Microstructure Formation in Freezing Nanosuspension Droplets

TL;DR

This study investigates the microstructure evolution in freezing nanosuspension droplets, revealing a two-step freezing process and the formation of stable porous silica spheres after multiple freeze-thaw cycles.

Contribution

It provides new insights into the microstructure formation during freezing of nanosuspensions and proposes a phenomenological model for particle behavior at ice fronts.

Findings

Fast dendrite growth occurs within milliseconds.

Microstructures become more flocculated after multiple freeze-thaw cycles.

Stable porous silica spheres form in dense suspensions after repeated freezing.

Abstract

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.