  The structural evolution of suspensions upon freezing is studied with optical
microscopy in a suspended droplet configuration. Droplets have millimeter size
and consist in an aqueous mixture of silica particles while the surroundings
phase is hexane. Freeze-thaw cycles are applied to this system and a two-step
freezing mechanism evidenced. A fast adiabatic growth of dendrites that invade
the full droplets is first observed, and occurs within a few milliseconds. Then
a slow process lasts for several seconds and corresponds to the release of
solidification latent heat into the hexane phase. The striking feature of this
work is to evidence that after the first freeze-thaw cycle flocculated
microstructures are generated. When a second cycle is performed,
microstructures further flocculate and generate, for dense silica suspensions,
stable porous spheres of the size of the droplets. A phenomenological
description based on repulsion or engulfment of particles by solidifying ice
fronts is proposed.
