Soret driven convection in a colloidal solution heated from above at very large solutal Rayleigh number

Convection in a colloidal suspension with a large negative separation ratio ? is studied experimentally by heating from above. Shadowgraph observations at very large solutal Rayleigh numbers 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?106, stable spoke-pattern planform with up and down columnar flows are observed up to R~?1.9×109. 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.