Deguang Yan

Visualizing the transient electroosmotic flow and measuring the zeta potential of microchannels with a micro-PIV technique

We have demonstrated a transient micro particle image velocimetry (micro-PIV) technique to measure the temporal development of electroosmotic flow in microchannels. Synchronization of different trigger signals for the laser, the CCD camera, and the high-voltage switch makes this measurement possible with a conventional micro-PIV setup. Using the transient micro-PIV technique, we have further proposed a method on the basis of inertial decoupling between the particle electrophoretic motion and the fluid electroosmotic flow to determine the electrophoretic component in the particle velocity and the zeta potential of the channel wall. It is shown that using the measured zeta potentials, the theoretical predictions agree well with the transient response of the electroosmotic velocities measured in this work.

Enhancement of electrokinetically driven microfluidic T‐mixer using frequency modulated electric field and channel geometry effects

This study reports improved electrokinetically driven microfluidic T‐mixers to enhance their mixing efficiency. Enhancement of electrokinetic microfluidic T‐mixers is achieved using (i) an active approach of utilizing a pulsating EOF, and (ii) a passive approach of using the channel geometry effect with patterned blocks. PDMS‐based electrokinetic T‐mixers of different designs were fabricated. Experimental measurements were carried out using Rhodamine B to examine the mixing performance and the micro‐particle image velocimetry technique to characterize the electrokinetic flow velocity field. Scaling analysis provides an effective frequency range of applied AC electric field. Results show that for a T‐mixer of 10 mm mixing length, utilizing frequency modulated electric field and channel geometry effects can increase the mixing efficiency from 50 to 90%. In addition, numerical simulations were performed to analyze the mixing process in the electrokinetic T‐mixers with various designs. The simulation results were compared with the experimental data, and reasonable agreement was found.

Diagnosis of transient electrokinetic flow in microfluidic channels

Measuring the electro-osmotic velocity distributions in microchannels is usually performed for steady-state electrokinetic flows. Characterizing time-dependent electrokinetic flows is of importance to the development of microfluidic devices such as rapid capillary electrophoretic separation system, ac pumps, novel micromixers, etc. In this paper, we use a micron-resolution particle image velocimetry (micro-PIV) based phase locking technique with an ordinary PIV charge coupled device (CCD) camera to carry out an experimental study of the transient electrokinetic flow in microchannels by synchronizing different trigger signals for the laser, CCD camera, and in-house designed high-voltage switch. With the transient micro-PIV technique, we further propose a method to decouple the particle electrophoretic velocity from the micro-PIV measured velocity and to determine the zeta potential of the channel wall. The time evolution of the full-field, electro-osmotic velocity distributions in both open- and closed-end...

Geometry Effect on the Electrokinetic Instability of the Electroosmotic Flow in Microfluidic Channels

To study the effect of geometry on electroosmotic flow in micro channels, we fabricated PDMS-glass microchannels of different designs, which have patterned channels with abrupt contraction of different sizes. Using fluorescent imaging technology, we demonstrated the effect of geometry on the instability of DC driven electroosmotic flow in microfluidic channels. For certain geometry and conductivity of the electrolyte solution (Sodium Bicarbonate), there is a threshold voltage for electroosmotic instability, exhibiting itself as “ripple”. Generally, the factors which affect the threshold voltage include channel width, channel geometry, and electrolyte conductivity. Narrower channel resulted in higher onset voltage. As conductivity of the electrolyte increases, the threshold voltage tends to increase. Early transition to unstable electroosmotic flow in microfluidic channels was observed under relatively low Re.Copyright

Diagnosis of Frequency-Dependent Electrokinetic Flow in Microfluidic Channels

The Study of the dynamic aspects of electrokinetic flow is of importance to the development of relevant microfluidic technologies. This paper presents a systematic investigation on the frequency-dependent electrokinetic flow. Experiments were carried out to measure the frequency-dependent electrokinetic flows in a rectangular microchannel by using the micro-PIV technique. Under the slip velocity approximation, the Stokes equation was solved analytically, and the closed-form solution for the velocity distributions of steady oscillating electroosmotic flow in a rectangular microchannel was obtained. With the measured channel zeta potential, the theoretical predictions of the frequency-dependent electroosmotic velocity fields were validated with the experimental data obtained in this study.

Deviation of Electroosmotic Flow From Plug-Like Profile: The Effect of Reservoir Size

In electrokinetically-driven microfluidic applications, reservoirs are indispensable and have finite sizes. During operating processes, as the liquid level in reservoirs keeps changing as time elapses, a backpressure is generated. Thus, the flow in microfluidic channels actually exhibits a combination of the electroosmotic flow and the time-dependent induced backpressure-driven flow. In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel. Important parameters that describe the effect of finite reservoir size on flow characteristics are discussed. A new concept termed as “effective pumping period” is introduced to characterize the reservoir size effect. The proposed model identifies the mechanisms of the finite-reservoir size effects and is verified by experiment using the micro-PIV technique. The results reported in this study can be used for facilitating the design of microfluidic devices.© 2007 ASME

Measurement of Transient Electrokinetic Flow in Microchannels Using Micro-PIV Technique

Measurement of the steady-state electroosmotic velocity distributions in microchannels has been reported in the literature. Characterizing time-dependent electrokinetic flows is of importance to the development of microfluidic devices such as rapid capillary electrophoretic separation systems, AC pumps, novel micromixers etc. In this paper, we report a novel technique for studying and quantifying the transient electrokinetic flow phenomena in microchannels using the micro-PIV system with an ordinary PIV CCD camera. This is achieved by synchronizing different trigger signals for the laser, CCD camera, and custom high-voltage switch. Using the transient micro-PIV technique, we further propose a method to determine the electrophoretic component in the particle velocity and the zeta potential of the channel wall. Then the time evolution of the full-field, electroosmotic velocity distributions in both open- and closed-end rectangular microchannels is obtained from the micro-PIV measurement data. Using the slip velocity approach and the measured channel zeta potential, the theoretical predictions of the transient electroosmotic flow in the open- and closed-end microchannels are obtained, and they are found in good agreement with the experimental results.Copyright