Onset of convection in colloids stratified by gravity.

Settling of the equilibrium (barometric) distribution of colloidal grains in a liquid requires a very long time in comparison with that necessary for temperature equilibrium establishment. This mismatch enables different scenarios for the onset of thermal convection. If the stratified state has had no time to be settled, the colloid behaves as a pure fluid where only stationary convection arises. On the contrary, if the barometric concentration profile has been reached before the temperature gradient is imposed, only oscillatory convection occurs. Thereafter, the oscillations last as long as it takes the convection to wash away the initial concentration profile. Then the fluid becomes homogeneous and stationary convection is eventually established. The influence of the Soret effect upon the onset of convection is also taken into account and discussed.