| When a small drop or bubble approaches a fluid-fluid interface, a thin liquid film forms between them and begins to drain. As the thickness of the draining film becomes sufficiently small (about 1000(ANGSTROM)), the effects of the London-van der Waals forces and of the repulsive force of any electrostatic double layer become important. We have developed a hydrodynamic theory for this coalescence process by including the effects of the London-van der Waals forces. Given only the drop radius and the required physical properties, we predict the configuration of the film as a function of time. We can also estimate the coalescence time, the time during which a small drop of bubble appears to rest at a phase interface before it coalesces under the influence of London-van der Waals forces.;Finally, these theories are applied to the unstable foam displacement of residual oil by carbon dioxide or nitrogen. The effects of system properties on three aspects of mobility control: distribution of bypassed oil, speed and displacement, and displacement efficiency, are investigated.;Some experimental studies suggest that the rate of coalescence of bubbles in foams and therefore the stability of foams is strongly affected by the surface viscosities. The effects of surface viscosities upon the stability of a draining plane parallel liquid film are examined. In an effort to provide a better understanding of these effects, we also have employed the detailed dimpled film model to explain the effects of surface viscosities more precisely. For a large intermediate range of surface viscosities, the coalescence time is a strong function of these parameters. Inclusion of the surface viscosities acts to moderate or even reverse trends previously established for the dependence of the coalescence time on the bubble radius, the viscosity of the film liquid, the interfacial tension, the strength of the London-van der Waals forces, and the density difference between the two phases. |
