(8g) Electrophoresis in Complex (non-Newtonian) Fluids: Theory and Experiments

The electrophoretic mobility of a colloidal particle in a Newtonian fluid is independent of both the particle size and shape in the thin Debye layer regime.  Notably, electric fields are used to drive particles in "complex" (non-Newtonian) fluids whose rheology differs from the Newtonian ideal, e.g. in microfluidic technologies and capillary electrophoresis.  However, there has been surprisingly little work on electrophoresis in non-Newtonian fluids.  To this end, here we discuss our modeling and experimental results on the electrophoresis of charged colloidal particles immersed in complex fluids.  Utilizing the Lorentz reciprocal theorem, we have developed an analytical framework to compute the electrophoretic mobility of a freely suspended charged colloid of arbitrary shape in a complex fluid possessing a shear-dependent viscosity.  The mobility includes contributions from the fluid inside and outside the thin electrical double layer surrounding the particle.  Notably, the latter endows the mobility with an explicit dependence on particle size, in stark contrast to electrophoresis in a Newtonian fluid.  Sample calculations are performed for particles in power-law and Carreau fluids.  To complement the theory, we have undertaken experiments with colloids of various sizes and surface chemistry in dilute polymer solutions.  Our experiments highlight the need to carefully account for additional physiochemical effects (e.g. suppression of particle zeta potential via polymer adsorption) due to the microstructural constituents of the complex fluid.  This in turn provides motivation to refinement of our theory to account for such factors.