Influence of porosity on Rayleigh-Taylor instabilities in reaction-diffusion systems

We analyze the effect of porosity in a porous medium on hydrodynamic instabilities in reaction-diffusion fronts. We use an experimental device to create an effective two-dimensional porous medium which is vertically orientated. In this system the molecular diffusion coefficients and the acid autocatalysis of the chlorite-tetrathionate reaction satisfy the appropriate conditions to produce a chemical front that advances through the cell leading to the products overlaying the reactants. The reactants have a lower density than the products and therefore a buoyantly unstable front develops. To evaluate the influence of the porosity on the formation and propagation of such instabilities, media with different porosities were used in the experiments. The amplitude of the instability is found to reduce as the porosity of the medium is decreased. For sufficiently small porosity, the instability can almost disappear leading to a planar front.