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.