Jacky S.H. Lee

Bio: Jacky S.H. Lee is an academic researcher from University of Toronto. The author has contributed to research in topics: Asset allocation & Portfolio. The author has an hindex of 9, co-authored 15 publications receiving 332 citations.

DC-dielectrophoretic separation of microparticles using an oil droplet obstacle

Abstract: A new dielectrophoretic particle separation method is demonstrated and examined in the following experimental study. Current electrodeless dielectrophoretic (DEP) separation techniques utilize insulating solid obstacles in a DC or low-frequency AC field, while this novel method employs an oil droplet acting as an insulating hurdle between two electrodes. When particles move in a non-uniform DC field locally formed by the droplet, they are exposed to a negative DEP force linearly dependent on their volume, which allows the particle separation by size. Since the size of the droplet can be dynamically changed, the electric field gradient, and hence DEP force, becomes easily controllable and adjustable to various separation parameters. By adjusting the droplet size, particles of three different diameter sizes, 1 µm, 5.7 µm and 15.7 µm, were successfully separated in a PDMS microfluidic chip, under applied field strength in the range from 80 V cm−1 to 240 V cm−1. A very effective separation was realized at the low field strength, since the electric field gradient was proved to be a more significant parameter for particle discrimination than the applied voltage. By utilizing low strength fields and adaptable field gradient, this method can also be applied to the separation of biological samples that are generally very sensitive to high electric potential.

Electroosmotic flow at a liquid–air interface

Abstract: This paper presents the first experimental evidence on electroosmotic flow at a liquid–air interface. A PDMS microchannel with an opening to air was created to allow for the formation of a liquid–air interface. Polystyrene particles were used to visualize the liquid motion and the experiments found that the particle velocity at the liquid–air interface was significantly slower than the particle velocity in the bulk. This result agrees with a mathematical model that considers the effects of electrical surface charges at the liquid–air interface in electroosmotic flow.

Electrokinetic flow in a free surface-guided microchannel

Abstract: The purpose of this study is to investigate electro-osmotic flow in a free surface-guided microchannel. Although multiphase microfluidics has attracted interests over the past few years, electro-osmotic flow involving free surfaces has yet to be studied in great detail. Several proposed theoretical models describing this type of electro-osmotic flow need to be verified by experiments. In this work, a surface-guided microchannel was fabricated using an innovative fabrication process. Because the liquid stream was confined by surface properties, solid sidewalls did not exist in this microchannel. Instead, the sidewalls were water-air or water-oil interfaces. Using this microchannel, two systems were investigated: water-air system and water-oil system. The experimental results were compared against three proposed models in order to gain more understandings on this type of electro-osmotic flow. Experimental results show that the liquid velocity near the liquid-fluid interface resembles a pluglike profile for both water-air and water-oil systems. Computer simulation results show that with the consideration of the electrical double layer and the surface charges, the electric forces inside the electrical double layer are counteracted by the surface forces at the liquid-fluid interface, also resulting in a pluglike velocity profile in the microchannel. Therefore, the model that considers both the electrical double layer and the surface charges at the liquid-fluid interface best describe the physical phenomenon observed in experiments.

Micromachining a miniaturized capillary electrophoresis-based chemical analysis system on a chip

Abstract: Micromachining technology was used to prepare chemical analysis systems on glass chips (1 centimeter by 2 centimeters or larger) that utilize electroosmotic pumping to drive fluid flow and electrophoretic separation to distinguish sample components. Capillaries 1 to 10 centimeters long etched in the glass (cross section, 10 micrometers by 30 micrometers) allow for capillary electrophoresis-based separations of amino acids with up to 75,000 theoretical plates in about 15 seconds, and separations of about 600 plates can be effected within 4 seconds. Sample treatment steps within a manifold of intersecting capillaries were demonstrated for a simple sample dilution process. Manipulation of the applied voltages controlled the directions of fluid flow within the manifold. The principles demonstrated in this study can be used to develop a miniaturized system for sample handling and separation with no moving parts.

Micro total analysis systems. Latest advancements and trends.

Abstract: Review

Continuous flow separations in microfluidic devices

Abstract: Biochemical sample mixtures are commonly separated in batch processes, such as filtration, centrifugation, chromatography or electrophoresis. In recent years, however, many research groups have demonstrated continuous flow separation methods in microfluidic devices. Such separation methods are characterised by continuous injection, real-time monitoring, as well as continuous collection, which makes them ideal for combination with upstream and downstream applications. Importantly, in continuous flow separation the sample components are deflected from the main direction of flow, either by means of a force field (electric, magnetic, acoustic, optical etc.), or by intelligent positioning of obstacles in combination with laminar flow profiles. Sample components susceptible to deflection can be spatially separated. A large variety of methods has been reported, some of these are miniaturised versions of larger scale methods, others are only possible in microfluidic regimes. Researchers now have a diverse toolbox to choose from and it is likely that continuous flow methods will play an important role in future point-of-care or in-the-field analysis devices.

Particle focusing in microfluidic devices

Abstract: Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.

Dielectrophoresis in microfluidics technology

Abstract: Dielectrophoresis (DEP) is the movement of a particle in a non-uniform electric field due to the interaction of the particle's dipole and spatial gradient of the electric field. DEP is a subtle solution to manipulate particles and cells at microscale due to its favorable scaling for the reduced size of the system. DEP has been utilized for many applications in microfluidic systems. In this review, a detailed analysis of the modeling of DEP-based manipulation of the particles is provided, and the recent applications regarding the particle manipulation in microfluidic systems (mainly the published works between 2007 and 2010) are presented.