Functionalizing slender structures with fluid interfaces: A recipe for solid-liquid mechanical hybrids

Par Hervé Elettro, Ecole Polytechnique Fédérale de Lausanne (EPFL),
Department of Mechanical Engineering, Laboratory of Fluid Mechanics and Instabilities (LFMI)

Mardi 13 Novembre, 14h00, Salle des séminaires (215), 2ème étage, Bâtiment A4N

Abstract :

Solids and liquids are on the opposite sides of the mechanical spectrum: the former is stiff while the latter is extremely stretchable. Can we get the best of both worlds ?
In this talk, I will show how the combination of liquid interfaces and slender structures can lead to unexpected hybrid properties. Proof of concepts will be given with two original systems.

The first part of my talk will be dedicated to a specific type of spider silk fibres, naturally covered with glue droplets. While the glue primary function is to maintain insects in the web after impact, we found that the droplets are also capable of reeling and coiling excess fibre. The mechanical response of the natural system is two-fold: solid-like (linear response) upon stretching but liquid-like (constant response) upon compression. The evolutionary advantage of this mechanism is to secure minimal tension in the web structure and maximize resilience under harsh natural conditions. We explained this behaviour in terms of transfer of surface energy into bending energy, and further showed that a fully synthetic system made of a soft thin structure and a wetting fluid can have the same exotic mechanical response than the natural spider silk.

In the second part of my talk, I will build upon the recent works that have shown the richness of fluid-carbon structures interactions, such as frictionless flows and exceedingly fast diffusion. We discovered that fluid adsorption can modify the mechanical properties of monolayer graphene, mainly by disrupting the out-of-plane \pi bonds. We measured a bending stiffness of 0.01 kT for graphene floating on water, almost 4 orders of magnitude below the expected theoretical value. Moreover, we found that the adsorption of carbon-based fluids drives the appearance of spontaneous wrinkles, explained by an intrinsic frustration due to lattice mismatch. We applied this result and showed that a simple optical measurement leads to quantitative details of the interaction of graphene with various molecules.