Understanding and enabling the control of the properties of foams is important for a variety of commercial processes and consumer products. In these systems, the role of surface active compounds has been the subject of many investigations using a wide range of techniques. The study of their influence on simplified geometries such as two bubbles in a liquid or a thin film of solution (such as in the well-known Scheludko cell), has yielded important fundamental understanding. Similarly, in this work an interferometric technique is used to study the dynamic evolution of the film formed by a single bubble being pressed against a planar air-liquid interface. Here interferometry is used to dynamically measure the total volume of liquid contained within the thin-film region between the bubble and the planar interface. Three different small-molecule, surfactant solutions were investigated and the data obtained via interferometry were compared to measurements of the density of bulk foams of the same solutions. The density measurements were collected with a simple, but novel technique using a conical-shaped bubbling apparatus. The results reveal a strong correlation between the measurements on single bubbles and complete foams. This suggests that further investigations using interferometric techniques can be instrumental to building a more detailed mechanistic understanding of how different surface-active compounds influence foam properties. The results also reveal that the commonly used assumption that surfactant-laden interfaces may be modeled as immobile, is too simplistic to accurately model interfaces with small-molecule surfactants.

Depletion-attraction induced adhesion of two giant (∼ 40 μm), charged multilamellar vesicles is studied using a new Cantilevered-Capillary Force Apparatus, developed in this laboratory. The specific goal of this work is to investigate the role of dynamics in the adhesion and de-adhesion processes when the vesicles come together or are pulled apart at a constant velocity. Hydrodynamic effects are found to play an important role in the adhesion and separation of vesicles at the velocities that are studied. Specifically, a period of hydrodynamically controlled drainage of the thin film between vesicles is observed prior to adhesion, and it is shown that the force required to separate a pair of tensed, adhering vesicles increases with increasing separation velocity and membrane tension. It is also shown that the work done to separate the vesicles increases with separation velocity, but exhibits a maximum as the membrane tension is varied.

The effect of interfacial slip on the rheology of a dilute emulsion of drops for small capillary numbers J. Rheol. 56, 1555 (2012); 10.1122/1.4749836The effect of interfacial slip on the dynamics of a drop in flow: Part I. Stretching, relaxation, and breakup Hydrodynamic interactions between deformable particles such as drops or vesicles are an integral part of the rheology of emulsions and suspensions. In addition, the drainage of the thin film separating two colliding drops or vesicles is crucial for understanding the dynamics of coalescence or adhesion, which can lead to phase separation. However, despite several decades of study, this phenomenon is still not well understood and existing analytical theories do not agree quantitatively with experimental and numerical results. In this article, new scaling arguments are presented to analyze the drainage process, once the film becomes sufficiently thin. In particular, it is shown that the length over which the pressure varies in the film changes as the film drains, and follows a specific scaling relation. The mass balance in the film is then revisited in light of the new scaling for the pressure gradient. Numerical simulations are conducted to test the new scaling arguments and evaluate the revised mass balance. In the case of vesicles, they exhibit an excellent fit with the new scaling theory. The theory is also found to apply well to drops, but only when the flow inside the drops is determined predominantly by the flow in the thin film rather than by the ambient flow. C 2013 AIP Publishing LLC. [http://dx.