Mixing of concentrated oil-in-water emulsions measured by nuclear magnetic resonance imaging

Mixing of concentrated oil-in-water emulsions in a horizontal, concentric-cylinder geometry was studied using nuclear magnetic resonance imaging. Time-of-flight and chemical shift imaging methods were used to measure velocity profiles and concentration maps in an emulsion that was mixed after being allowed to cream for several hours. The results revealed detailed information about mixing in concentrated emulsions. In the initial state, before mixing, the emulsion system formed three layers: an upper, oil-rich, “creamed” layer; a lower, water-rich layer; and a bulk concentration layer in between. It was found that the thickness of the creamed layer remained constant during mixing, while the oil concentration in that layer decayed exponentially as a function of time. It was also observed that most of the emulsion is quiescent while mixing occurs; the only motion detected occurs in a thin layer close to the rotating, outer cylinder. The velocity profile only begins to transition to its steady-state configuration after the emulsion becomes well mixed. A simple model is introduced that gives a plausible explanation of these experimental observations. These results indicate that the mixing mechanism in concentrated emulsions is significantly different from that in single-phase liquids.