Convection in a colloidal suspension with a large negative separation ratio $\psi$ is studied experimentally by heating from above. Shadowgraph observations at very large solutal Rayleigh numbers $\tilde{R}$ are reported as a function of time. Fast relaxation oscillations are reported for the root mean square value of the shadowgraph intensity. While pure fluids exhibit a transition to turbulent convection for Rayleigh number $R\simeq {10}^6,$ stable spoke-pattern planform with up and down columnar flows are observed up to $\tilde{R}\simeq 1.9\times {10}^9.$ It is suggested that the surprising stability of the planform against turbulence is due to nonlinear focusing arising from the concentration dependence of the diffusion coefficient.

• Received 1 March 2002

DOI:https://doi.org/10.1103/PhysRevE.66.055301