Strong effect of fluid rheology on electrokinetic instability and subsequent mixing phenomena in a microfluidic T-junction

Abstract: This study presents a detailed investigation of how the rheological behaviour of fluid could influence the electrokinetic instability (EKI) phenomenon in a microfluidic T-junction. The non-Newtonian power-law model with different values of the power-law index (n) is used to obtain fluids of different rheological behaviours. We find that as the fluid rheological behaviour changes from shear-thickening (n > 1) to shear-thinning (n < 1) via the Newtonian (n = 1) one, the EKI phenomenon is significantly influenced under the same conditions. In particular, the intensity of this EKI phenomenon is found to be significantly higher in shear-thinning fluids than in Newtonian and shear-thickening fluids. As a result, the corresponding mixing phenomenon, often achieved using this EKI phenomenon, is also notably enhanced in shear-thinning fluids compared to that achieved in Newtonian and shear-thickening fluids. A detailed analysis of both the flow dynamics and mixing phenomena in terms of streamlines, velocity fluctuations, concentration field, mixing efficiency, etc., is presented and discussed in this study. We also employ the data-driven dynamic mode decomposition (DMD) technique to analyze the flow field in more detail. In particular, the information on the coherent flow structures obtained with different values of the power-law index facilitates the understanding of both the EKI-induced chaotic convection and mixing phenomena in a better way; for instance, why the mixing efficiency is higher in shear-thinning fluids than that in Newtonian and shear-thickening fluids. Moreover, we observe that the spatial expanse and intensity of these coherent structures differ significantly as the power-law index changes, thereby providing valuable insights into certain aspects of the underlying flow dynamics that otherwise are not clearly apparent from other analyses.