Periodic precipitation banding of metal hydroxides in agarose gels via cyclic-voltage-driven reaction–transport–reaction process

Abstract

This study investigated how cyclic switching between high and low voltages can generate Liesegang-like (but not conforming to the scaling laws) periodic precipitation bands of metal hydroxides in agarose gel columns, aiming to uncover the generality and underlying mechanism of this reaction–transport–reaction process. The process is generalizable to various combinations of metal anodes and cathodes, with the number of bands typically increasing with cycle number. The spatiotemporal evolution of the bands exhibits considerable anode-element dependence, but is less dependent on the cathode element, as well as the dimensions of the gel column, demonstrating the generality of this banding phenomenon. Analyses of the time-dependence of the electric current and the morphology and composition of the anode-derived deposits indicate that OH⁻ ions react with the anode surface after their transport from the cathode. Numerical simulations based on extended Nernst–Planck equations suggest that periodic banding is possible if (i) the generation of reactant ions is periodically suppressed, (ii) the diffusion coefficients of the reactant ions differ considerably, and (iii) a quantitative imbalance exists between the produced and precipitated ions. A qualitative mechanism for the observed banding phenomenon is proposed, in which the tunneling effect of OH⁻ ions, owing to the Grotthuss mechanism, is suggested as a possible cause of this quantitative imbalance. These findings offer new insights into electrochemically induced pattern formation and highlight the potential of reaction–transport–reaction systems for advancing our understanding of coupled reaction–transport phenomena.