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Propulsion and friction of non wetting objects
ESPCI ParisTech
We investigate special dynamics generated by non wetting objects. On a substrate textured with asymmetric sawtooth-like grooves, a Leidenfrost liquid or solid is propelled horizontally. The textures rectify the vapor flow in a well-defined direction so that the mobile is driven by viscosity. On a flat substrate, if the solid is made asymmetric, the non homogenous mass distribution tilts the bottom surface, and with it, the pressure force due to the vapor flow : its horizontal component moves the object. A Leidenfrost drop between two parallel plates also shows a peculiar behavior : beyond a critical radius, it destabilizes into a liquid ring which grows and breaks up into smaller pieces. These objects undergo very little friction on flat surfaces. It comes from inertial dissipation in the surrounding air, and from a liquid boundary layer for the droplets. However, on a crenelated substrate, the impacts on the textures greatly emphasized the dissipation.
To finish, we are interested in several natural non wetting objects : the Argyroneta aquatica and a few superhydrophobic aquatic plants. The first one is a spider which lives entirely underwater. It houses in a big air bubble it creates by capturing smaller bubbles at the surface, with a dynamic motion of its superhydrophobic abdomen and its legs. As for the second ones, they use their non wetting surfaces to survive in case of flooding.