MSc Students Presentations - Pantelis Moschopoulos

We develop a comprehensive model for the electro-osmotic flow of a polyethylene oxide (PEO) solution in a NaCl aquatic solvent under steady-state conditions in a microchannel. Based on the experimental data of Huang et al. (2016) for microchannels made of poly-dimethylsiloxane (PDMS) with bonded glass interior, we show that (a) the solvent contribution in the viscosity of the fluid, (b) the polymer migration and ionic steric effects and (c) the physicochemical description of the zeta potential are essential features of the model for capturing the complex rheological behavior adequately, whereas neglecting them leads to inaccurate predictions. The formation of a polymer depletion layer along the microchannel wall maintains the polymeric chains away from the area of the highest shear-rates, thus preserving their integrity. This makes the shear-stress at the solvent/solution interface a critical quantity for the characterization of the effect of the flow on the integrity of a polymeric chain and the design of microdevices for PEO transport. The values of this interfacial shear stress are found to be almost 30% lower than estimations of the wall shear stress based on a simplified single-phase formulation, which neglects the formation of the polymer depletion layer. However, the critical stress component is not the shear, but the elongational one at this interface. The latter is found to be an order of magnitude larger than the former (!) and almost 50% lower than the elongational stress at the wall that the single-phase model predicts for the experimental conditions of Huang et al. (2016). Sweeping a wide range of values of the applied potential (340 V/cm- 500 V/cm) and the ionic concentration (0.001 mM – 10 mM), the model shows that they affect the dynamics of the depletion layer in a non-monotonic way. Indicatively, the system can also achieve higher electroosmotic mobilities and lower shear and elongational stresses at the interface of the phases by increasing the bulk ionic concentration.