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This work focuses on three microhydrodynamics problems, in which one or several soft structures interact with a viscous fluid. These structures are flexible filaments, inspired from biological flagella. In each case, a quantitative experimental study is combined with a minimal theoretical model, in order to capture the main features of the physics involved. The first system is a macroscopic elastic filament, rotating in a viscous fluid. As the rotation frequency increases, the filament undergoes a continuous but sharp transition, from an almost straight state to a helical shape, tightly wrapped around the rotation axis. Depending on the anchoring conditions, this shape transition may be associated with an unstable branch in the force/torque characteristic. The second system consists in magnetic artificial microcilia, made of self-assembled superparamagnetic colloids. We present the fabrication of these microfilaments, and their spatial organization into arrays of tunable geometry in microfluidic channels. The filaments precess around the vertical axis. We find a critical inclination between the magnetic field and the precession axis, above which the high-frequency response of a filament becomes asynchronous. This dynamic transition is due to a purely geometrical criterium, arising from the nature of the magnetic dipolar interaction. Last, due to long-range hydrodynamical coupling, the trajectories of the free ends of an array of precessing microfilaments show an unexpected shape transition. A minimal, two-body model brings physical insight, in semi-quantitative agreement with the experimental data.
Mlle Naïs Coq , doctorante au laboratoire Physique et mécanique des milieux hétérogènes ( PMMH ) soutiendra sa thèse vendredi 19 novembre 2010 à 11h en l’amphithéâtre Langevin de l’ESPCI ParisTech