Microstructure Formation in Freezing Nanosuspension Droplets

The structural evolution of suspensions upon freezing is studied with optical microscopy in a suspended droplet configuration. Droplets are of millimeter size and consist of an aqueous mixture of silica particles, while the surrounding phase is hexane. Freeze thaw cycles are applied to this system, and a two-step freezing mechanism is 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.