Fast-onset Soret-driven convection in a colloidal suspension heated from above

Convection in a colloidal suspension with a large negative separation ratio b.Psi is studied experimentally by heating the sample from above. Shadowgraph observations at large solutal Rayleigh numbers (R)tilde are reported as a function of time. Stable spoke-pattern planforms with up and down columnar flows are observed at (R)tilde ap10 6 . The convective flow starts in a time much smaller than that needed for the concentration gradient to reach steady state. Fast relaxation oscillations are reported for the rms value of the shadowgraph intensity. The oscillation period is in good agreement with the predictions of a recent work on self-oscillation behaviour induced by the Soret effect. However, in contrast with the prediction of the theory, the oscillations are damped and the rms attains a steady state value. Moreover, at very high stresses, the oscillations are overdamped.