Ionic Origin of Electro-osmotic Flow Hysteresis.

Chun Yee Lim,An Eng Lim,Yee Cheong Lam

doi:10.1038/srep22329

Abstract

Electro-osmotic flow, the driving of fluid at nano- or micro- scales with electric field, has found numerous applications, ranging from pumping to chemical and biomedical analyses in micro-devices. Electro-osmotic flow exhibits a puzzling hysteretic behavior when two fluids with different concentrations displace one another. The flow rate is faster when a higher concentration solution displaces a lower concentration one as compared to the flow in the reverse direction. Although electro-osmotic flow is a surface phenomenon, rather counter intuitively we demonstrate that electro-osmotic flow hysteresis originates from the accumulation or depletion of pH-governing minority ions in the bulk of the fluid, due to the imbalance of electric-field-induced ion flux. The pH and flow velocity are changed, depending on the flow direction. The understanding of electro-osmotic flow hysteresis is critical for accurate fluid flow control in microfluidic devices, and maintaining of constant pH in chemical and biological systems under an electric field.

Highlights

Electro-osmotic flow, the driving of fluid at nano- or micro- scales with electric field, has found numerous applications, ranging from pumping to chemical and biomedical analyses in microdevices
Electro-osmotic flow (EOF) or electro-osmosis is the flow of fluid in a micro-/nano-sized channel or a porous material under an externally applied electric field
If the electrical double layer (EDL) is thin as compared to the size of the channel, the fluid flow velocity U is given by the Helmholtz-Smoluchowski slip velocity equation: U = −εoεrEζ /μ, where E is the electric field, εo is the permittivity of free space, εr is the relative permittivity of the liquid, ζ is the zeta potential and μ is the viscosity of the liquid

Summary

Introduction

Electro-osmotic flow, the driving of fluid at nano- or micro- scales with electric field, has found numerous applications, ranging from pumping to chemical and biomedical analyses in microdevices. EOF has been exploited in various applications including drug delivery[1,2], fuel cell[3,4], sludge treatment[5,6], deoxyribonucleic acid (DNA) focusing and manipulation[7,8], disease diagnosis from blood[9], chemical species separation[10], pumping[11] and mixing of fluids[12,13] in various microfluidic devices In many of these applications, the fluid driven by EOF is inhomogeneous, differing in conductivity and concentration. As EOF is a surface-driven phenomenon, it is only intuitive to examine surface-related effects for the cause of EOF hysteresis Contrary to this obvious notion, we demonstrate in this investigation that EOF hysteresis originates from the ionic behavior in the bulk of the fluid. The understanding of EOF hysteresis is critical to the accurate manipulation of fluids and solutes in real world applications, where the fluids involved are typically inhomogeneous

Methods

Results

Conclusion