Multiphase fluids are ubiquitous in foods and industrial manufacturing. In some cases it is desirable to create stable mixtures of immiscible phases, like mayonnaise or shampoo, while in other cases an efficient phase separation is required, such as the extraction of oil from naturally occurring oil sands. In both cases, a fundamental understanding of the physics governing the phase separation process is needed to improve the final product and reduce the manufacturing cost.

In this dissertation, the physical mechanisms leading to phase separation in foams, emulsions, and suspensions are investigated from a fundamental point of view. As such, the experimental techniques employed herein are applied to simplified systems, usually consisting of just two, micron-sized particles (e.g. droplets or vesicles) within a suspending fluid. In fact, a new experimental apparatus called a Cantilevered-Capillary Force Apparatus (CCFA) was developed as a major part of this work, that is specifically designed to make unique measurements on just one or two of these particles. The new apparatus is particularly useful for testing theoretical predictions on these types of simplified systems.

The CCFA was used to make the first reported force measurements of the adhesion of two vesicles in suspension. The work of adhesion was found to be dependent on the degree of deformability of the vesicles. The CCFA was also used to characterize the adhesion mechanism of model food emulsion particles and to measure rheological properties of individual particles. The individual particles with a high volume fraction of internal solids were found to behave like a yield stress fluid and to adhere by forming capillary bridges.

A third study was performed using the CCFA to measure the time scale for emulsion droplets to coalesce in a quiescent fluid when compressed together with a constant force. Here the time scale for coalescence is referred to as the "drainage time'' because the rate-limiting step is the drainage of suspending fluid from the thin film of fluid that separates the drops when they collide. The drainage time was found to be only weakly dependent on the magnitude of the compressive force, but quite sensitive to the droplet size. An additional study of the drainage time between expanding, foam bubbles in a quiescent fluid was performed. The dependence of the drainage time on the bubble size and growth rate was found to differ dramatically from dependence for expanding drops under similar conditions.

Work was also done as a part of this dissertation to improve the existing theoretical predictions that employ a scaling approach to predicting the dynamics of the phase separation process. In conjunction with this theoretical work, boundary-integral simulations were utilized to test some of the assumptions built into the theory. It was demonstrated that there is a physical constraint on the length scale over which the pressure gradient in a thin film with a free interface can occur under conditions relevant to emulsion droplets and vesicles in suspension.