Electroosmosis of viscoelastic fluids in pH-sensitive hydrophobic microchannels: Effect of surface charge-dependent slip length

We analytically investigated the electroosmotic flow characteristics of complex viscoelastic liquids within a charged hydrophobic microchannel, considering the pH and salt concentration-dependent surface charge effects in our analysis. We examined the variation of the electric-double layer (EDL) potential field, the surface charge-dependent slip (SCDS) length, the flow field, the viscosity ratio, and both normal and shear stresses in relation to the bulk pH, bulk salt concentration, and Deborah number of the solution. Our current findings indicate that, under strong flow resistance due to increased electrical attraction on counter ions, a highly basic solution with a high EDL potential magnitude results in a significant decrease in the slip length. Neglecting the effect of SCDS leads to an overestimation of flow velocity, with this overprediction being more pronounced for highly basic solutions. This overestimation diminishes as bulk salt concentration increases, particularly when compared to strongly acidic solutions. Furthermore, a noticeable increase in average velocity is observed as the Deborah number rises for highly basic solutions compared to highly acidic ones. This is attributed to the substantial reduction in apparent viscosity caused by the shear-thinning nature of the liquid at higher shear rates, supported by a larger zeta potential modulated strong electrical force for basic solutions. Additionally, we found that the intensity of shear and normal stresses tends to increase with bulk pH, primarily due to the rise in electric body force at higher zeta potential. These results can potentially inform the design and development of a compact, nonmoving electroosmotic pump for transporting biological species with varying physiological properties, such as solution pH. This technology could be applied in subsequent processes involving mixing, separation, flow-focusing for cell sorting, and other related applications.