Investigating the effect of finite ionic size and solvent polarization on induced charge electro-osmosis around a perfectly polarizable cylinder

Many industrially relevant microfluidic applications use concentrated solutions of macro-molecular solutes dissolved in polar solvents like water, which are typically deployed at high voltages. In this study, we investigate the effect of finite ionic sizes and solvent polarization on induced charge electro-osmotic flow around a perfectly polarizable cylinder, at high electric field strengths and ionic concentrations. The flow is actuated by means of a direct current electric field, and the step response of various flow parameters are studied numerically. Finite ionic sizes, defined through a steric factor ν, are modeled using the modified Poisson–Nernst–Planck model. Additionally, a field-dependent permittivity, characterized by a solvent polarization number A, accounts for molecular re-orientation effects. Our findings reveal an ion-size modulated decrement in charge concentration in the electrical double layer and an augmentation in the electric field. Remarkably, the resulting flow velocities increase with ion size. Solvent polarization, on the other hand, results in a marked reduction in flow velocities. Steric effects, however, dominate over a large range of parameter space (applied voltage and bulk ionic concentration) as compared to solvent polarization. Finally, we demonstrate that unequal ionic sizes result in flow asymmetries at the steady-state, thereby generating net electro-phoretic motion of suspended particles.