Taming ring morphology in 2D Co(OH)2 Liesegang patterns

Abstract

We present here novel expermients on the formation of Liesegang rings in 2D. We consider the system of propagating Co(OH)₂ bands studied extensively in 1D, from the viewpoint of a large number of considerations. In this paper, we focus on morphological aspects and, in the first part, we seek to steer the appearance of the pattern to achieve a pre-determined morphology. We aim at attaining three main features: minimizing the re-dissolution of Co(OH)₂ at the back of the propagating pattern, clearing the fuzzy precipitate region lagging behind, and increasing the ring spacing. We vary three experimental parameters to achieve that threefold purpose: 1. decreasing the concentration of the diffusing (outer) electrolyte (NH₄OH), 2. applying a constant electric field radially across the circular pattern, and, 3. increasing the gel concentration to a moderately high value. The best pattern was obtained under the conditions: 9% gelatin, [NH₄OH]₀ = 1.33 M, and applied potential V = 4.0 V, for a 0.100 M CoCl₂ taken as constant throughout the whole set of experiments performed. The observations are discussed in relation to the effects that cause them, and the known properties of Liesegang patterns. In the second part of the study, we monitor distortions of the ring pattern from circular symmetry, by applying a constant linear electric field across the circular medium. Elliptical distortions are obtained, which become notably important as the applied potential increases through 1.75 V. The variation of the ring curvature with applied potential is quantified and discussed.