Numerical Investigation of an Active Micromixer Utilizing Alternating Rotating Obstacles for Non-Newtonian Fluids at Ultra-low Reynolds Numbers

Achieving rapid mixing in microfluidic systems is notoriously difficult at ultra-low Reynolds numbers (Re << ۱), where slow molecular diffusion dominates species transport. This study presents a numerical investigation of an active micromixer designed to overcome this limitation. Our design incorporates five cylindrical obstacles that rotate at alternating angular velocities (۱۰۰ to ۸۰۰ RPM) to induce chaotic advection. The mixing efficiency quantified using the Degree of Mixing (DOM) across a critical range of molecular diffusivities (D = ۱۰⁻¹۰ to ۱۰⁻¹۲ m²/s) and for Power-Law non-Newtonian fluids with indices from n=۰.۶ to n=۱.۴. The results demonstrate that the rotating obstacles successfully achieve DOM > ۹۰% for all tested diffusivities when the angular velocity reaches ۸۰۰ RPM. However, the most significant finding concerns fluid rheology: contrary to conventional expectations, mixing efficiency consistently improved with increasing Power-Law index (n). Shear-thickening fluids (n > ۱) demonstrated superior performance, consistently achieving DOM > ۹۰% across the entire diffusivity range. In contrast, the most shear-thinning fluid (n=۰.۶) failed to reach the ۹۰% threshold at the lowest diffusivities. Hydrodynamic analysis revealed that this enhancement stems from increased flow asymmetry in shear-thickening fluids, where the peak axial velocity shifted approximately ۵.۵% from the centerline, thereby strengthening the convective transport crucial for efficient mixing. This work provides practical operational and design guidelines for achieving high-efficiency mixing (≥۹۰%) under severe diffusional limitations and complex rheological conditions. All simulations were conducted in the creeping-flow regime. An analysis based on a rotational Reynolds number confirms that inertial effects and flow instabilities remain negligible even at the highest rotation rates considered.

کلیدواژه ها:

Active micromixer ، Non-Newtonian fluid ، Power-law model ، Chaotic advection ، Low Reynolds number