The uniform film model for the gravitional approach of fluid drops to plane and deformable interfaces

Axisymmetric drainage within thin planar or spherical films of viscosity μ, area A and uniform thickness δ at time t is governed by the equation where F is the force pressing on the film and n the number of immobile interfaces. Assuming that the film ruptures at a critical thickness which is independent of the physical properties and applied force, expressions of the form may be obtained for drops approaching rigid and deformable interfaces and rigid spheres approaching deformable interfaces. The values of the exponents a, b and c suggest that an increase in drop volume, V, and density difference, δρ, and a decrease in interfacial tension, σ, should increase the coalescence time, τ, in liquid‐liquid systems. However in gas‐liquid systems an increase in δρ should decrease the coalescence time. The variation of δ with time was obtained experimentally from capacitance measurements. For drops approaching a deformable interface, the corresponding values of n are always close to 1 and in the region between 0 and 2. The values increase when the interfaces are made less mobile by adding surface active agents; they are also larger for drops approaching a rigid interface and for rigid spheres approaching a deformable interface. Consequently the uniform film model adequately describes the approach of drops to rigid and deformable interfaces.