Steady axial electric field may lead to controllable cross-stream migration of droplets in confined oscillatory microflows

Abstract

Cross-stream migration of a droplet in an incipient flow turns out to be of outstanding importance in several emerging applications encompassing chemistry, engineering and biology. Here, we bring out the confluence of confinement, oscillatory axial pressure gradient and steady axial electric field towards controlling spatiotemporal characteristics of cross-stream migration of droplets in a micro-confined fluidic environment, bearing immense implications in in vitro modelling of bio-analytical procedures. Under the sole influence of an oscillatory axial pressure gradient, the time taken by a droplet to achieve a steady-state transverse position is significantly long and the direction of the droplet's motion cannot be altered at will. However, confinement-modulated electrohydrodynamic interactions enable overcoming this constraint, even when the applied electric field is orthogonal to the intended direction of droplet migration, a proposition that is not feasible in an unbounded domain. Our results reveal that depending on the relative electrical properties of the droplet and the carrier phases and a competing influence of electrical, viscous and capillary stresses, the rate of transverse migration can be controlled by effectively modulating the axial oscillations in its cross-stream motion. Beyond a threshold value of the applied electric field, simultaneous enhancement in the droplet migration rate and reversal in the direction of its lateral migration become possible, which cannot otherwise be achieved by the oscillatory pressure field alone. Furthermore, the oscillatory characteristics in the droplet migration can be dampened out completely by exploiting the addressed physical interplay. Results from in-house experiments corroborate our theoretical conjecture.