Electroosmosis at inhomogeneous charged surfaces

The Helmholtz-Smoluchowski (HS) relation between the local electrostatic potential and lateral solvent velocity in a continuum fluid at a planar charged surface under the action of a tangential electric field is exact for smooth surfaces, even in the presence of ion correlations. At rough surfaces, the interplay of hydrodynamics and surface friction due to electrostatic and excluded-volume interactions between surface groups and counterions gives rise to pronounced deviations from HS-theory. This is demonstrated by hydrodynamic simulations of ions at charged corrugated surfaces. Derived effective electrokinetic surface charges are highly reduced, in agreement with the experimental trend.