Stationary electro-osmotic flow driven by AC fields around charged dielectric spheres

Raúl Fernández-Mateo,Antonio Ramos,Pablo García-Sánchez,Víctor Calero,Hywel Morgan

doi:10.1017/jfm.2021.650

Raúl Fernández-Mateo, Antonio Ramos + Show 3 more

Open Access

https://doi.org/10.1017/jfm.2021.650

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Abstract

Highlights

Solid particles immersed in aqueous electrolytes usually carry a net surface charge which is screened by a diffuse ionic layer on the electrolyte side of the interface
In this work we focus on the simplest system, in which stationary flows for alternating current (AC) fields arise due to surface conduction, where there is already an asymmetry between counter-ions and co-ions
Experimental data on the stationary electro-osmotic flow around microparticles are in agreement with the predictions of the model. These flows could be important in controlling the interaction between microscopic particles subjected to low-frequency AC electric fields and in general for any technique relying on AC fields for electrical manipulation of dielectric particles

Summary

Introduction

Solid particles immersed in aqueous electrolytes usually carry a net surface charge which is screened by a diffuse ionic layer on the electrolyte side of the interface. The gradients in electrolyte concentration occur due to particle surface conduction The model for these phenomena predicts the previously reported stationary flows observed around dielectric micropillars (20 μm diameter) subjected to AC fields (Calero et al 2021). We extend the analysis of Schnitzer & Yariv (2012) to the case of AC electric fields and describe the stationary flow around a charged colloidal sphere as a function of frequency, albeit with the restriction of a binary electrolyte with equal ionic diffusivities. The latter assumption is valid for our experiments with KCl aqueous solutions and greatly simplifies the theoretical treatment of the problem. These flows could be important in controlling the interaction between microscopic particles subjected to low-frequency AC electric fields (around 1 kHz or less) and in general for any technique relying on AC fields for electrical manipulation of dielectric particles

Theory

Linear response

Quadratic response

Experiments

Discussion and conclusions