Exact expressions for the mobility and electrophoretic mobility of a weakly charged sphere in a simple electrolyte

TL;DR

This paper derives explicit asymptotic formulas for the mobility and electrophoretic mobility of weakly charged spheres in electrolytes, reconciling classical theories and extending them with new coefficients and corrections.

Contribution

It provides fully explicit, asymptotically exact expressions for particle mobility in electrolytes, bridging classical limits and introducing new coefficients and corrections.

Findings

Reproduces classical Debye and Onsager limits for small particles.

Recovers non-monotonous charge dependence for large particles.

Adds previously unknown coefficients and corrections to existing models.

Abstract

We present (asymptotically) exact expressions for the mobility and electrophoretic mobility of a weakly charged spherical particle in an $1:1$ electrolyte solution. This is done by analytically solving the electro and hydrodynamic equations governing the electric potential and fluid flow with respect to an electric field and a nonelectric force. The resulting formulae are cumbersome, but fully explicit and trivial for computation. In the case of a very small particle compared to the Debye screening length ($R \ll r_D$) our results reproduce proper limits of the classical Debye and Onsager theories, while in the case of a very large particle ($R \gg r_D$) we recover, both, the non-monotonous charge dependence discovered by Levich (1958) as well as the scaling estimate given by Long, Viovy, and Ajdari (1996), while adding the previously unknown coefficients and corrections. The main applicability condition of our solution is charge smallness in the sense that screening remains linear.