Instability of Electrokinetic Microchannel Flows With Conductivity Gradients (original) (raw)

Electrokinetic Flow Instability in High Concentration Gradient Microflows

Fluids Engineering, 2002

This paper documents the scalar imaging of an electrokinetic flow instability that is directly relevant to microfluidic systems that aim to handle and analyze heterogeneous sample streams. The instability occurs in simple T-junctions where two streams of different ionic concentration flow into a common channel. Using neutral dye visualizations, general qualitative behavior of the instability is documented including the formation of a wave in the stream/stream material line that originates at the junction of the two channels and propagates downstream. Several quantitative properties of this phenomenon are measured including wave speed and the extent of the perturbation boundary. This work is part of an ongoing project to identify the physics of this instability and determine the regime of stability, with an ultimate goal of developing methods to either enhance or suppress the instability.

Convective instability of electrokinetic flows in a cross-shaped microchannel

2006

We present a parametric experimental study of convective electrokinetic instability (EKI) in an isotropically etched, cross-shaped microchannel using quantitative epifluorescence imaging. The base state is a three-inlet, one-outlet electrokinetic focusing flow configuration where the centre sample stream and sheath flows have mismatched ionic conductivities. Electrokinetic flows with conductivity gradients become unstable when the electroviscous stretching and folding of conductivity interfaces grows faster than the dissipative effect of molecular diffusion. Scalar images, critical applied fields required for instability, and temporal and spatial scalar energy are presented for flows with a wide range of applied d.c. electric field and centre-tosheath conductivity ratios. These parameters impose variations of the electric Rayleigh number across four orders of magnitude. We introduce a scaling for charge density in the bulk fluid as a function of local maximum conductivity gradients in the flow. This scaling shows that the flow becomes unstable at a critical electric Rayleigh number (Ra e, = 205) and applies to a wide range of applied field and centre-tosheath conductivity ratios. This work is relevant to on-chip electrokinetic flows with conductivity gradients such as field amplified sample stacking, flow at the intersections of multi-dimensional assays, electrokinetic control and separation of sample streams with poorly specified chemistry, and low-Reynolds number micromixing. † Present address:

Electrokinetic instabilities in thin microchannels

2005

An important class of electrokinetic, microfluidic devices aims to pump and control electrolyte working liquids that have spatial gradients in conductivity. These high-gradient flows can become unstable under the application of a sufficiently strong electric field. In many of these designs, flow channels are thin in the direction orthogonal to the main flow and the conductivity gradient. Viscous stresses due to the presence of these walls introduce a stabilizing force that plays a major role in determining the overall instability. A thin channel model for fluid flow is developed and shown to provide good agreement with a complete three-dimensional model for channel aspect ratios Շ0.1.

Electrokinetic flow : characterization, control and application in microfluidic systems

Advanced microfluidic devices can perform complete biochemical analysis in a single fabricated chip. One of the crucial issues in developing these microfluidic devices is to transport reagents and electrolytes to specified destinations without external intervention. The electrokinetic (EK) pumping can provide a kinetic source to route the liquid through microchannel networks. The EK pumping has numerous advantages including ease of fabrication, no need for moving parts, high reliability, and no noise, so that it has been extensively implemented in many microfluidic systems. The present study focuses on the characterization, control and manipulation of electrokinetic flows in microchannel network in order to optimally design and effectively control microfluidic devices. Specifically, due to strong relevance to the development of novel microfluidic devices such as millisecond capillary electrophoretic separation systems, AC pumps, advective chaotic micromixers etc., the time-dependent and frequencydependent electroosmotic flows (EOF) are thoroughly investigated. Having advantages of obtaining the whole-field information of fluid flow in microfluidic channels, the micro-PIV technique is used to characterize the EOF in microfluidic channels. Since the tracer particles used in micro-PIV measurements and channel wall are charged in liquids, electrokinetic mobilities and zeta potentials of the tracer particles and the channel surfaces are crucial to the design, control, and characterization of microfluidic devices. A new method, which combines the electrokinetic flow theory and the micro-PIV experiment, is developed to simultaneously determine the zeta potentials of both the channel wall and the tracer particles. With the known zeta potentials, the EOF velocity field can be obtained by subtracting the electrophoretic effects on the tracer particles, and hence the theoretical model can be validated using the micro-PIV technique. A micro-PIV based phase locking technique is developed to measure the transient electrokinetic flow in microchannels. With the transient micro-PIV technique, a method is further proposed to decouple the particle electrophoretic velocity from the micro-PIV measured velocity and to determine the zeta potential of the channel wall.

Electrokinetic microflow instability with conductivity gradients

2003

We have experimentally identified and quantified an electrokinetic flow instability that occurs in DC-electric-field driven microfluidic channels with significant conductivity gradients. We have, for the first time, developed a physical model for this instability which captures the interactions between bulk charge accumulation, electromigration, convection, and diffusion. A linear stability analysis based on this model captures key physics of this convective instability with a threshold electric field, The model and experiments show conductivity gradients and their associated bulk charge accumulation are crucial for such instabilities.

Convective Instabilities of Electrokinetic Flows in a Cross-Shaped Microchannel

Fluids Engineering, 2004

Application of electrokinetic instability flow for enhanced micromixing in cross-shaped microchannel

Biomedical Microdevices, 2006

This paper proposes a cross-shaped micromixer featuring a pair barrier within the mixing channel. The proposed device obtains a rapid mixing of two sample fluids by means of the electrokinetic instability-induced shedding effects which are produced when a DC electric field of an appropriate intensity is applied. The proposed device uses a single high-voltage power source to simultaneously drive and mix the sample fluids. The effectiveness of the mixer is characterized experimentally as a function of the applied electric field intensity and the extent to which a pair barrier obstruct the mixing channel. The experimental results indicate that the mixing performance reaches 96% at a cross-section located 1 mm downstream of the cross-junction when an electric field of 300 V/cm is applied. The micromixing method presented in this study provides a simple low-cost solution to mixing problems in lab-on-a-chip systems.

Numerical simulation of electrokinetic flow in microfluidic chips

Microfluidic chips are miniaturized analytical devices used in chemical, biological and medical applications. In most cases, fluids are conducted through microchannels by applying electric potentials and/or pressure gradients. This growing lab-on-a-chip technology requires numerical simulations to assist the design, control and optimization of analytical manipulations. The present work deals with FEM-based calculations of the dynamics of electrolyte solutions in cross-shaped microchannels, where the flow is driven by the action of external electric fields. A theoretical modeling of electrokinetic and transport phenomena in the system is carried out in the framework of continuum fluid mechanics. Calculations ground on conservation equations of mass, momentum and electrical charge, considering effects in three dimensions. Operations normally performed in analytical systems are discussed, such as loading, focusing, and injection of samples. Numerical simulations carried out in this work can be a valuable tool to control and optimize practical manipulations in microfluidic chips.

Numerical Modelling of Electrokinetic Flow in Microchannels: Streaming Potential and Electroosmosis

2020

Investigating the flow-behavior in microfluidic systems has become of interest due to the need for precise control of the mass and momentum transport in microfluidic devices. In multiphase flows, precise control of the flow behavior is much more challenging as it depends on multiple parameters. The following thesis focuses on two aspects of microfluidics discussed in two chapters: the flow reversal phenomenon in streaming potential flows and the magnetic fields generated by electroosmotic and streaming potential flows. In the first chapter, the proposed microfluidic system consists of an aqueous solution between a moving plate and a stationary wall, where the moving plate represents a charged oil-water interface. A numerical model was developed to predict the streaming potential flow created due to the shear-driven motion of the charged upper wall along with its associated electric double layer (EDL) effect. Additionally, analytical expressions were derived by solving the nonlinear Poisson-Boltzmann equation along with the simplified Navier-Stokes equation in order to describe the effect of the EDL on the sheardriven flow of the aqueous electrolyte solution. Results show that the interfacial charge of the moving interface greatly impacts the velocity profile of the flow and can reverse its overall direction. The numerical results were validated by the analytical expressions, where both models predicted that flow can reverse its overall direction when the surface potential of the oil-water interface exceeds 120mV. For the second chapter, models were constructed for the transient electrokinetics, for both the electroosmotic flow and for the shear driven streaming potential flow, in a charged nanocapillary channel. Additionally, the transient effects of ionic currents and the magnetic field generated both inside and outside the microchannel were evaluated, and the results compared with known iii analytical solutions for verification purposes. In order to correctly simulate the above models, the following partial differential equations are solved together for the electrolyte continuum to capture the physics of the problem: a) the Navier-Stokes equation for the fluid flow b) Poisson-Nernst-Planck equations for the electric potential distribution and ion transport and c) Ampere-Maxwell's law for the associated magnetic field. The obtained results showed that the magnetic field detected outside of the nanochannels can be used as a secondary electromagnetic signal for biomolecules as a part of a sequencing technique.

A depth-averaged electrokinetic flow model for shallow microchannels

2008

Electrokinetic flows with heterogeneous conductivity configuration occur widely in microfluidic applications such as sample stacking and multidimensional assays. Electromechanical coupling in these flows may lead to complex flow phenomena, such as sample dispersion due to electro-osmotic velocity mismatch, and electrokinetic instability (EKI). In this work we develop a generalized electrokinetic model suitable for the study of microchannel flows with conductivity gradients and shallow-channel geometry. An asymptotic analysis is performed with the channel depth-to-width ratio as a smallness parameter, and the three-dimensional equations are reduced to a set of depth-averaged equations governing in-plane flow dynamics. The momentum equation uses a Darcy-Brinkman-Forchheimer-type formulation, and the convectivediffusive transport of the conductivity field in the depth direction manifests itself as a dispersion effect on the in-plane conductivity field. The validity of the model is assessed by comparing the numerical results with full three-dimensional direct numerical simulations, and experimental data. The depth-averaged equations provide the accuracy of three-dimensional modelling with a convenient two-dimensional equation set applicable to a wide class of microfluidic devices.

Instability of Electrokinetic Microchannel Flows With Conductivity Gradients (original) (raw)

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