María Laura Olivares

Electrokinetic flow of non-Newtonian fluids in microchannels.

A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cylindrical microchannels is presented. Calculations are based on constitutive models of the fluid viscosity, and take into account wall depletion effects of colloids and polymer solutions. The resulting equations allow one to predict the flow rate and electric current as functions of the simultaneously applied electric potential and pressure gradients. It is found that (i) nonlinear effects induced by the shear-dependent viscosity are limited to the pressure-driven component of the flow, and (ii) the reciprocity between electroosmosis and streaming current is complied. Thus a generalized form of the force-flux relations is proposed, which is of interest in microfluidic applications.

The EOF of polymer solutions

The EOF of polymer solutions is analysed in the framework of continuum fluid mechanics and the standard electrokinetic model. Two key aspects are taken into consideration: the non‐Newtonian character of the fluid and the polymer concentration near the interface, which greatly modify the fluid viscosity in the region where electroosmosis takes place. A satisfactory mathematical model is derived for the electroosmotic mobility of solutions that present polymer depletion at the wall. The case of solutions containing polymers that adsorb onto the wall is briefly reviewed, and a preliminary approach is discussed for the limit of strong polymer adsorption. In order to illustrate the theoretical discussions, experimental data obtained from aqueous solutions of carboxymethyl cellulose in fused‐silica capillaries are presented. Relevant results are observed, which are appropriately captured by the modelling proposed. The fundamental phenomena discussed in this work are of interest in microfluidics and electrophoresis.