  We report new chemoconvective pattern formation phenomena observed in a
two-layer system of miscible fluids filling a vertical Hele-Shaw cell. We show
both experimentally and theoretically that the concentration-dependent
diffusion coupled with the frontal acid-base neutralization can give rise to
formation of the local unstable zone low in density resulting in a perfectly
regular cell-type convective pattern. The described effect gives an example of
yet another powerful mechanism which allows the reaction-diffusion processes to
govern the flow of reacting fluids under gravity condition.
