Pattern observations and heat-transport measurements of convection in ethanol-water mixtures at positive values of the separation ratio $\psi$ are presented. Close to onset, the convective flow manifests itself in a stationary square pattern with negligible change in heat transport compared with the conductive state. Far from threshold, convection selects the usual roll structure with a strong change in heat transport. In a crossover region, the competition between square and roll patterns leads to oscillations.

• Received 19 June 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2018