Rotational dynamics of a soft filament: wrapping transition and   propulsive forces
  We analyze experimentally the shape of a long elastic filament rotating in a viscous liquid. We identify a continuous but sharp transition from a straight to an helical shape, resulting from the competition between viscous stresses and elastic forces. This induced helicity generates a propulsive force along the axis of rotation. In addition, we show that the shape transition is associated with an unstable branch in the force-torque relation. A linearized model of the fluid-structure interaction is proposed to account for all the features of the non-linear filament dynamics. 
Na\"is Coq (PMMH), Olivia Du Roure (PMMH), Joel Marthelot (PMMH),   Denis Bartolo (PMMH), Marc Fermigier (PMMH)