Transition between spiral and target states in Rayleigh-Bénard convection

Rayleigh-Benard convection in SF6 near its thermodynamic critical point resulted in a continuous transition between states containing many spirals and many targets. This form of convection is generally considered a paradigm for pattern formation under nonequilibrium conditions. The formation of hexagonal arrays of Benard cells is well known, but more complex patterns, namely spirals and targets, have also been observed. The Prandtl number--the ratio between the thermal and viscous timescales in the fluid--determined which pattern was observed. The patterns were probably generated by the same or similar mechanisms, as the transformation from one pattern into another was continuous, and patterns coexisted under some conditions. Of theoretical interest is the question of how patterns of such different symmetry and topology are related.