TRANSITIONS IN 2-DIMENSIONAL PATTERNS IN A FERROCYANIDE-IODATE-SULFITE REACTION

Transitions in two-dimensional (2D) spatial patterns were investigated in a ferrocyanide-iodate-sulfite (FIS) reaction in a circular thin,ee l reactor. The state of the gel reactor was maintained by contact of o ne side of the gel with a continuously refreshed well-stirred reservoi r. For long residence times of the chemicals in the reservoir, the gel reactor was in a spatially uniform state of low pH (about 4), while a t short reservoir residence times the reactor was in a uniform state o f high pH (about 7). At intermediate residence times the spatiotempora l 2D structures observed include a large low pH oscillating spot, smal l metastable high pH oscillating spots, shrinking rings, spirals that formed when the axisymmetry of shrinking rings was broken, self-replic ating spots that either grew and divided or died from overcrowding, an d highly irregular, stationary lamellae. Transitions among the differe nt patterns were examined as a function of gel thickness (0.2-0.6 mm), reservoir residence time (0.6-4 min), and ferrocyanide concentration (12-80 mM). Iodate and sulfite concentrations were held fixed at 75.0 and 89.0 mM, respectively. Several transitions were examined in detail : from a stationary spot to an oscillating spat; from an oscillating s pot to a shrinking ring or spirals; the onset of replicating spots; an d the transition from a homogeneous state to lamellar patterns. The ob served phenomena can all be described in terms of a parity-breaking fr ont bifurcation (nonequilibrium Ising-Bloch bifurcation). (C) 1996 Ame rican Institute of Physics.