Non-Newtonian Stress in an Electrolyte

Analogies are drawn between the dynamics of electrolyte solutions and those of dilute suspensions of charged colloidal particles. The viscosity of both electrolytes and suspensions is a function of the ionic concentration c and of the Peclet number Pe characterizing the ratio of applied shear rate that tends to deform the ionic charge clouds, to diffusion that allows them to relax back to equilibrium. In particular, previously published results on the rheology of colloidal suspensions ( Lever D. A. J. Fluid Mech. 1979, 92, 421−433 ) imply not only that the Falkenhagen O(c 1/2) electrical contribution to viscosity is shear thinning, as shown by H. Wada ( J. Stat. Mech.: Theory Exp. 2005, P01001), but also that this contribution to the stress is elastic, with normal stress differences appearing at O(Pe2). In practice, the shear rates required for substantial departure from a Newtonian rheology are large, typically 109 s−1.