Electroosmotic Flow of Shear-Thinning Xanthan Gum Solutions in a Rectangular Microchannel

Electroosmotic flow is commonly used with other electrokinetic phenomena to manipulate aqueous samples in micro/nanofluidic devices. However, most studies of electroosmosis have been focused upon Newtonian fluids, although many chemical and biological samples are complex fluids with shear-thinning and viscoelastic characteristics. These rheological properties have been demonstrated to enhance electroosmotic pumping, mixing, etc., for which a fundamental understanding of electroosmotic velocity is crucial. We develop here a numerical model to understand and predict the experimentally measured electroosmotic velocity of xanthan gum solutions in a rectangular microchannel from our previous paper (J. Bentor et al., Langmuir 2024, 40, 20113-20119). This model couples interfacial electrokinetics with shear-thinning rheology by considering a polymer depletion layer (PDL), which is a Newtonian fluid free of polymer due to the polymer-wall interactions, adjacent to each channel wall and a power-law fluid in the bulk. It predicts with good accuracy the experimental electroosmotic velocity in xanthan gum solutions with varying polymer and buffer concentrations across the tested electric fields. The only fitting parameter is PDL thickness, which decreases with the increasing polymer or buffer concentration but is independent of the electric field magnitude. The PDL to electricdouble-layer thickness ratio is found to play a critical role in understanding the electric field nonlinearity of the electroosmotic velocity.