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

TL;DR

This study investigates convection in a colloidal suspension heated from above at very high solutal Rayleigh numbers, revealing stable patterns and oscillations that challenge typical turbulence onset expectations.

Contribution

It provides experimental observations of stable convection patterns and oscillations at extremely high solutal Rayleigh numbers in colloidal solutions, highlighting nonlinear focusing effects.

Findings

Stable spoke-pattern planforms observed up to Ra ~10^9

Fast relaxation oscillations in shadowgraph intensity

Suppression of turbulence due to nonlinear focusing effects

Abstract

Convection in a colloidal suspension with a large negative separation ratio is studied experimentally by heating from above. Shadowgraph observation at very large solutal Rayleigh numbers 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 of about 10^6, stable spoke-pattern planform with up and down columnar flows are observed up to solutal Rayleigh numbers of the order of 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.