Complex flow transitions in a homogeneous, concentrated emulsion

Results are presented for the flow of homogeneous, concentrated, oil-in-water emulsions subjected to a shear flow between rotating, horizontal concentric cylinders. Nuclear magnetic resonance imaging (NMRI) was used to measure velocity profiles. This technique allows velocity profiles to be measured noninvasively within a flowing, concentrated emulsion. It was observed that below a critical velocity, in a portion of the gap, the fluid moves in a direction opposite to the outer, rotating cylinder. Above this critical velocity, the emulsion corotates with the outer cylinder. Theoretical analysis suggests that the transitions are driven by buoyancy effects. The corotating and counter-rotating flow states at different rotation speeds can be characterized by a single dimensionless parameter ̂C, which relates buoyancy and viscous effects.