Search

The apparent hydrodynamic slip of polymer solutions is a result of polymer depletion at channel walls, and its fluid dynamic effects are well known. This work reviews the evidences of apparent slip in electrokinetics, and discusses practical consequences in the following fields: (i) electrokinetic transport of polymer solutions in microchannels, which is of interest for the design and operation of microfluidic chips, particularly for electrophoresis; (ii) electroosmotic pumping, where it has been observed that the employment of polymer solutions greatly enhances the output pressure; and (iii) electrokinetic energy conversion, where the apparent slip also contributes to improve the conversion efficiency. In all cases, critical discussions are taken from basic physical concepts., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.

A theoretical relation between the zeta potential of silica capillaries and the composition of the background electrolyte (BGE) is presented in order to be used in capillary zone electrophoresis (CZE). This relation is derived on the basis of the Poisson-Boltzmann equation and considering the equilibrium dissociation of silanol groups at the capillary wall as the mechanism of charge generation. The resulting model involves the relevant physicochemical parameters of the BGE-capillary interface. Special attention is paid to the characterization of the BGE, which can be either salt or/and buffer solutions. The model is successfully applied to electroosmotic flow (EOF) experimental data of different aqueous solutions, covering a wide range of pH and ionic strength. Numerical predictions are also presented showing the capability of the model to quantify the EOF, the control of which is relevant to improve analyte separation performance in CZE.