A new growth instability in colloidal crystallization

Abstract

SUSPENSIONS of monodisperse colloidal particles are useful model systems for studying order–disorder transitions. The bright iridescence of colloidal crystals, resulting from diffraction of visible light, is well-known^1–5. We have shown recently that this diffraction process depolarizes light sufficiently to reveal the orientation of the primary diffraction planes through the colours observed in a polarizing microscope^6,7. In the course of these investigations we have observed a new crystallization instability that can be explained, in part, by a diffusion-limited crystallization process. Although it is analogous to the mechanism of pattern formation in molecular systems, here the instability occurs for particles interacting solely by repulsive forces. We provide a qualitative explanation of the process using a classic Mullins–Sekerka⁸ stability analysis. Our observations suggest that colloidal suspensions can provide a useful model not only for the study of crystallization thermodynamics but also for investigations of ordering dynamics.