Numerical Investigation of Nanostructure Orientation on Electroosmotic Flow.

An Eng Lim,Yee Cheong Lam

doi:10.3390/mi11110971

Abstract

Electroosmotic flow (EOF) is fluid flow induced by an applied electric field, which has been widely employed in various micro-/nanofluidic applications. Past investigations have revealed that the presence of nanostructures in microchannel reduces EOF. Hitherto, the angle-dependent behavior of nanoline structures on EOF has not yet been studied in detail and its understanding is lacking. Numerical analyses of the effect of nanoline orientation angle θ on EOF to reveal the associated mechanisms were conducted in this investigation. When θ increases from 5° to 90° (from parallel to perpendicular to the flow direction), the average EOF velocity decreases exponentially due to the increase in distortion of the applied electric field distribution at the structured surface, as a result of the increased apparent nanolines per unit microchannel length. With increasing nanoline width W, the decrease of average EOF velocity is fairly linear, attributed to the simultaneous narrowing of nanoline ridge (high local fluid velocity region). While increasing nanoline depth D results in a monotonic decrease of the average EOF velocity. This reduction stabilizes for aspect ratio D/W > 0.5 as the electric field distribution distortion within the nanoline trench remains nearly constant. This investigation reveals that the effects on EOF of nanolines, and by extrapolation for any nanostructures, may be directly attributed to their effects on the distortion of the applied electric field distribution within a microchannel.

Highlights

Electroosmotic flow (EOF) in a porous material or micro-/nanodimensional channel is an electric-field-induced fluid flow, and is an electrokinetic phenomenon
When a solid surface is exposed to an aqueous solution, the developed negative charges next to the channel wall attract and repel positive and negative ions, respectively. This leads to the formation of a net positively charged layer known as the electrical double layer (EDL) of nanometer thickness
When the EDL thickness is thin compared to the dimensions of the fluidic channel, the EOF velocity may be calculated by the Helmholtz–Smoluchowski equation: VEOF = − r oζE/μ, (1)

Summary

Introduction

Electroosmotic flow (EOF) in a porous material or micro-/nanodimensional channel is an electric-field-induced fluid flow, and is an electrokinetic phenomenon. The EOF velocity profiles correspond to those with overlapping EDLs [37,38,39], and the flow rate was found to increase with increasing period and decrease with increasing amplitude of surface roughness This coincides with the investigations carried out by Kang and Suh [32] and Masilamani et al [33]. The effects of varying the nanoline width and depth in conjunction with the variation of orientation angle were carried out This investigation aims to deepen the understanding of EOF in microfluidic devices with patterned nanostructured surfaces to gain precise and accurate flow manipulation for applied analytical purposes in areas such as biochemistry and life science

Simulation Domain

Boundary Conditions

Results and Discussion