Turing patterns in a simple gel reactor

We introduce a very simple open two-dimensional gel reactor for studying chemical patterns that arise in reaction-diffusion systems. The reactor consists of a thin disk-shaped layer of polyvinyl alcohol gel with one face in direct contact with a well-stirred flow reactor containing reactants of the chlorite-iodide-malonic acid system; the other face of the gel is in contact with a piece of transparent plexiglass. We have used this reactor to study the transition from a uniform state to a hexagonal pattern; for other concentrations, striped patterns were observed. The wavelength of these structures is greater than the thickness of the gel layer, which indicates that the patterns are two-dimensional (a single layer). This reactor provides a promising new tool for studying chemical patterns since the diffusion time of reactants into and out of the gel can be made small compared to the total residence time in the stirred flow reactor. This feature facilitates the comparison between theory and experiment since the chemical concentrations leading to pattern formation are close to those in the stirred flow reactor and hence can be directly measured. Pattern formation in reaction-diffusi on systems has attracted the interest of experimentalists and theorists alike during the last few decades. Until recently experimental work centered on spatio-temporal patterns in closed systems. Typically, a thin layer of reactive solution was placed in a petri dish and patterns such as spiral waves and target patterns were observed. However, since these appeared in a closed system, they were only short-lived transients. Several open chemical reactor designs have been introduced in order to allow the study of chemical patterns at conditions maintained far from equilibrium. Most of these designs employ an inert gel reaction medium in which convection is suppressed; such a reactor has been called a CFUR (continuous flow unstirred reactor), in analogy with the acronym CSTR used for continuous flow stirred tank reactors. In 1987 Noszticzius et al. [1] used a CFUR with an annular gel that was fed at the inner and outer rims to study traveling waves in the Belousov-Zhaboti nskii (BZ) reaction. Another CFUR design consists of a thin gel layer that is sandwiched between two thin porous glass plates [2]. The face of each porous glass plate opposite to the gel is in contact with a stirred