Hydrodynamics, growth and interfaces

We review a number of situations where the formation and evolution of an interface from fluctuations are modified by the presence of an external (shear) flow. The process which derives the interface formation is spinodal decomposition. Experiments are performed by using light scattering techniques in binary liquids close to their critical miscibility point. Hydrodynamics can alter the fluctuations, thus defining "high (low) shear" regions where the fluctuations are strongly (weakly) affected. The situations in which the initial and final states of the system are in the low shear region lead to a very particular growth where the domains are thinned in one direction and ultimately homogenized. The domains still evolve in the other directions, thus leading to a kind of two-dimensional growth. The situations where final state is in the high shear region lead to a permanent state, in which the domains cannot grow above a shear-dependent length: when this length is made smaller than the correlation length, the system becomes homogeneous. Stopping the flow makes the phase separation to proceed in the same manner as it does after a thermal quench. Switching off the high shear thus provides a mean to quench a system. The situations where the initial state is one of high shear and the final state is one of low shear raises many questions: here fluctuations must evolve from an initial state where they are already out of equilibrium.