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.
Precipitate systems consist of particles of various sizes distributed in space. The determination of the particle size distribution (PSD) in a precipitate allows a better understanding of the spatio-temporal dynamics of such a system. The Liesegang method of growing precipitates in gels slows down the kinetics of precipitation to a diffusion-controlled limit and yields exotic patterns of parallel salt bands. This promotes the possibility of measuring the PSD, especially when bands with well-dispersed particles in space are obtained. Such an experiment was realized in our laboratory on cobalt(II) oxinate yielding appropriate patterns with exactly the desired features. We here attempt to improve and quantify these experiments, to measure the PSD using different analysis methods. The measurements are performed via two routes: in the laboratory under the microscope and by image analysis of 2D pictures of the various bands. The two methods show a good qualitative agreement in that the trends are almost perfectly reproduced. The band morphology is studied notably as a function of initial supersaturation, and novel features related to particle size and number density are observed. Two-salt metal oxinate patterns are prepared and analyzed.
Tree-like aggregates (dendrites) have been reported in a variety of precipitate systems. We here explore different routes for the growth of dendrites of PbF2 and Pb(NO3)2. The PbF2 ramifications form via the interdiffusion of coprecipitates (F- into Pb2+) in microslides (Liesegang-type experiments) and via the infiltration of electrolyte through cracks, thus simulating geochemical fractals. The Pb(NO3)2 dendrites are grown by evaporation of dilute solutions of the salt. Images of the various patterns obtained are analyzed and their fractal dimensions are determined. The location of the edges of successive "cascades" in dendritic patterns obeys a spacing law similar to that of Liesegang bands.
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