Transitions in two-dimensional patterns in a ferrocyanide–iodate–sulfite reaction

Abstract

Transitions in two-dimensional (2D) spatial patterns were investigated in a ferrocyanide–iodate–sulfite (FIS) reaction in a circular thin gel reactor. The state of the gel reactor was maintained by contact of one side of the gel with a continuously refreshed well-stirred reservoir. 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 at short reservoir residence times the reactor was in a uniform state of high pH (about 7). At intermediate residence times the spatiotemporal 2D structures observed include a large low pH oscillating spot, small metastable high pH oscillating spots, shrinking rings, spirals that formed when the axisymmetry of shrinking rings was broken, self-replicating spots that either grew and divided or died from overcrowding, and highly irregular, stationary lamellae. Transitions among the different 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 spot; from an oscillating spot to a shrinking ring or spirals; the onset of replicating spots; and the transition from a homogeneous state to lamellar patterns. The observed phenomena can all be described in terms of a parity-breaking front bifurcation (nonequilibrium Ising-Bloch bifurcation).