Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage (≤10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be generated inside the droplet within the standard regimes of operating parameters, bearing far-reaching consequences towards enhancing internal mixing in multifarious droplet based microfluidic applications. An inherent integrability with the existing electrowetting on dielectric platforms renders the process ideal to be used in conjunction with digital microfluidic technology.

The present study deals with the effect of interfacial slip on the deformation and emulsion rheology of a dilute suspension of droplets in a linear flow. The droplets are laden with surfactants that are bulk-insoluble and get transported only along the interface. An asymptotic approach is adopted for the present analysis in order to tackle the nonlinearity present due to deformation of droplets. The analysis is carried out under two different limiting scenarios namely: surface diffusion-dominated-surfactant transport and surface convection-dominated surfactant transport. For either of the limiting cases we look into the droplet dynamics for two commonly encountered bulk flows -uniaxial extensional and simple shear flow. Under the assumption of negligible fluid inertia in either phase, it is shown that slip at the droplet interface significantly affects the surfactant-induced Marangoni stress and hence droplet deformation and emulsion rheology. Presence of interfacial slip not only brings about a decrease in the droplet deformation but also reduces the effective viscosity of the emulsion. The fall in both droplet deformation and effective viscosity is found to be more severe for the limiting case of surface convection-dominated surfactant transport. For the case of an imposed simple shear flow, the normal stress differences generated due to droplet deformation are affected as well due to the presence of interfacial slip. † E-mail address for correspondence: suman@mech.iitkgp.ernet.in studies deals with the effect of interfacial slip on the droplet dynamics 9,30-32 . Ramachandran and Leal 9 studied the effect of interfacial slip on the rheology and dynamics of a dilute emulsion of droplets suspended in linear flows. They showed that increase in the slip at the interface reduces both the droplet deformation as well as the effective viscosity of the emulsion.Recently, Vlahovska et al. and Mandal et al. 12,14 investigated the effect of surfactants on the deformation characteristics as well as the emulsion rheology of a dilute suspension of droplets in linear flows. However, there is no study available in the literature that puts forward the effect of interfacial slip on the dynamics of a surfactant-laden droplet. Surfactants at the droplet surface bring in alterations in the interfacial rheology that promotes the presence of interfacial slip, which is quite realistic in any multiphase system and is found to possess a significant effect on droplet dynamics 9,31 . This is the prime objective of the present study. It is seen that interfacial slip induces a greater stability to the emulsion by reducing the droplet deformation brought about by the imposed linear flow. Also, it reduces the effective viscosity of the suspension. It is observed that there is a good match between our theoretical prediction and previously performed experiments when the effect of interfacial slip is taken into consideration in our analysis. For the limiting case of convection dominated surfactant transport, the influence of slip is found to be h...