from a continuous driving. If a strong electric field is applied to a silicon carbide nanowire, a significant DC current is estalished that may eventually lead to self-oscillation. This spontaneous oscillation is characterized by a mechanical oscillation and an oscillating AC component in the current at the same frequency selected by the electromechanical system.

Response to phase shift

Phase-time diagram of a synchronized nanowire. A discontinuity is introduced in the external forcing signal to observe the dynamics of the synchronized phase.