Spatial distribution of electrokinetically driven flow measured by micro-PIV (an evaluation of electric double layer in microchannel) (original) (raw)
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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.
Optically sliced measurement of velocity and pH distribution in microchannel
Experiments in Fluids, 2007
A simultaneous measurement technique for the velocity and pH distribution was developed by using a confocal microscope and a 3CCD color camera for investigations of a chemical reacting flow field in a microchannel. Micron-resolution particle image velocimetry and laser induced fluorescence were utilized for the velocity and pH measurement, respectively. The present study employed fluorescent particles with 1 lm diameter and Fluorescein sodium salt whose fluorescent intensity increases with an increase in pH value over the range of pH 5.0-9.0. The advantages of the present system are to separate the fluorescence of particles from that of dye by using the 3CCD color camera and to provide the depth resolution of 5.0 lm by the confocal microscope. The measurement uncertainties of the velocity and pH measurements were estimated to be 5.5 lm/s and pH 0.23, respectively. Two aqueous solutions at different pH values were introduced into a T-shaped microchannel. The mixing process in the junction area was investigated by the present technique, and the effect of the chemical reaction on the pH gradient was discussed by a comparison between the proton concentration profiles obtained from the experimental pH distribution and those calculated from the measured velocity data. For the chemical reacting flow with the buffering action, the profiles from the numerical simulation showed smaller gradients compared with those from the experiments, because the production or extinction of protons was yielded by the chemical reaction. Furthermore, the convection of protons was evaluated from the velocity and pH distribution and compared with the diffusion. It is found that the ratio between the diffusion and convection is an important factor to investigate the mixing process in the microfluidic device with chemical reactions.
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.
An experimental study of electro-osmotic flow in rectangular microchannels
Experimental studies were carried out on fully developed and steady electro-osmotic flow in a rectangular channel where the channel height h is comparable to its width and the thickness of the electric double layer characterized by the Debye length is much less than h. The nano-particle image velocimetry technique was used to measure the two components of the velocity field parallel to and within about 100 nm of the channel wall for h 6 25 µm. The mobility of the particle tracers was calculated from averaged velocity data for various electric field strengths. The experimentally determined mobility values are compared with analytical predictions for dilute aqueous solutions of sodium tetraborate.
Time-resolved velocity and pH mapping in T-shaped microchannel
A simultaneous measurement technique in time series for the velocity and pH distributions was developed by using fluorescence, a confocal microscope and a 3CCD camera for the investigation of an unsteady mixing flow field in a microchannel. Spatially averaged time-resolved particle tracking velocimetry (SAT-PTV) and laser induced fluorescence (LIF) were utilized for the velocity and pH measurement, respectively. SAT-PTV can detect temporal velocity variations of the fluid flow due to spatial-averaging velocity vectors obtained by PTV. LIF gives the pH distribution from the fluorescent intensity by the calibration technique. The present study employed particles with 1 μm diameter and Fluorescein sodium dye whose fluorescent intensity increases with an increase in pH value over the range of pH 5.0−9.0. The particles and dye absorbed excitation light of the almost same wavelength range, while emitted fluorescence of the different wavelength range. The simultaneous measurement was achieved by separating the emission wavelength of the particles from that of the dye using the 3CCD camera. Two aqueous solutions at different pH values were introduced into a T-shaped microchannel. Velocity-vector maps were obtained by applying SAT-PTV to instantaneous images. A two-dimensional distribution for the mixing flow field with a pH gradient was detected by the LIF technique whose a pH resolution was estimated to be 0.12. The spatial resolutions for the velocity and pH measurements were 15 μm × 15 μm and 5 μm × 5 μm, respectively. The depth of field for the confocal microscope is 1.4 μm which is one fifth of the depth of field of a conventional microscope, because out-of-focus fluorescence was excluded from the captured images by the spatial filtering technique of the confocal microscope. The validation of the simultaneous measurement technique was evaluated by a comparison between proton concentration profiles obtained from the experiments and the numerical simulation. The present technique was applied to an unsteady mixing flow controlled by a syringe pump and a pulse generator, and the mass fluxes due to the convection and diffusion were evaluated by the experimental results.
PIV Measurement for Electroosmotic Flow in Sio 2 pdms Surface Modified Microchannel
Electroosmotic is a flow created under the application of electric field on the fluid. Recently, PDMS exhibits great potential in microfluidic device for many applications. The less ability to support electroosmotic flow and flow instability with time are the phenomena associated with the fluid flow in PDMS microchannel. To improve fluid flow in PDMS, microchannel surface modified with SiO 2 , different thicknesses, by PECVD was employed. Surface characterizations were carried out using atomic force microscope (AFM) and scanning electron microscopy (SEM). Whereby; UV-visible spectrometer and X-ray diffraction (XRD) were employed to identify optical properties and structural phase of the modified PDMS respectively. Particle image velocimetry (PIV) was used to track particles in PDMS microchannel, native and modified. We demonstrated that PDMS surface modified with SiO 2 slowing down fluid flow comparing to native PDMS allowing reaction and/or measurements taken place in microchannel. Fluid stability in PDMS, native and surface modified, were monitored via electrical resistancetime measurements with the aid of Cd 2+ aqueous solution mixed with AuNP colloidal. SiO 2 PDMS surface modified microchannel achieved stable time-fluid flow.
Journal of the Brazilian Chemical Society, 2011
A instrumentação para a determinação fotométrica do fluxo eletrosmótico (EOF) em dispositivos microfluídicos é descrita neste trabalho. A instrumentação é baseada em componentes acessíveis e consiste em um microscópio trinocular e no fotodiodo integrado OPT101. Um diodo emissor de luz (LED) de alta intensidade foi utilizado como fonte de radiação. Para as determinações foram utilizadas soluções aquosas dos corantes azul patente V e azul de metileno. O sistema foi utilizado no monitoramento do EOF em microdispositivos de poli(dimetilsiloxano) (PDMS) e híbridos de toner/vidro. As mobilidades do EOF determinadas em pH 7,0 foram (5,75 ± 0,01)10 -4 cm 2 V -1 s -1 e (3,2 ± 0,1)10 -4 cm 2 V -1 s -1 para os dispositivos de toner/vidro e PDMS, respectivamente. Medidas reprodutíveis foram obtidas em todos os experimentos, levando a uma alta precisão na determinação. O método proposto foi comparado com o método tradicional de determinação do EOF que envolve a medida da corrente nos microcanais.
Experiments in Fluids, 2002
This paper presents global and point-wise comparisons of experimental measurements and numerical simulation results of electro-osmotically driven flows in two elementary micro-channel configurations, a straight channel with a groove and a T-junction channel. The microchannels are made by photolithography using poly-dimethylsiloxane (PDMS), a type of silicon product, which is transparent, and electro-osmotically permeable to a variety of liquids. A microscopic particle image velocimetry system has been developed to measure full field velocity distributions by tracking the fluorescence images of 500-nm diameter fluorescene dye particles. The numerical algorithm is based on a spectral element formulation of incompressible Navier-Stokes equations with electro-osmotic forcing. The algorithm utilizes modal expansions in mixed quadrilateral and triangular meshes, enabling complex geometry discretizations with spectral accuracy. Comparisons of experimental and numerical results show good agreements, validating both numerical and experimental methodologies.
Effects of applied electric field and microchannel wetted perimeter on electroosmotic velocity
Microfluidics and Nanofluidics, 2008
Parameters which affect electroosmotic flow (EOF) behavior need to be determined for characterizing flow in miniature biological and chemical experimental processes. Several parameters like buffer pH, ionic concentration, applied electric field and channel dimensions influence the magnitude of the electroosmotic flow. We conducted numerical and experimental investigations to determine the impact of electric field strength and wetted microchannel perimeter on
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.