A simple yet efficient approach for electrokinetic mixing of viscoelastic fluids in a straight microchannel

C Sasmal

doi:10.1038/s41598-022-06202-x

Abstract

Many complex fluids such as emulsions, suspensions, biofluids, etc., are routinely encountered in many micro and nanoscale systems. These fluids exhibit non-Newtonian viscoelastic behaviour instead of showing simple Newtonian one. It is often needed to mix such viscoelastic fluids in small-scale micro-systems for further processing and analysis which is often achieved by the application of an external electric field and/or using the electroosmotic flow phenomena. This study proposes a very simple yet efficient strategy to mix such viscoelastic fluids based on extensive numerical simulations. Our proposed setup consists of a straight microchannel with small patches of constant wall zeta potential, which are present on both the top and bottom walls of the microchannel. This heterogeneous zeta potential on the microchannel wall generates local electro-elastic instability and electro-elastic turbulence once the Weissenberg number exceeds a critical value. These instabilities and turbulence, driven by the interaction between the elastic stresses and the streamline curvature present in the system, ultimately lead to a chaotic and unstable flow field, thereby facilitating the mixing of such viscoelastic fluids. In particular, based on our proposed approach, we show how one can use the rheological properties of fluids and associated fluid-mechanical phenomena for their efficient mixing even in a straight microchannel.

Highlights

Many complex fluids such as emulsions, suspensions, biofluids, etc., are routinely encountered in many micro and nanoscale systems
The number of corresponding studies on these elastic instabilities in electrokinetic flows is very limited as compared to that available for pressuredriven flows. Note that these instabilities are different from that of electrokinetic instability (EKI)[22], which is often seen during the electrokinetic flows. The former one is originated in viscoelastic fluids due to the interaction between the elastic stresses generated in the system and the presence of streamline curvature in the system[23,24], whereas the latter one is generally developed in Newtonian fluids due to the presence of electrical conductivity gradient[25–27]
After solving the aforementioned governing equations based on the open-source computational fluid dynamics (CFD) code OpenFOAM, we discuss the corresponding flow and mixing phenomena results

Summary

Introduction

Many complex fluids such as emulsions, suspensions, biofluids, etc., are routinely encountered in many micro and nanoscale systems. Some numerical and experimental studies have found the existence of an instability, known as the electro-elastic instability (EEI), in electrokinetic flows of viscoelastic fluids in various systems These instabilities are associated with an unstable and fluctuating flow field and originated once the Weissenberg number, Wi, Datta et al.[21] presented an extensive discussion and review on the origin, mechanism and properties of these elastic instabilities originated in various systems ranging from simple cross-slot geometry to complex porous media They discussed both the experimental and numerical perspectives of these elastic instabilities and provided the future scope for studying and applying them in many practical applications in detail. We aim to show how the mixing process of these complex fluids can be achieved even in a straight microchannel based on our proposed technique by using the phenomena of electro-elastic instability and electro-elastic turbulence

Objectives

Methods

Results

Discussion

Conclusion