Electrically modulated cross-stream migration of a compound drop in micro-confined oscillatory flow

Abstract

In this work, we present a means of controlling the cross-stream trajectory of a compound drop under the combined action of a transverse electric field and an oscillatory axial pressure gradient in a microfluidic channel. We bring out the decisive role of the flow pulsation in delaying the drop's attainment of a steady-state transverse position. With an enhancement in the frequency of oscillation, both the axial and transverse movement of the drop may be preferentially attenuated, with its dynamic traversal occurring in a locus offset to the central axis as precisely set in by the action of transverse electric forcing, to bring in exclusive controllability on the drop trajectory along with its eventual position of settlement. Moreover, our results also decipher that the value of the viscosity ratio between inner-to-outer droplet phases having less than unity delays the compound drop migration, whereas the converse enhances the same. In addition, we observe that a leaky dielectric compound drop having the electrical permittivity ratio of the inner-to-outer droplet phase surpassing their respective electrical conductivity ratio is not only capable of selectively altering the direction of the resulting drop trajectory from toward the channel centerline to away from the same but at the same time facilitates a precise settling of the same at an intermediate transverse location by harnessing the interplay of electrical and hydrodynamic shear. We further identify the key dimensionless parameters along with their desirable ranges accountable for the directional switching of the drop trajectory with high specificity. These findings open up novel perspectives of controllable maneuvering of the double emulsion system in a confined microenvironment bearing decisive implications in engineering and biology.