Ion-partitioning effect promotes the electroosmotic mixing of non-Newtonian fluids in soft-patterned microchannels.

Abstract

We numerically investigate the mixing characteristics of non-Newtonian fluids under the ion-partitioning effect in a micromixer having a built-in patterned soft polyelectrolyte layer (PEL) on its inner wall surfaces. We show that the mixing phenomenon is greatly modulated by the migration of counter-ions triggered by the Born energy difference caused by the electrical permittivity differences between the PEL and bulk electrolyte. We demonstrate counter-ion concentration field, flow velocity variation, species concentration distribution, mixing efficiency and neutral species dispersion by varying the electrical permittivity ratio and rheological parameters. In contrast to the scenario of no ion-partitioning, results show that a decrease in counter-ions in the PEL permits a greater prediction of the induced potential field therein by the ion-partitioning effect. This phenomenon results in a higher electrical body force in the PEL at a lower permittivity ratio when the ion-partitioning effect is considered. Notably, for a lower permittivity ratio (= 0.2), the ion-partitioning effect results in an electrical body force that is significantly higher than that in the no ion-partition case. Consequently, when the ion-partitioning effect is present, we find that flow velocity and recirculation strength are an order of magnitude higher than those in the no ion-partitioning case. Furthermore, we revealed that because of the ion-partitioning effect, higher vortex strength at lower permittivity ratios leads to better species homogeneity and mixing efficiency. Thus, mixing efficiency surpasses 90% for lower permittivity ratio values. Neutral species dispersion is faster owing to the ion-partitioning effect, especially for higher Carreau numbers. Utilizing the ion-partitioning effect, the results of this study can be utilized to design and develop efficient micromixers intended for the mixing of non-Newtonian fluids for diagnostic applications.