Abstract
We have applied imaging ellipsometry/reflectometry to the study of coalescence phenomena. Specifically, we have shown that film drainage between a deformable oil droplet and a solid surface can be monitored using imaging ellipsometry/reflectometry which yields the real time profile of a droplet as film drainage occurs, on approach of the droplet to an interface. The technique has the advantages that it is sensitive to the measurement of both thick and thin films and yields a film profile without the need to scan across the surface. We have profiled the shape of an oil droplet in a continuous aqueous phase as the droplet approached a silica surface. During film drainage the droplet was observed to dimple, evolving to a flattened profile. The addition of small concentrations of surfactant destabilised the film. Film drainage was non-uniform until the point of film rupture. At very low surfactant concentrations droplet attachment and incomplete rupture were observed.
Highlights
This paper describes the use of imaging ellipsometry/ reflectometry for profiling an oil droplet in an aqueous medium as it approaches a solid surface
We describe the application of imaging ellipsometry/reflectometry to study the system of an oil droplet approaching a hydrophilic, negatively charged silica plate in a continuous aqueous medium
This paper shows that the technique of imaging ellipsometry/ reflectometry is capable of following the evolution of droplet profiles as a function of time as a droplet approaches an interface
Summary
This paper describes the use of imaging ellipsometry/ reflectometry for profiling an oil droplet in an aqueous medium as it approaches a solid surface. In stable liquid/liquid systems, the film will thin to an equilibrium thickness at which point no further thinning of the intervening fluid between the droplet and the surface occurs. This equilibrium thickness is dependent upon the magnitude of the disjoining pressure in the film which, in this case, acts to stabilise the film. If the system is unstable, the film continues to thin to what is termed the critical thickness, at which point an instability develops, the film ruptures, and the droplet coalesces at the interface
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