Understanding the Freezing of Colloidal Suspensions: Crystal Growth and Particle Redistribution

Abstract

The solidification of colloidal suspensions is ubiquitous in nature and technology. Whereas we want to control the microstructure and architectures of materials obtained through freezing routes, understand natural phenomena, or finely control the texture of frozen food, a solid understanding of the phenomena and the underlying mechanisms is essential. The basic principles are remarkably simple and can be described with only a few words: crystals grow, and repel and concentrate particles between them. The reality turns out to be rapidly more complex once we start digging into the details and the many parameters that control the phenomenon. What do we have to understand? What are the exact mechanisms and phenomena that take place during the different stages? What do we want to predict and why? When do the suspensions freeze? What controls the freezing point? What controls the morphology of the crystals? Are particles repelled by the crystals? How do particles organise in the frozen body? All these questions are tackled in this chapter. The chapter is organised by following the chronological order of the mechanisms that successively take place during the freezing of colloids: nucleation, growth, particle redistribution and concentration, and eventually recrystallisation. The chapter goes beyond the simple case of colloidal, rigid particles and discuss the case of soft objects such as bubbles or droplets.