Electro-osmotic flow in coated nanocapillaries: a theoretical investigation

TL;DR

This paper presents a theoretical study of electro-osmotic flow in coated nanocapillaries, revealing how polymer coatings and charges influence flow direction and magnitude through numerical simulations and analytical models.

Contribution

It introduces a combined numerical and theoretical framework to analyze electro-osmotic flow modifications due to charged polymer coatings in nanocapillaries.

Findings

Flow reduction observed with polymer coatings.

Flow inversion occurs when polymer charge opposes surface charge.

Phase diagram maps flow regimes based on polymer density and surface charge.

Abstract

Motivated by recent experiments, we present a theoretical investigation of how the electro-osmotic flow occurring in a capillary is modified when its charged surfaces are coated by charged polymers. The theoretical treatment is based on a three dimensional model consisting of a ternary fluid-mixture, representing the solvent and two species for the ions, confined between two parallel charged plates decorated by a fixed array of scatterers representing the polymer coating. The electro-osmotic flow, generated by a constant electric field applied in a direction parallel to the plates, is studied numerically by means of Lattice Boltzmann simulations. In order to gain further understanding we performed a simple theoretical analysis by extending the Stokes-Smoluchowski equation to take into account the porosity induced by the polymers in the region adjacent the walls. We discuss the nature of the velocity profiles by focusing on the competing effects of the polymer charges and the frictional forces they exert. We show evidence of the flow reduction and of the flow inversion phenomenon when the polymer charge is opposite to the surface charge. By using the density of polymers and the surface charge as control variables, we propose a phase diagram that discriminates the direct and the reversed flow regimes and determine its dependence on the ionic concentration.