Influence of surfactants on the electrohydrodynamic stretching of water drops in oil

- The electrohydrodynamic stretching of a salt water drop in oil is studied with simulations and experiments.
- We account for the surfactants present in the oil, and study their effect of the deformation.
- We find that addition of surfactants enables stretching slightly beyond Taylor's stability limit.
- Studying the overshoot of drops responding to a square voltage, we demonstrate the dual effect of surfactants.
- In experiments, an unexplained hysteresis effect is found, pointing to non-idealities in the interface.

In this paper we present experimental and numerical studies of the electrohydrodynamic stretching of a sub-millimetre-sized salt water drop, immersed in oil with added non-ionic surfactant, and subjected to a suddenly applied electric field of magnitude approaching 1  kV/mm. By varying the drop size, electric field strength and surfactant concentration we cover the whole range of electric capillary numbers (CaE) from 0 up to the limit of drop disintegration. The results are compared with the analytical result by Taylor (1964) which predicts the asymptotic deformation as a function of CaE. We find that the addition of surfactant damps the transient oscillations and that the drops may be stretched slightly beyond the stability limit found by Taylor. We proceed to study the damping of the oscillations, and show that increasing the surfactant concentration has a dual effect of first increasing the damping at low concentrations, and then increasing the asymptotic deformation at higher concentrations. We explain this by comparing the Marangoni forces and the interfacial tension as the drops deform. Finally, we have observed in the experiments a significant hysteresis effect when drops in oil with large concentration of surfactant are subjected to repeated deformations with increasing electric field strengths. This effect is not attributable to the flow nor the interfacial surfactant transport.