Finite-amplitude traveling-wave convection in binary fluid mixtures.

We have performed flow-visualization and heat-transport measurements of convection in ethanol-water-mixtures. For experimental parameters in the range studied, the Soret effect supresses the onset of convection relative to that in a pure fluid. A simplified model predicts that flow will begin via a forward bifurcation to an oscillatory state of spatially fixed rolls in which the direction of circulation periodically reverses. For larger Rayleigh numbers, nonlinear theory predicts a hysteretic transition to steady convection. Contrary to the expectations based on this model, we find no forward bifurcation at onset, but rather, an exponentially growing oscillatory transient which triggers a finite-amplitude convective state. The nonlinear state is not steady but instead consists of convective rolls which move laterally in the plane of the fluid layer, as traveling waves.