Crystallization and Melting of Thermoresponsive Colloids Confined in Microcapsules

Abstract

Phase behaviors of metals have been modeled with colloidal systems. Inspired by the thermal treatment of metals, we design colloidal counterparts confined in microcapsules that show cooling rate-dependent crystallization behavior and optical properties. Thermoresponsive colloidal particles are highly concentrated in microcapsules to make colloidal crystals. The crystals melt upon heating and particles recrystallize upon cooling due to reversible change of the volume fraction. The crystallization behavior strongly depends on the cooling rate, similarly to metals. For slow cooling, large single-crystalline grains are grown from the inner wall of microcapsules by heterogeneous nucleation and merging. By contrast, fast cooling causes both homogeneous and heterogeneous nucleation, resulting in small crystallites with random orientations in the inner volume. This structural variation changes optical properties originated from Bragg diffraction. The slowly cooled microcapsules display structural colors from the entire projection that are bright and orientation-dependent. The fast-cooled microcapsules show axisymmetric, rotation-independent color patterns in the projection. As the thickness of crystalline grains that align parallel to the inner wall decreases along with the cooling rate, so does the color brightness and reflectance.