Propagating fronts in 2D Cr(OH)(3) precipitate systems in gelled media

Diffusion fronts propagate as two co-precipitate ions inter-diffuse in a ge l medium. Liesegang bands of precipitate form periodically behind the diffu sion front of an outer electrolyte. The precipitation of Cr(OH)(3) from NaO H diffusing into a Cr3+ gel matrix is known to yield a single band that pro pagates in a one-dimensional (1D) tube - Cr(OH)(3) dissolves in excess OH- forming Cr(OH)(4)(-). We perform similar experiments on the Cr(OH)(3) syste m in two dimensions (2D), wherein we obtain a perfectly circular Cr(OH)(3) ring that grows larger and thicker as time advances. Using a specially desi gned Petri dish, ring propagation is monitored both in the absence and the presence of a constant electric field. The field is applied along a radial direction, and the front velocities with the field on are compared with the field-free case. When the field is applied against the direction of front propagation ("negative" field), wave saturation is obtained, characterized by a slight increase in the velocity of propagation with field strength, un til it reaches a constant value as the field strength is further increased. In a positive field situation, the wave velocity increases with field stre ngth and exhibits some other interesting features: (1) wave stopping indica ted by a freeze in the ring position at a certain characteristic time; (2) annihilation of the ring formation above a critical value of the field stre ngth. Electrical effects in 2D are also studied when electrodes with differ ent potentials are planted at various locations in the electrolyte peripher y. Interesting patterning structures including the distortion of the circul ar symmetry and the birth of multiple rings are reported. (C) 2001 Elsevier Science B.V. All rights reserved.