ELECTROVISCOUS EFFECTS IN PRESSURE-DRIVEN FLOW OF ELECTROLYTE LIQUIDS THROUGH MICROFLUIDIC DEVICES

Abstract

The importance of microfluidic technology is continuously increasing because of its broad applications in many areas of biomedical and engineering fields, such as micro-heat pumps, micro-heat sinks, DNA analysis, drug screening, drug delivery systems, Lab-on-a-chip, bioanalysis, and cell cultivation. In general, higher-resistance convective flows in microfluidic devices can explained by the electroviscous effects (EVEs). Further, microfluidic elements are commonly featured as non-uniform geometries due to their wide uses in practical microfluidic applications. In this work, the investigation of surface (i.e., charge-dependent slip, chargeasymmetry, and charge-heterogeneity) and geometrical (i.e., contraction-ratio) features effect on the electroviscous flow of electrolyte liquids through microfluidic devices has been carried out for a fixed volumetric flow rate (Q). A finite element method (FEM) is used to solve the mathematical model consisting of the Poisson’s, Nernst-Planck (NP), Navier-Stokes (NS), and continuity equations. The mathematical model is represented by electrostatic (es), transport of dilute species (tds), and laminar flow (spf) modules of computational fluid dynamics (CFD) solver COMSOL Multiphysics. Mathematical correlations are proposed for flow fields, i.e., total electrical potential (U), excess charge (n∗), induced electric field strength (Ex), and pressure (P) as a function of flow governing parameters (K, Si, B0, Sr, Srh, and dc). Further, simple pseudo-analytical models to predict the pressure drop has been presented for no-slip (B0 = 0), and charge-dependent slip (B0 ̸= 0) flow through symmetrically/homogeneously (Sr = Srh = 1), asymmetrically (Sr ̸= 1), and heterogeneously (Srh ̸= 1) charged microfluidic devices by accounting for the pressure drop in the Poiseuille flow through the uniform channel, and that in creeping flow through a thin orifice. These models overestimate the pressure drop within ±2–5% than their numerical results. The summary of the five problems explored in this work is as follows: The first part of thesis explores the charge-dependent slip (0 ≤ B0 ≤ 0.20) effects in electroviscous flow (EVF) of electrolyte liquid through contraction-expansion (dc = Wc/W = 0.25) slit microfluidic geometry for inverse Debye length (2 ≤ K ≤ 20) and surface charge iii Abstract iv density (4 ≤ S ≤ 16). The detailed analysis of flow fields, i.e., total electrical potential, excess charge, induced electric field strength, and pressure, are carried out for wide ranges of flow governing parameters (K, S, B0). Results show that electroviscous correction factor (i.e., Y = μeff/μ = ΔP/ΔP0) maximally enhances by 72% for charge-dependent slip (B0 ̸= 0) than no-slip (B0 = 0) flow, over the ranges of conditions explored herein. Subsequently, the influence of charge-asymmetry (0 ≤ Sr ≤ 2) in electroviscous flow of electrolyte liquid through contraction-expansion (dc = 0.25) slit microchannel has been analyzed numerically for 2 ≤ K ≤ 20 and 4 ≤ St ≤ 16. In-depth insights of flow fields (ΔU, n∗, Ex, ΔP) dependence on governing parameters, such as K, St, and Sr are presented in the microfluidic device. Further, the maximum increment in the correction factor (Y ) is noted as 39.13% with the overall change in the charge-asymmetry (0 ≤ Sr ≤ 2) at fixed K and St. This part of the thesis focuses on the influence of contraction-ratio (0.25 ≤ dc ≤ 1) in electroviscous flow of electrolyte liquid through slit microfluidic device for 2 ≤ K ≤ 20 and 4 ≤ S ≤ 16. The dependence of flow fields on the governing parameters (K, S, dc) is analyzed in detail by normalizing the flow fields with their minimum and maximum values and reference case dc = 1. Results depict that Y enhances maximally by 46.99% with overall variation in contraction-ratio (0.25 ≤ dc ≤ 1) at fixed K and S Subsequently, the effect of charge-heterogeneity (0 ≤ Srh ≤ 2) in the EVF of electrolyte liquid through uniform (dc = 1) slit microfluidic device has been investigated for 2 ≤ K ≤ 20 and 4 ≤ S1 ≤ 16. The complex dependence of ΔU, n∗, Ex, and ΔP on dimensionless parameters, such as K, S1, and Srh is analyzed in-depth in the device. Further, maximum enhancement in Y is recorded as 40.98% with overall change in the charge-heterogeneity (0 ≤ Srh ≤ 2) at fixed K and S1. Finally, the influence of charge-heterogeneity (0 ≤ Srh ≤ 2) in the electroviscous flow of electrolyte liquid through contraction-expansion (dc = 0.25) slit microchannel has been explored for 2 ≤ K ≤ 20 and 4 ≤ S1 ≤ 16. The ΔU, n∗, Ex, and ΔP variation with governing parameters (K, S1, Srh) in the microfluidic device is analyzed in detail. Results show that Y increases maximally by 45.73% with overall variation in the charge-heterogeneity (0 ≤ Srh ≤ 2) at fixed K and S1. The insights obtained from the present work help design reliable and efficient microfluidic devices for practical microfluidic applications, i.e., mixing, heat and mass transfer processes, etc. Further, the analysis of surface and geometrical features in this work helps to provide a precise understanding of flow dynamics in microfluidic devices for high-resistance convective flows.