The effect of axial concentration gradient on electrophoretic motion of a charged spherical particle in a nanopore

Abstract

The electrophoretic motion of a charged spherical nanoparticle along the axis of a nanopore connecting two fluid reservoirs, subjected to an axial electric field and electrolyte concentration gradient, has been investigated using a continuum model. The model consists of the Poisson and Nernst-Planck equations for the electric potential and ionic concentrations and the Stokes equations for the hydrodynamic field with zero gravity. In addition to the electrophoresis generated by the externally imposed electric field, the particle also experiences diffusiophoresis arising from the externally imposed concentration gradient. The effects of the diffusiophoresis on the axial electrophoretic motion are examined with changes in the ratio of the particle size to the thickness of the electric double layer (EDL), and the imposed concentration gradient. Since the EDL thickness, the particle size, and the nanopore size are of the same order of magnitude, the diffusiophoresis is dominated by the induced electrophoresis driven by the generated electric field arising from the double-layer polarization (DLP). For a relatively small κa _p, the ratio of the particle size to the EDL thickness, the diffusiophoresis is dominated by the induced electrophoresis from the type II DLP, which propels the particle toward regions with lower salt concentration. Depending on the magnitude and direction of the externally imposed concentration gradient, the electrophoretic motion can be accelerated, decelerated, and even reversed by the diffusiophoresis.