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.
The present study is focused on micromixing enhancement techniques for electroosmotic flows in a modulated microchannel with a modified topology by utilizing heated blocks on the surface of the microchannel. The heated blocks carry higher temperatures as compared to the other portions of the channel wall, resulting in a sharp variation in the temperature of the fluid. The species transport is governed by the Nernst–Planck equation in a modified form by adding a thermo-electrochemical migration term due to the temperature variation in the ions, justifying the electrochemical equilibrium conditions. The fluid considered for the study is non-Newtonian and is governed by a power-law model. The Navier–Stokes equations, along with the thermal energy equation, are simulated numerically in a coupled form utilizing a finite volume-based semi-implicit method for the pressure-linked equation algorithm to interpret the behavior of the electric potential distribution, the external electric field, the flow field, the temperature distribution, and the species concentration, which are the major contributors for the mixing efficiency. The numerically simulated results are varied with the analytical results for the simple electroosmotic flow in the microchannel, indicating that the mixing efficiency can be enhanced by increasing the temperature of the heated blocks. Due to the thermo-electrochemical migration, ions are redistributed along the heated blocks, oscillating the flow velocity by creating vortices, resulting in the mixing enhancement. The effects of the geometrical parameters, the Debye–Hückel parameter, the temperature gradient, the power-law index, and the Nusselt number are elaborated for the effective flow rate and micromixing. The mixing efficiency is found to be optimum for higher temperature gradients and higher power-law indices. The net throughput analysis that combines the geometrical modulation and wall temperature variation will aid in improving the design and fabrication of microfluidic mixers.
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