(251w) Can a Neutral Particle Translate in an Electric Field?

We introduce the self energy of mobile ions to the standard continuum model of electrokinetic processes. The self energy captures ion solvation, ion-ion correlation, and spatial permittivity variation effects in inhomogeneous dielectric systems. Differences in anion and cation self energies can engender local charge separation and hydrodynamic flow not observed in the mean-field theory of electro-osmosis.

We illustrate such behavior for a semi-infinite plate geometry. The Helmholtz-Smoluchowski formula for electro-osmotic slip is recovered; importantly, the mean zeta potential is self-consistent with an equilibrium charge distribution that also depends on ion self energies. If the surface is neutral, then classical theory anticipates the potential and fluid velocity are everywhere zero. We predict, however, that a valence asymmetric salt solution (e.g. CaCl₂) adjacent to a neutral surface will induce a non-zero zeta potential and, therefore, a finite slip velocity. This unintuitive result suggests a finite electrophoretic mobility for a uniformly neutral, non-polarizable particle in an asymmetric salt solution. Our analysis challenges the present understanding of the phoretic particle charge-mobility relationship, namely coincidence of zero charge and zero mobility. So yes, under certain conditions, a neutral particle can translate in an electric field!