Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheet Hydrogels

Abstract

Anisotropic chemical wave propagation of self-oscillating Belousov–Zhabotinsky (BZ) reaction was demonstrated in the poly(N-isopropylacrylamide) gel films embedded with macroscopically aligned liquid crystalline inorganic nanosheets. Although the average propagation rate of chemical wave v̅ was 3.56 mm min–1 in the gels without nanosheets, the propagation was retarded in the gels with 1 wt % of nanosheets: v∥̅ = 1.89 mm min–1 and v⊥̅ = 1.33 mm min–1 along the direction parallel and perpendicular to the nanosheet planes, respectively. Thus, the wave propagation is anisotropic with the anisotropy ratio v∥/v⊥̅ = 1.42 in these gels and the periodic patterns formed by the BZ reaction were concentric ellipses, different from circles seen in isotropic gels. Furthermore, the propagation rate and degree of anisotropy were controllable by nanosheet concentration. These phenomena can be explained that the diffusion of molecules inside the gel is effectively hindered along the direction perpendicular to the nanosheet planes due to the very large aspect ratio of the aligned nanosheets. The present systems will be applicable for anisotropic self-oscillating soft actuators with one-dimensional motions as well as for ideal model system of BZ reactions.