Influence of contraction ratio on electroviscous flow through the slit-type non-uniform microfluidic device

Abstract

The electroviscous effects are relevant in controlling and manipulating the fluid, thermal, and mass transport microfluidic processes. The existing research has mainly focused on the fixed contraction ratio (dc, i.e., the area ratio of contraction to expansion) concerning the widely used contraction–expansion geometrical arrangement. This study has explored the influence of the contraction ratio (dc) on the electroviscous flow of electrolyte liquids through the charged non-uniform microfluidic device. The numerical solution of the mathematical model (Poisson's, Nernst–Planck, and Navier–Stokes equations) using a finite element method yields the local flow fields. In general, the contraction ratio significantly affects the hydrodynamic characteristics of microfluidic devices. The total electrical potential and pressure drop maximally change by 1785% (from −0.2118 to −3.9929) and 2300% (from −0.0450 to −1.0815), respectively, as the contraction ratio (dc) varies from 1 to 0.25. Furthermore, an electroviscous correction factor (Y, i.e., the ratio of apparent to physical viscosity) maximally enhances by 11.24% (at K = 8, S = 16 for 0.25≤dc≤1), 46.62% (at S = 16, dc=0.75 for 20≥K≥2), 22.89% (at K = 2, dc=0.5 for 4≤S≤16), and 46.99% (at K = 2, dc=0.75 for 0≤S≤16). Thus, the electroviscous effect is obtained maximum at dc=0.75 for the considered ranges of conditions. Finally, a pseudo-analytical model has been developed for a charged microfluidic device with variable contraction size (0.25≤dc≤1), based on the Hagen–Poiseuille flow in the uniform slit, which calculated the pressure drop within ±3% of the numerical results. The present numerical results may provide valuable guidelines for the performance optimization and design of reliable and essential microfluidic devices.