Theoretical Analysis of a Pulsatile Electroosmotic Flow in a Wavy Electrode (original) (raw)
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Journal of Microelectromechanical Systems, 2007
The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed. [2006-0114] Index Terms-Electrokinetic effect, microchannel, microfluidics, surface charge pattern, vortex. I. INTRODUCTION M ICROFLUIDIC systems have attracted major research interest due to promising potential applications in biotechnology, in particular in micro total analysis systems or the lab-on-a-chip. Typically, an assay carried out in such a microsystem involves flow of buffer solutions, reaction, separation, and detection [1]-[3]. Hence, the control of liquid flow is an integral element in the operation of such fluidic microsystems. Pressure and gravity are typically the forces applied to drive liquid flows in macrochannels. However, when the characteristic length scale of the channel is too small, surface forces acting on the flow (e.g., friction) become dominant in comparison to body forces (e.g., inertia). Consequently, flows driven by surface forces, such as electroosmosis, are recently receiving great attention for controlling liquid motion in microchannels [4]. Electroosmotic flow not only is more efficient as the channel size decreases but also requires no moving parts.
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.
DC electrokinetic particle transport in an l-shaped microchannel
Langmuir, 2010
Electrokinetic transport of particles through an L-shaped microchannel under DC electric fields is theoretically and experimentally investigated. The emphasis is placed on the direct current (DC) dielectrophoretic (DEP) effect arising from the interactions between the induced spatially nonuniform electric field around the corner and the dielectric particles. A transient multiphysics model is developed in an arbitrary Lagrangian-Eulerian (ALE) framework, which comprises the Navier-Stokes equations for the fluid flow and the Laplace equation for the electrical potential. The predictions of the DEP-induced particle trajectory shift in the L-shaped microchannel are in quantitative agreement with the obtained experimental results. Numerical studies also show that the DEP effect can alter the angular velocity and even the direction of the particle's rotation. Further parametric studies suggest that the L-shaped microfluidic channel may be utilized to focus and separate particles by size via the induced DEP effect.
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.
Effect of nanostructures orientation on electroosmotic flow in a microfluidic channel
Electroosmotic flow (EOF) is an electric-field-induced fluid flow that has numerous micro-/nanofluidic applications, ranging from pumping to chemical and biomedical analyses. Nanoscale networks/structures are often integrated in microchannels for a broad range of applications, such as electrophoretic separation of biomolecules, high reaction efficiency catalytic microreactors, and enhancement of heat transfer and sensing. Their introduction has been known to reduce EOF. Hitherto, a proper study on the effect of nanostructures orientation on EOF in a microfluidic channel is yet to be carried out. In this investigation, we present a novel fabrication method for nanostructure designs that possess maximum orientation difference, i.e. parallel versus perpendicular indented nanolines, to examine the effect of nanostructures orientation on EOF. It consists of four phases: fabrication of silicon master, creation of mold insert via electroplating, injection molding with cyclic olefin copolymer (COC), and thermal bonding and integration of practical inlet/outlet ports. The effect of nanostructures orientation on EOF was studied experimentally by current monitoring method. The experimental results show that nanolines which are perpendicular to the microchannel reduce the EOF velocity significantly (approximately 20%). This flow velocity reduction is due to the distortion of local electric field by the perpendicular nanolines at the nanostructured surface as demonstrated by finite element simulation. In contrast, nanolines which are parallel to the microchannel have no effect on EOF, as it can be deduced that the parallel nanolines do not distort the local electric field. The outcomes of this investigation contribute to the precise control of EOF in lab-on-chip devices, and fundamental understanding of EOF in devices which utilize nanostructured surfaces for chemical and biological analyses.
Electrokinetic Velocity Characterization of Microparticles in Glass Microchannels
The 2008 Annual …, 2008
Insulator-based dielectrophoresis (iDEP) is an efficient technique with great potential for miniaturization. It has been applied successfully for the manipulation and concentration of a wide array of particles, including bioparticles such as macromolecules and microorganisms. When iDEP is applied employing DC electric fields, other electrokinetic transport mechanisms are present: electrophoresis and electroosmotic flow. In order to achieve dielectrophoretic trapping of bioparticles, dielectrophoresis has to overcome electrokinetics (electroosmosis and electrophoresis). Therefore, to improve and optimize iDEP-based separations, it is necessary to characterize these electrokinetic mechanisms under the operating conditions employed for dielectrophoretic separations. The main objective of this work was to identify the operating conditions that will benefit dielectrophoretic trapping and concentration of particles when electrokinetics is present.
Scientific Reports, 2015
Electrokinetic phenomena are a powerful tool used in various scientific and technological applications for the manipulation of aqueous solutions and the chemical entities within them. However, the use of DC-induced electrokinetics in miniaturized devices is highly limited. This is mainly due to unavoidable electrochemical reactions at the electrodes, which hinder successful manipulation. Here we present experimental evidence that on-chip DC manipulation of particles between closely positioned electrodes inside micro-droplets can be successfully achieved, and at low voltages. We show that such manipulation, which is considered practically impossible, can be used to rapidly concentrate and pattern particles in 2D shapes in inter-electrode locations. We show that this is made possible in low ion content dispersions, which enable low-voltage electrokinetics and an anomalous bubble-free water electrolysis. This phenomenon can serve as a powerful tool in both microflow devices and digital microfluidics for rapid pre-concentration and particle patterning. Electrokinetic phenomena (EKP) are the induced movement of fluids, or of charged chemical entities immersed in them, due to an externally applied electric field 1,2. Since their first discovery in the 19th century 3 , EKP have attracted considerable scientific attention, and have been extensively studied 1. Nowadays, EKP are utilized in diverse fields of science and technology, to translate aqueous solutions and manipulate particles and molecules. Such uses range, e.g., from protein gel electrophoresis 4 , used in biochemical analysis, to industrial-scale electrophoretic deposition processes 5 used for coating inorganic surfaces. In recent years, the usage of EKP has expanded from macro-scaled applications (e.g. gel electrophoresis) to micro-scaled environments, i.e. to microfluidics 6,7 and "lab-on-a-chip" devices 8,9. EKP in macro-scaled applications is predominantly DC-based, and mostly relies on the classical DC-induced EKP of electroosmosis (EO) and electrophoresis (EP). However, in miniaturized devices the use of DC EKP is limited to one type of device only, in which liquid is translated along micro-channels 10. In theory, DC-induced EKP could serve as an excellent tool for liquid and particle actuation in micro-scale environments. This is since the magnitude of an electric field, which is the driving force in EKP, is inversely proportional to distance. Device miniaturization should have hence become an
Induced-Charge Electrokinetic Phenomena: Theory and Microfluidic Applications
Physical Review Letters, 2004
We give a general, physical description of "induced-charge electro-osmosis" (ICEO), the nonlinear electrokinetic slip at a polarizable surface, in the context of some new techniques for microfluidic pumping and mixing. ICEO generalizes "AC electro-osmosis" at micro-electrode arrays to various dielectric and conducting structures in weak DC or AC electric fields. The basic effect produces micro-vortices to enhance mixing in microfluidic devices, while various broken symmetries-controlled potential, irregular shape, non-uniform surface properties, and field gradients-can be exploited to produce streaming flows. Although we emphasize the qualitative picture of ICEO, we also briefly describe the mathematical theory (for thin double layers and weak fields) and apply it to a metal cylinder with a dielectric coating in a suddenly applied DC field.
Electroosmotic flow: From microfluidics to nanofluidics
ELECTROPHORESIS, 2021
Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nanoscale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applicationsfrom biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so-called electrical double layer (EDL), which forms in the vicinity of a charged solid-liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of electrical double layer models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid-liquid interface. Moreover, we discuss the physicochemical phenomena owing to the interaction of electrical double layer when the characteristic length of the conducting media is decreased from the microscale to the nanoscale. Finally, we highlight the pioneering studies and the most recent works on electro osmotic flow devoted to both theoretical and experimental aspects.
Electrokinetic generation of microvortex patterns in a microchannel liquid flow
Journal of Micromechanics and Microengineering, 2003
The technology developed for micropatterning the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distribution under an externally applied, steady electric field. We present design methodologies to electrokinetically drive vortical flows in any desired direction. In particular, we investigate surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices. Finally, in view of its potential application to microscale mixing, we discuss a surface charge pattern that can give rise to streamwise vorticity.