TRANSITION BETWEEN SPIRAL AND TARGET STATES IN RAYLEIGH-BENARD CONVECTION

RAYLEIGH-BENARD convection1-4, which occurs when a shallow fluid layer is heated from below, is commonly regarded as a paradigm for pattern formation under non-equilibrium conditions. The formation of hexagonal arrays of Benard cells is well known, but more complex patterns such as targets5 and spirals5-11 have also been reported. Similar patterns have been seen in electrohydrodynamical convection12,13, oscillatory chemical reactions14-19 and biological systems19,20. In general, the spiral and target states are found for different experimental conditions. Here we report the observation of a continuous transition between states containing many spirals and many targets, in a fluid undergoing Rayleigh-Benard convection near the gas-liquid critical point. Whether spirals or targets are observed depends on the Prandtl number, the ratio between the thermal and viscous timescales in the fluid. Neither of these states seems to be predicted by the hydrodynamic equations that describe the fluid motions1-4, 21. The fact that the transformation of one pattern into the other is continuous, and that under some conditions they can coexist, suggests that they may be generated by the same or a similar mechanism.