Effects of hydrophobic slips in non-uniform electrokinetic transport of charged viscous fluid in nozzle-diffuser

Abstract

The mixing efficiency and electro-osmotic flow enhancement over a hydrophobic structured microchannel with nozzle-diffuser under an external electric field is studied. The interfacial surface zeta potential is varied to generate a strong convection effect between two injecting fluids from the reservoirs for a wide range of Reynolds numbers. The Poisson–Nernst–Plank model is incorporated to deal with thick and thin diffuse layers formed by a non-Newtonian electrolyte solution for the numerical simulation of the mobility of ions. To avoid the high-pressure drop complications in the computation, we have scaled the mixing efficiency with the average pressure drop. The analytical validation of the velocity and potential for thin and thick electric double layer with the existing experimental results corroborated and bridged the performance of the present model to achieve faster mixing by reducing pressure gradient. It is demonstrated that hydrodynamic slip increases the flow velocity producing larger mobility; however, the heterogeneous zeta potential generates a backflow that prevents the driving fluids leading to higher mixing efficiency, discussed in the first phase of the work. It is found that the mixing performance of Newtonian fluid is maximum for a nozzle-diffuser-shaped microchannel when all other physical parameters are constant. In the next phase, the mixing performance of shear thickening, shear thinning, and Newtonian fluid has been discussed for various physio-chemical parameters, i.e., slip length, Debye parameter, channel conical angle/slope, and heterogeneous zeta potential strength. The mixing efficiency and the pressure gradient decrease with the increase in the Debye parameter and the slip length. It is observed that the mixing efficiency can further be enhanced by increasing the heterogeneity of zeta potential strength and channel conical angle. This study can be used as a benchmark model for fabrication of chaotic mixers in hydrophobic slips with wall-mounted heterogeneous zeta potential and can be suitable for handling the power-law fluids.

Effect of boundary slip on electroosmotic flow in a curved rectangular microchannel

“…The EOF of a two-layer fluid in a hydrophobic microchannel was investigated by Shit et al, [34] who considered the influence of wall slip effects on the flow. A study conducted by Majhi et al [35] demonstrated that the enhancement of EOF in hydrophobic structured microchannels with nozzle-diffuser configurations could be attributed to the boundary slip phenomenon occurring within these channels. By considering the combined influence of ion size and slip effects, Sujith et al [36] developed a theoretical model for EOF and entropy generation in a hydrophobic microchannel.…”

“…The application of a constant external electric potential induces varying electric field forces on the fluid in different regions of the channel, leading to a more non-uniform velocity distribution. Additionally, despite the contributions of numerous researchers to the investigation of EOF in hydrophobic microchannels, [33][34][35][36][37][38][39][40][41][42][43][44] almost all studies on EOF in rectangular microchannels have predominantly employed no-slip boundary conditions, even in straight rectangular microchannels. Accordingly, the present study numerically investigates the creeping Dean EOF in curved rectangular microchannels with slip boundary conditions for the first time.…”