  The behaviour of a single sub-millimetre-size water drop falling through a
viscous oil while subjected to an electric field is of fundamental importance
to industrial applications such as crude oil electrocoalescers. Detailed
studies, both experimental and computational, have been performed previously,
but an often challenging issue has been the characterization of the fluids. As
numerous authors have noted, it is very difficult to have a perfectly clean
water-oil system even for very pure model oils, and the presence of trace
chemicals may significantly alter the interface behaviour. In this work, we
consider a well- characterized water-oil system where controlled amounts of a
surface active agent (Span 80) have been added to the oil. This addition
dominates any trace contaminants in the oil, such that the interface behaviour
can also be well-characterized. We present the results of experiments and
corresponding two-phase- flow simulations of a falling water drop covered in
surfactant and subjected to a monopolar square voltage pulse. The results are
compared and good agreement is found for surfactant concentrations below the
critical micelle concentration.
