Electrohydrodynamics of Dispersed Drops of Conducting Liquid: From Drops Deformation and Interaction to Emulsion Evolution
P. Atten
Grenoble Electrical Engineering laboratory (G2Elab), CNRS, Grenoble INP & Joseph Fourier University, France
Abstract- The interaction and behavior of droplets of conducting liquid suspended into an insulating medium and subjected to an electric field are considered, in particular with relevance to electrical treatment of water-in-oil emulsions. After recap of the influence of an electric field on a single conducting drop, the action of a field on two close drops is examined. In the static case of anchored drops which attract each other, results are recalled concerning the interfaces deformation and disruption under the field action. In the dynamical case of free drops, their spacing decreases with time until an interface instability creates a bridge; in the asymptotic case of very close small droplets, an approximate model is briefly presented, which predicts the time for draining of the oil film between the droplets. The formation of a bridge between the drops does not necessarily lead to their merging. Examples of partial coalescence and bouncing with charge exchange are presented and qualitative explanations are proposed. The two-drop behavior under field leads to some insight on the evolution of a water-in-oil emulsion subjected to an AC electric field. In a stagnant emulsion, electrocoalescence results first in a fast increase of the droplets mean size and, finally, in a quasi steady arrangement of nearly equally spaced big drops in rows aligned with the field. In a flowing emulsion, the shear and turbulence play the major role in promoting quasi collisions of droplets, the electric field leading to coalescence if the film draining time is lower than the time of close proximity of drops.
Keywords-Electrocoalescence, electrical drops bouncing, water-in-oil emulsions