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

In most emulsions, a density difference between the dispersed and the continuous phases leads to separation of the components by gravity, known as 'creaming.' Typically, a uniform emulsion is desirable, and hence it is important to examine the kinetics and mechanism of emulsion mixing required to achieve this uniform state. In addition, previous mixing research has focused on the impact of known flow fields on the microstructure of the system, where the microstructures do not modify the flow field in the process. In contrast, in the case of concentrated emulsions, it is shown here that the evolution of the nonuniform droplet concentration profile has a major impact on the observed flow field. Mixing of concentrated oil-in-water emulsions in a horizontal, concentric-cylinder geometry was studied using nuclear magnetic resonance imaging (NMRI). The NMRI technique provides droplet concentration and velocity profiles noninvasively and in situ within a flowing, concentrated emulsion of isooctane and water stabilized with nonionic surfactant. An initial nonuniform concentration profile is established by creaming of a homogeneous emulsion. We then measure the time-dependent effect of slow shear flow on the concentration and velocity profiles. Time-of-flight and chemical shift imaging methods were used to measure velocity profiles and concentration maps during the mixing process, respectively. The results obtained show detailed information about the mixing process in concentrated emulsions. It was found that the thickness of the cream layer remains constant during mixing while the concentration in that layer decays exponentially as a function of time. It was also observed that while mixing occurs, most of the emulsion is quiescent, the only detectable motion being in a thin moving layer close to the rotating outer cylinder wall. A simple model is introduced that is able to give a reasonable explanation of these experimental observations. These results indicate that the mixing mechanism and kinetics in concentrated emulsions are significantly different from those in single-phase liquids.