title: The Influence of Dielectric Decrement on Electrokinetics.
abstract: We treat the dielectric decrement induced by excess ion polarization as a source of ion specificity and explore its impact on electrokinetics. We employ a modified Poisson-Nernst-Planck (PNP) equations accounting for the dielectric decrement. The dielectric decrement is determined by the excess ion polarization parameterαand whenα= 0 the standard PNP model is recovered. Our model shows that ions saturate at large zeta potentials (ζ). Because of ion saturation, a condensed counterion layer forms adjacent to the charged surface, introducing a new length scale, the thickness of the condensed layer (lc). For the electro-osmotic mobility, the dielectric decrement weakens the electro-osmotic flow owing to the decrease of the dielectric permittivity. At largeζ, whenα≠ 0, the electro-osmotic mobility is found to be proportional toζ/2, in contrast toζpredicted by the standard PNP model. This is attributed to ion saturation at largeζ. In terms of the electrophoretic mobilityMe, we carry out both an asymptotic analysis in the thin-double-layer limit and solve the full modified PNP model to computeMe. Our analysis reveals that the impact of the dielectric decrement is intriguing. At small and moderateζ, the dielectric decrement decreasesMewith an increasingα. At largeζ, it is well known that the surface conduction becomes significant and plays an important role in determiningMe. It is observed that the dielectric decrement effectively reduces the surface conduction. Hence in stark contrast,Meincreases asαincreases. Our predictions of the contrast dependence of the mobility onαat different zeta potentials qualitatively agree with experimental results on the dependence of the mobility among ions and provide a possible explanation for such ion specificity. Finally, the comparisons between the thin-double-layer asymptotic analysis and the full simulations of the modified PNP model suggest that at largeζthe validity of the thin-double-layer approximation is determined bylcrather than the traditional Debye length.
authors.affiliation: Department of Mechanical Engineering, University of Nevada, Las Vegas, NV, 89154.
authors.forename: Hui
authors.initials: H
authors.lastname: Zhao
authors.affiliation: Department of Mechanical Engineering, University of Nevada, Las Vegas, NV, 89154.
authors.forename: Shengjie
authors.initials: S
authors.lastname: Zhai
keywords: colloids; micro-/nano-fluidic dynamics; microfluidics