Émilie Verneuil

 

Science et Ingénierie de la Matière Molle (SIMM, ESPCI)

 

Contact and friction on hydrogels

Hydrogels consist in hydrophilic polymer networks swollen by water : as such, they stand as a good model system of poroelastic materials, often found in living matter. Hence, when a sphere is pressed against a thin layer of hydrogel, water is expelled out of the contact zone. Under set load, measuring the increase of the contact area over time provides a tool to assess both its permeability and elastic modulus. With this in mind, when the sphere now slides at the hydrogel surface, the subsequent poroelastic transport of water leads to a coupling between the friction force and the normal load depending on the sliding velocity. We provide a quantitative description of the friction in this case. Finally, by developing an original experimental setup where the sphere rotates at the hydrogel surface, we achieve much simple sliding conditions where poroelastic flows of water become negligible. We show that friction arises from a Schallamach’s sequence at molecular scale in which the polymer chains at the interface undergo sequences of pinning, stretching, and depinning events, the latter step dissipating energy. We are then able to precisely link dissipation at macro and molecular scales. Our results are found consistent for a series of molecules we graft at the sphere interface. In this regard, the present work is closely connected to force spectroscopy measurements performed in biophysics or surface physics.