Surface instability of a thin electrolyte film undergoing coupled electroosmotic and electrophoretic flows in a microfluidic channel

Abstract

We consider the stability of a thin liquid film with a free charged surface resting on a solid charged substrate by performing a general Orr-Sommerfeld (O-S) analysis complemented by a long-wave (LW) analysis. An externally applied field generates an electroosmotic flow (EOF) near the solid substrate and an electrophoretic flow (EPF) at the free surface. The EPF retards the EOF when both the surfaces have the same sign of the potential and can even lead to the flow reversal in a part of the film. In conjunction with the hydrodynamic stress, the Maxwell stress is also considered in the problem formulation. The electrokinetic potential at the liquid-air and solid-liquid interfaces is modelled by the Poisson-Boltzmann equation with the Debye-Hückel approximation. The O-S analysis shows a finite-wavenumber shear mode of instability when the inertial forces are strong and an LW interfacial mode of instability in the regime where the viscous force dominates. Interestingly, both the modes are found to form beyond a critical flow rate. The shear (interfacial) mode is found to be dominant when the film is thick (thin), the electric field applied is strong (weak), and the zeta-potentials on the liquid-air and solid-liquid interfaces are high (small). The LW analysis predicts the presence of the interfacial mode, but fails to capture the shear mode. The change in the propagation direction of the interfacial mode with the zeta-potential is predicted by both O-S and LW analyses. The parametric range in which the LW analysis is valid is thus demonstrated.

Electric field mediated spraying of miniaturized droplets inside microchannel

“…In this direction, a number of recent studies indicated that the use of external field can also be an efficient alternative to digitize the flow patterns . For example, an alternating current (AC) field could be integrated with the microchannels to form emulsions with miniaturized droplets .…”

Contact line dynamics of electroosmotic flows of incompressible binary fluid system with density and viscosity contrasts

“…It should be duly noted that none of the aforementioned studies investigated the effects of electric field as the actuating force, on the dynamics of the contact line. However, previous researchers have addressed electrically actuated two phase fluid motion in the form of capillary filling and displacement, 33 thin film processes, 53 characterization of porous media, 54 and soil dewaterification, 35 to name a few.…”

Localized electric field induced transition and miniaturization of two‐phase flow patterns inside microchannels

“…Further, the interfacial tension of the liquid–liquid configuration is at least an order of magnitude lower than the similar gas–liquid systems. In addition to this, the external field induced transport behaviors of the liquid–liquid flows find important applications in microscale pumping, mixing, separation, reaction, heat, and mass transfer . Recent works indicate that the influence of an external electric field can cause interesting transitions in the liquid–liquid multiphase flow patterns due to the electrohydrodynamic (EHD) stress at the interface originating from the accumulation of induced dipoles or free charges .…”