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

TL;DR

This study investigates how surfactants influence the deformation and stability of water drops in oil under electric fields, combining experiments and simulations to reveal damping effects, deformation limits, and hysteresis phenomena.

Contribution

It provides new insights into surfactant effects on electrohydrodynamic drop behavior, including damping oscillations and extending deformation limits beyond classical predictions.

Findings

Surfactants damp transient oscillations of water drops.

Drops can be stretched beyond Taylor's stability limit with surfactants.

Hysteresis observed in repeated deformation cycles with high surfactant concentrations.

Abstract

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 ($Ca_E$) 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 $Ca_E$. 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.