Electrokinetic Flow and Particle Manipulations in Microfluidics
Charles Chun Yang
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

An electrokinetic phenomenon is referred to as effects associated with the movement of ionic solutions near charged interfaces under applied electric field. Implementation of electrokinetic phenomena in microfluidic operations has been demonstrated for pumping liquid phase based on electroosmosis and for separating charged species (e.g., DNA sequencing) based on electrophoresis. Electroosmotic flow, offers a useful alternative to pressure-driven flow, and has numerous advantages including ease of fabrication and control, no need for moving parts and hence no noise, and high reliability. In this talk, I will first report the experimental characterization of the time-dependent and steady-state electroosmotic flows used for actuation, flow switch and mixing enhancement. I will then discuss the Joule heating induced heat transfer and its effects on electroosmotic flow and capillary electrophoresis transport, and particularly its application in concentration of sample solutes. Furthermore, I will talk the deposition of colloidal particles in electrokinetic flow. Finally, I will conclude by presenting electrokinetics based sorting, separation and concentration of microparticles. 

Bio:
Professor Chun Yang obtained his B.Sc. degree from the Department of Thermal Engineering at Tsinghua University in 1985, Master’s degree in Engineering Thermophysics from University of Science and Technology of China in 1988, and Ph.D. degree in Mechanical Engineering from University of Alberta in 1999. In 1999, he joined Nanyang Technological University, Singapore, and now is an Associate Professor in the School of Mechanical and Aerospace Engineering. He is the author and coauthor of about 100 publications in referred international journals. He has five US patens in his name and has co-authored one text book entitled “Elementary Electrokinetic Flow”. He services as a member of editorial advisory board for Journal of Microfluidics and Nanofluidics and International Journal of Emerging Multidisciplinary Fluid Sciences. He is a reviewer for the Research Grant Council of Hong Kong, Research Grant Council of Australia, National Science and Engineering Research Council of Canada and Dutch Technology Foundation.

Thermally Driven Gas Flows and Their Applications in Actuation
Wenjing Ye
Department of Mechanical Engineering, The Hong Kong University of Science and Technology

One unique feature of rarefied gas is that steady-state motion can be induced by a thermal gradient in the absence of any externally applied force. Crookes radiometer is perhaps the first demonstration of such a phenomenon. With the advent of micromachining technology, devices with their feature size on the order of micron or even nanometer can be routinely fabricated. As a consequence, gas encountered in micro/nano devices is rarefied even at the atmospheric environment. This has created an opportunity to utilize thermal loading as a passive driving mechanism to manipulate flows. In this talk, I will first discuss some fundamentals about rarefied gas and its unique behavior. Next, I will present some recent work on the modeling of thermal transpiration and Knudsen force. Applications such as Knudsen pump will also be discussed.

Bio:
Professor Ye received her B.S. degree from University of Science and Technology of China, her M.S. degree from University of California at San Diego and her Ph.D. degree from Cornell University. Before she joined the faculty of Hong Kong University of Science and Technology, she was a postdoctoral associate at Massachusetts Institute of Technology and then an assistant Professor at Georgia Institute of Technology. Her research interests include Numerical techniques: integral methods, Direct Simulation Monte Carlo (DSMC), Molecular Dynamics (MD), Multiscale numerical methods, Modeling and design of micro/nano systems, micro resonator design and application, and Rarefied gas transport.

Concentration of Sample Solutes in Microfluidic Structures using Temperature Gradient Focusing with Joule Heating Effects
Zhengwei Ge
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

Microfluidics utilizing the well-developed microfabrication technologies is promising to next generation of analytical instruments for chemical analysis and biomedical diagnosis. However, one of the challenges posed by microfluidics lies in the difficulty of detecting very dilute solutions of analytes with ultrasmall volumes in microchannels. Sample concentration techniques are developed to enhance the detection sensitivity and resolution, and to facilitate separation and reaction as well. In the literature, the reported techniques for sample concentration can be mainly classified into the stacking methods and equilibrium field gradient focusing methods. Of them, Temperature Gradient Focusing (TGF) is a recently developed technique based on balancing the electrophoretic motion of analyte molecules against the bulk electroosmotic flow of solution under a temperature gradient either generated by Joule heating or external heating. The possibly achieved high concentration and required relatively short channels (e.g., 4mm) make TGF well suitable for the development of integrated microfluidic systems, with a promising in combination with other concentration and separation techniques. Compared with external heating, there are several advantages of using Joule heating to generate the required temperature gradient. It consumes less power. The electric field used for generating Joule heat can also be used to drive the flow, which makes the design simple. Importantly, the device is more portable without need of external heating units.

In this talk, a systematic study of the TGF with Joule heating effect will be reported. Microfluidic concentration is achieved in a microchannel with a step change in cross-section. A comprehensive mathematical model is developed to describe the complex TGF processes. The proposed mathematical model includes a set of governing equations for the applied electric field, electroosmotic flow field Joule heating induced temperature field, and the sample solute concentration distributions as well. Since the thermophysical and electrical properties including the liquid dielectric constant, viscosity, and electric conductivity and electrophoretic mobility are all temperature-dependent, these governing equations are strongly coupled. Hence, scaling analysis is conducted to estimate time scales so as to simplify the mathematical model. Numerical computations are performed to obtain the temperature, velocity and sample concentration distributions which allow us to reveal the insightful TGF mechanisms.

Experiments are carried out to study the effects of applied voltage, buffer concentration, and channel size on TGF processes in two types of PDMS/Glass and PDMS/PDMS microfluidic channels. These experimental parametric effects are summarized using a dimensionless Joule number. The general trend is that increasing the Joule number would enhance the TGF, and thus improve concentration efficiency. A more than 450-fold concentration enhancement was obtained within 75 seconds in the PDMS/PDMS micro-device. A comparison of the numerical simulation results with the experimental data shows reasonable agreement in the Joule number effect. Profiles of velocity, temperature and concentration obtained numerically were implemented to describe the focusing mechanisms. In addition, TGF experiments are also carried out under the combined AC and DC electric field. The use of AC field which contributes to produce the temperature gradient can greatly reduce the required DC voltage to produce much higher concentration enhancement. A 2500-fold enhancement is demonstrated within 14 min under the combined AC and DC mode.