The Experimental Observation and Modelling of Film Thinning and Film Retraction during the Interfacial Coalescence of Biodiesel and Glycerol Droplets

This paper is concerned with the way interfacial coalescence of a single drop of biodiesel or glycerol occurs at a glycerol/biodiesel interface. Two stages of interfacial coalescence were studied: the thinning of the trapped liquid film between the rising or falling droplet and bulk fluid interface, and the retraction of the film after the film had ruptured. Unexpectedly, the thinning time for the high viscosity glycerol film around a rising biodiesel droplet was found to be much shorter than that for a low viscosity biodiesel film around a sedimenting glycerol droplet. Squeeze flow modelling showed that the film of glycerol around a biodiesel droplet was bounded by relatively inviscid biodiesel and therefore flowed with high slip at its interfaces, resulting in rapid film thinning. The biodiesel film around a glycerol droplet was bounded by highly viscous glycerol and flowed with little slip at the interfaces, resulting in slower film thinning. After rupture, film retraction was found to be much faster for biodiesel droplets than for glycerol droplets. The drag exerted by the fluid surrounding the film was found to control the retraction kinetics. The results are of particular relevance to the separation of glycerol from biodiesel and of general relevance to coalescence kinetics for immiscible drops at an interface.