Eccentricity-induced dielectrophoretic migration of a compound drop in a uniform external electric field

Abstract

Eccentric compound drops, which are ubiquitous in many naturally inspired and engineering systems, can migrate under the sole presence of a uniform electric field, unlike the case of isolated single drops. Here, we report the migration of eccentric compound drops under a uniform electric field, imposed parallel to the line of centres of the constituting drops, by developing an approximate analytical model that applies to low Reynolds number limits under negligible droplet deformation, following axisymmetric considerations. In contrast to the sole influence of the electrohydrodynamic forces that has thus far been established to be emphatic for the eccentric configuration, here we report the additional effects induced by the dielectrophoretic forces to result in decisive manipulation in the drop migration. We show that the relative velocity between the inner and outer drops, which is a function of the eccentricity itself, dictates the dynamical evolution of the eccentricity variation under the competing electrohydrodynamic and dielectrophoretic interactions. This brings out four distinct regimes of the migration characteristics of the two drops based on their relative electro-physical properties. Our results reveal that an increase in eccentricity and the size ratio of the inner and outer droplets may induce monotonic or non-monotonic variation in the drop velocities, depending on the operating regime. We show how the interplay of various properties holds the control of selectively increasing or suppressing the eccentricity with time. These findings open up various avenues of electrically manipulative motion of encapsulated fluidic phases in various applications encompassing engineering and biology.

Slippery rheotaxis: new regimes for guiding wall-bound microswimmers

“…In order to solve the above system of governing equations and boundary conditions, together with the force-free constraint, we use eigenfunction expansion of the Stokes flow problem in the bispherical coordinates (ξ, η, φ) (Happel & Brenner 1983). In the bispherical system the plane boundary is located at ξ = 0 and the spherical swimmer surface corresponds to ξ = ξ 0 (Behera, Poddar & Chakraborty 2023;Poddar 2023). In this solution method the expressions for the velocity components contain a set of unknown coefficients…”