  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.
