Sedimentation of a surfactant-laden drop under the influence of an electric field

TL;DR

This paper presents a theoretical analysis of how surfactants and electric fields influence the sedimentation and deformation of viscous drops, revealing complex interactions that affect settling velocity and shape.

Contribution

It introduces a coupled model considering surfactant convection, electrohydrodynamics, and deformation, highlighting the impact of Marangoni stresses on drop dynamics under electric fields.

Findings

Marangoni stress can significantly oppose electrohydrodynamic motion.

High surfactant mobility immobilizes the drop surface, eliminating charge convection effects.

Drop shape and deformation are strongly affected by surfactant sensitivity and electric forces.

Abstract

The sedimentation of a surfactant-laden deformable viscous drop acted upon by an electric field is considered theoretically. The convection of surfactants in conjunction with the the combined effect electrohydrodynamic flow and sedimentation leads to a locally varying surface tension, which subsequently alters the drop dynamics via the interplay of Marangoni, Maxwell and hydrodynamic stresses. Assuming small capillary number and small electric Reynolds number, we employ a regular perturbation technique to solve the coupled system of governing equations. It is shown that when a leaky dielectric drop is sedimenting in another leaky dielectric fluid, Marangoni stress can oppose the electrohydrodynamic motion severely, thereby causing corresponding changes in internal flow pattern. Such effects further result in retardation of drop settling velocity, which would have otherwise increased due to the influence of charge convection. For highly mobile surfactants (high Peclet number limit), the drop surface becomes immobilized and the charge convection effect gets completely eliminated. For non-spherical drop shapes, the effect of Marangoni stress is overcome by the 'tip stretching' effect on the flow field. As a result, the drop deformation gets intensified with increment in sensitivity of surface tension to the local surfactant concentration. Consequently, for oblate type of deformation the elevated drag force causes further reduction in velocity. Owing to similar reasons, prolate drops experience lesser drag and settles faster than the surfactant-free case. In addition to this, with increased sensitivity of interfacial tension on the surfactant concentration, the asymmetric deformation about the equator gets suppressed.