Large Scale, Unidirectional Convection during Phase Separation of a Density Matched Liquid Mixture

Complete phase segregation may occur on a 10 centimeter scale even in the absence of buoyancy, due to unidirectional, large scale rapid bulk flows driven by chemical potential gradients. Using a hexadecane-acetone nearly density-matched liquid mixture in a 20 cm-long condenser tube with a 1 cm diameter, we observed the rapid axial migration of the acetone-rich drops towards the warmer regions of the condenser. Conversely, the hexadecane-rich drops moved in the opposite direction, therefore ruling out thermocapillary effects as a possible explanation of the phenomenon. These flows lead to a complete phase segregation within 10 seconds, with the formation of a single interface perpendicular to the axial direction. Changing the temperature gradient along the tube from 0.25 C/cm to 1 C/cm no change was detected, with typical drop speeds up to 6 cm/s, irrespectively of the distance of the drop from the wall, showing that the phenomenon is not due to a flow instability.