A novel gravity-induced flow transition in two-phase fluids

Abstract

Experimental results are reported that show a gravity-induced flow transition in well-mixed suspensions and emulsions, even when the buoyancy-driven velocity of isolated drops or particles is several orders of magnitude smaller than the imposed velocity. The experiments were conducted with emulsions of isooctane in water and suspensions of polymethyl-methacrylate particles in water. Both the drop and particle diameters were approximately 3–5μm, and concentrations of the dispersed phases ranged from dilute (2%) to concentrated (40%). The two-phase fluids were confined to a horizontal, concentric-cylinder apparatus in which the outer cylinder was rotated, and the velocity profiles were measured by nuclear magnetic resonance imaging. The results show that the flow transition is relatively insensitive to the volume fraction of the dispersed phase. The flow transition occurs because, although the buoyancy-driven velocity is relatively small on the length scale of the particle or drop dimension, the flow itself induces a slight variation in the suspension concentration and, hence, density. Although only on the order of 10−4g∕cm3, this density difference spans a macroscopic length scale, making the buoyancy effect competitive with the imposed flow. These arguments yield a dimensionless parameter that predicts very closely the nonequilibrium phase diagram generated by the experiments.