Particles of different density in thermocapillary liquid bridges under the action of travelling and standing hydrothermal waves

Abstract

We observed particles of different density ratio α = ρ p /ρ f in thermocapillary liquid bridges with steady and with time-dependent flow under normal- and under microgravity. Particle accumulation structures (PAS) visualize some features of the hydrothermal wave in the liquid bridge. Relatively fast formation of PAS from particles which are considerably less dense than the fluid (α = 0.42) in oscillatory thermocapillary flow of top-heated liquid bridges was observed and explained by an additional buoyancy-assisted mechanism which brings the particles into the surface flow. This PAS from particles with α = 0.42 will persist under normal gravity for infinite time. In contrast to these less dense particles the heavier particles with α > 1 settle down under normal gravity on the lower end face of the liquid bridge after some time and are no longer in suspension and PAS will fade out. On the other hand, particles with α = 0.42 will be less suited for experiments under microgravity than particles with α > 1 because most of them will be trapped in the vortex centre of the thermocapillary flow. The sedimented particles with α > 1 are a means to visualize some features of standing hydrothermal waves which are visualzed and discussed for the first time.