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 (NH 4 OH), 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, [NH 4 OH] 0 ¼ 1.33 M, and applied potential V ¼ 4.0 V, for a 0.100 M CoCl 2 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.

Rhythmic precipitate patterns and fractal structure

“…It is shown that for a typical Liesegang pattern, the band spacing increases as B 0 increases, but decreases as A 0 increases. This formulation is consistent with experimental observation [25,26].…”

Effect of an alternating current electric field on Co(OH)2 periodic precipitation

“…9. In contrast with the field free and DC cases [1,13,14,36], the spacing is distinctly larger at lower Co 2+ concentrations.…”

“…Periodic precipitation can be used for potential applications in material science [12], if effective techniques to control and design these patterns can be found. Several studies treated special effects in this broad dynamical system, such as the variation of the concentrations of the electrolytes [13,14], the application of an electric field [15][16][17][18][19], the addition of impurities to the gel medium [20], or the change in the pH field [21,22]. Other studies compared and contrasted the cases where direct [24,23] or revert [25] spacing is obtained, and considered the formation of polymorphic structure [26,27].…”

Morphology of a 2D Mg2+/NH4OH Liesegang pattern in zero, positive and negative radial electric field

“…In Liesegang landscapes, the morphological appearance of the bands is governed by the coupling between diffusion, chemical reactions and the field response of the co-precipitate ions. A diversity of wave propagation features were demonstrated in the presence of an electric field, in HgI 2 [7,8], Cr(OH) 3 [9], Ag 2 Cr 2 O 7 [10,11] and Co(OH) 2 [12,13] precipitate systems. A wealth of wave structures and exotic behavior was also obtained from the conjecture of the dynamics of diffusion-precipitation patterning media subjected to applied electric fields [11,[14][15][16].…”