(181v) Coupling Electrokinetics and Rheology: Electrophoresis In Non-Newtonian Fluids
Microfluidic technologies routinely employ electric fields to manipulate and transport fluids containing colloidal-scale entities such as macromolecules, colloidal particles, cells, and self-assembled structures. The rheology of these complex fluids often differs from the Newtonian ideal, exhibiting viscoleasticity, shear thinning, and normal stress differences, for example. These properties can drive counter-intuitive and visually dramatic phenomena when complex fluids are subject to hydrodynamic flows. However, there is comparatively little knowledge of electrically driven, or electrokinetic, flows of complex fluids, which is surprising in light of the importance of electric fields in micro-scale transport of complex fluids. Here, we present modeling and experimental results on the electrophoresis of charged colloidal particles immersed in complex (non-Newtonian) fluids. We have developed a theory based on the Lorentz reciprocal theorem that allows computation of the electrophoretic motion of particles in complex fluids with an (arbitrary) shear-rate dependent viscosity or with normal stress differences. Notably, there are two distinct contributions to the electrophoretic motion, arising from the non-Newtonian electro-osmotic flow in the (thin) electrical double layer and stresses in the electroneutral bulk fluid, respectively. Importantly, the latter endows the electrophoretic mobility with an explicit dependence on particle size and shape, in stark contrast to electrophoresis in Newtonian fluids. We further illustrate the influence of non-Newtonian rheology by computing the mobility of a single spherical particle using several constitutive equations, including the power-law and Carreau models. These predictions are compared with experimental measurements of electrophoresis in shear-thinning and Boger fluids.