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Interplay between Spinodal Decomposition and Gelation and Their Role in Two- and Three-Dimensional Pattern Formation at the Gelatin Gel Surface

Sasaki, Kazuto, Itatani, Masaki, Sato, Daisuke, Unoura, Kei, Nabika, Hideki
Journal of physical chemistry 2019 v.123 no.22 pp. 13782-13788
fluorescence, gelatin, gelatinization temperature, gelation, microscopy, models, starch, sugars
Understanding the mechanisms underlying the spontaneous formation of Liesegang patterns based on the prenucleation and postnucleation models is of critical importance for understanding pattern formation in nature. In contrast to the rapid experimental and theoretical advances in understanding within the framework of the prenucleation model, discussion of the postnucleation model is mostly limited to numerical analysis. To construct a standard model for a chemical experiment discussed in terms of the postnucleation model, we have investigated the pattern formation mechanism in a mixed system containing gelatin, starch, and sugar. Fluorescence and differential interference contrast microscopy revealed a process of two-dimensional spinodal decomposition into gelatin-rich and gelatin-poor phases. Since the gelation temperature of the gelatin-rich phase was higher, spatially periodic gelation proceeded selectively in the gelatin-rich phase. As both two-dimensional spinodal decomposition and three-dimensional gelation occurred above the gelation temperature of the initial sol solution, we propose that these patterns form by two-dimensional spinodal decomposition into gelatin-rich and gelatin-poor phases followed by gelation in the gelatin-rich phase, which yields the spatially periodic, three-dimensional patterns of the gelatin gel. Pattern geometry was found to be dependent on sample size or shape. Although three geometries—labyrinth, one-directional, and intermediate patterns—appeared in a circular Petri dish, only a one-directional banded pattern formed in a rectangular cell. We present a good experimental system, in which the correlation of the spacing and other empirical laws with a theoretical basis is possible within the framework of the postnucleation model.