The Bdle of Surfactants on the Coalescences of Emulsion Droplets

The coalescence of droplets and their life-time are shown to depend strongly on the surfactant properties _aod concentration. The following more important effects are predicted: 1) With pure liquids the energy is mainly dissipated in the droplets and the velocity of thinning, V, sho~ld not depend on the film viscosity. 2) The surfactant partition coefficient should playa crucial role on the emulsion stability. When it is soluble in the continuous phase, the surface mobility and the rate of thinning should be small and the flow in the droplet(s) could be disregarded; When the surfactant is soluble in the droplet(s), it should have no effect on the surface mobility and hence on the rate of thinning which must be much higher than it would be in the former case. 3) While the rate of thinning virtually should not depend on the surface viscosity. even values as small as 10-7 kg. s-1 would be sufficient to consider the interfaces as tangentially immobile with respect to the wave motion. 4) The film profile and hence the critical thickness at which the rupture occurs, should not depend on factors affecting to the same extent both the thinning and the wave motion. 5) The effect of the surfactant on the lifetime of the droplets is mainly due to its influence on the velocity of thinning rather th~ on the critical thickness of rupture.