Moving blue band caused by cooperation of diffusion and phase separation

Abstract

Diffusions of Cu 2+ and Fe 3+ in gelatin generate a moving blue band. It is formed by a diffusion of Cu 2+ and a phase separation of gelatin with diffusing Fe 3+. The diffusing Fe 3+ forms Fe(OH) 3 colloids and gathers gelatin molecules from the surroundings. The diffusion of gelatin molecules generates the concentration gradient, resulting in a gel/sol transition in the dilute phase. In the region where the concentration of Fe 3+ is high enough, the gel remains hard, while a sol phase appears under the hard gel. The absorption spectrum of Cu 2+ depends on the concentration ratio of Cu 2+ to gelatin. As a consequence, we can see a blue band in the restricted region between the diffusing front of Cu 2+ and the phase separation front. The movement of the blue band is caused by a coupling of a simple diffusion and the phase separation.