Foams and Concentrated Emulsions: Dynamics and "Phase" Behavior

Abstract

Drainage of the continuous phase plays an important role in the collapse of foams and concentrated liquid-liquid emulsions. A general model for the drainage in initially homogeneous (uniform volume fraction of each phase) foams and concentrated emulsions is presented. Separation of the dispersed phase is accounted for by considering the collapse of films in the upper part of the system. Expressions for the movement ofthe two boundaries (emulsion/continuous phase and emulsion/dispersed phase) are formulated as a function of the velocity of the continuous phase fluid. The model successfully predicts the trend of drainage lag times observed in initially homogeneous foams. Phase behavior based on a drainage equilibrium, which implies a balance between the opposing effects of gravity and the plateau border suction gradient, is investigated. There are three possibilities : separation of a single phase (continuous or dispersed), separation of both phases, and no phase separation. It is shown that the phase behavior depends on a single dimensionless group which is a measure of the relative magnitudes of the gravitational and capillary forces. A generalized phase diagram is presented which can be used to determine the phase behavior. It is shown that separation of both phases occurs only if the column length exceeds a critical value.