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 psi is studied experimentally by heating from above. Shadowgraph observations at very large solutal Rayleigh numbers (R) over tilde 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 Rsimilar or equal to10(6), stable spoke-pattern planform with up and down columnar flows are observed up to (R) over tilde similar or equal to1.9x10(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.