Sang Youl Yoon

3D particle position and 3D velocity field measurement in a microvolume via the defocusing concept

This paper reports on a method for detecting three-dimensional particle positions and conducting three-dimensional microflow diagnostics in a microvolume via a three-pinhole defocusing concept. A simple setup and an easy detection method are described. The calibration-based defocusing method is suggested in place of formulae introduced through geometric analyses in previous studies. Depth calibration was performed in a microvolume, and X–Y compensation functions were obtained. By using the calibration functions, three-dimensional particle positions can be calculated at a sub-micron depth resolution. The effects of pinhole masks made with different pattern sizes are also described. The developed method was applied to a microflow in a micro backward-facing step. Time-resolved particle trajectories and three-dimensional volumetric velocity fields at a depth of 50 µm were obtained and are presented here.

Cross-type optical particle separation in a microchannel

A continuous, real-time optical particle separation, which was previously delineated theoretically, is successfully implemented experimentally for the first time. In this method, particles suspended in a flowing fluid are irradiated with a laser beam propagating in a direction perpendicular to direction of fluid flow. Upstream of the laser beam, the particles move parallel to the direction of fluid flow. When the particles pass through the laser beam, the scattering force pushes them in the direction of laser beam propagation, causing the particles to be displaced perpendicular to the fluid flow direction. This displacement, known as the retention distance, depends on the particle size and the laser beam parameters. Finally, the particles escape from the laser beam and maintain their retention distances as they move downstream. In the present work, the trajectories and retention distances of polystyrene latex microspheres with three distinct diameters were monitored and measured using cross-type optical particle separation. The measured retention distances for different-sized particles were in good agreement with theoretical predictions.

A Highly Accurate and Consistent Microfluidic Viscometer for Continuous Blood Viscosity Measurement

A high-precision microfluidic viscometer with a microfluidic channel array composed of 100 indicating channels is demonstrated in this study. The relative viscosity of the sample fluid could be measured by simply counting the number of the indicating channels occupied by the sample and the reference fluids. Using lumped parameter modeling, an analytical solution of the relative viscosity is derived. In order to evaluate the performance of the developed microfluidic viscometer, the viscosity values obtained by the microfluidic viscometer are compared with the ones obtained by a conventional viscometer. In Newtonian fluid (sodium dodecyl sulfate [SDS] solution) tests, the normalized differences in the viscosities measured by two methods are less than 2.5%. In non-Newtonian fluid (whole blood, 45% hematocrit) tests at various shear rates, the viscosities measured by two methods are evaluated by a regression analysis via power law (). The k values for both the microfluidic viscometer and the conventional viscometer are 12.953 and 13.175, respectively; the n values are 0.797 and 0.807, respectively. The normalized differences in two parameters measured by two methods are less than 2%. Thus, it could be concluded that the microfluidic viscometer developed in this study is capable of measuring viscosity of both Newtonian fluid (SDS solution) and non-Newtonian fluid (whole blood) with a relatively high accuracy in a continuous and near real-time fashion. Furthermore, the viscometer could be potentially employed in cardiopulmonary bypass procedures by continuously monitoring viscosity changes due to blood damages and hemodilution.

Handheld mechanical cell lysis chip with ultra-sharp silicon nano-blade arrays for rapid intracellular protein extraction

This paper presents a handheld mechanical cell lysis chip with ultra-sharp nano-blade arrays fabricated by simple and cost effective crystalline wet etching of (110) silicon. The ultra-sharp nano-blade array is simply formed by the undercutting of (110) silicon during the crystalline wet etching process. Cells can be easily disrupted by the silicon nano-blade array without the help of additional reagents or electrical sources. Based on the bench-top test of the proposed device, a handheld mechanical cell lysis chip with the nano-blade arrays is designed and fabricated for direct connection to a commercial syringe. The direct connection to a syringe provides rapid cell lysis, easy handling, and minimization of the lysate dead volume. The protein concentration in the cell lysate obtained by the proposed lysis chip is quantitatively comparable to the one prepared by a conventional chemical lysis method.

Experimental Investigation of Pulsatility Effect on the Deformability and Hemolysis of Blood Cells

In this study, we investigated the differences between pulsatile cardiopulmonary bypass (CPB) procedure and nonpulsatile CPB procedure in terms of their effects on hemolysis and deformability of red blood cells (RBCs) under various shear stress conditions. In order to research the effects on hemolysis and deformability, four parameters--free hemoglobin (fHb) concentration, normalized index of hemolysis (NIH), deformability index (DI) of RBCs, and elongation index of RBCs--have been deeply investigated. For these investigations, two randomly assigned adult mongrel dog groups-nonpulsatile group (NP, n = 6) and pulsatile group (P, n = 6)--were examined. According to our results, both types of perfusion did not show any statistical differences in terms of the concentrations of fHb as well as NIH. In addition, there were no significant differences in RBC deformability between perfusion types within an operation time of 3 h. Therefore, our studies suggest that pulsatile perfusion has no significant difference from nonpulsatile perfusion in terms of hemolysis and deformability of RBCs.

PIV Measurements of the Flow and Turbulent Characteristics of a Round Jet in Crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.

Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application

This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5-15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (ζ) potential analysis. Bulk-heterojunction organic photovoltaic cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had good preservability and stability in working solution and the synthetic microfluidic system provided a low-cost, environmentally friendly approach to the continuous production of ZnO NPs.

Vacuum-assisted microcontact printing (μCP) for aligned patterning of nano and biochemical materials

We developed a novel vacuum-assisted microcontact printing (μCP) process that presents a powerful method for patterning functional materials with precise alignment. The printing pressure of the vacuum-assisted μCP was applied using the pressure difference between the inside and outside of an elastomeric stamp. A double exposure microfabrication process was adopted for manufacturing different height protrusions on the elastomeric stamps. The outer protrusion was designed to be higher than the printing patterns, thereby acting as a vacuum sealing wall. The printing pressure was easily applied and controlled using commercial syringes and motorized syringe controllers. A high printing pressure exceeding 10 psi was applied uniformly to the target substrate. Precision alignment was realized using a common optical alignment system. During the alignment process, damage to the previously patterned material and undesired printing patterns due to stamp dragging were avoided by imposing a separation distance between the printed pattern and the substrate. Several functional materials, including proteins and nanomaterials, could be successively patterned. Protein–protein, protein–nanowire, and three-dimensionally patterned nanowires are described. This versatile vacuum-assisted μCP process is a powerful means for implementing the large-scale fabrication in bio- and nano-technologies and related applications.

Resolution of Cross-Type Optical Particle Separation

A real-time, continuous optical particle separation method, termed cross-type optical particle separation, is investigated theoretically and experimentally. The trajectory of a particle subject to cross-type optical particle separation is predicted by solving the particle dynamic equation and compared with experimental data. For various flow velocities and particle sizes, the retention distances are measured where the displacement perpendicular to the fluid flow direction is referred to as the retention distance. The measured retention distances are in good agreement with theoretical predictions. The retention distance increases as the particle size increases due to the radiation force, but decreases as the flow velocity increases since the residence time of a particle in the laser beam decreases with increasing flow velocity. To evaluate the performance of the cross-type optical particle separation method, size-based separation resolution is derived theoretically in terms of the refractive index of the particle and instrumental fluctuations. Furthermore, an expression for the maximum resolution is derived.

Simultaneous mixing and pumping using asymmetric microelectrodes

This study proposes ideas for simultaneous mixing and pumping using asymmetric microelectrode arrays. The driving force of the mixing and pumping was based on electroosmotic flows induced by alternating current (ac) electric fields on asymmetric microelectrodes. The key idea was to bend/incline the microelectrodes like diagonal/herringbone shapes. Four patterns of the asymmetric electrode arrays were considered depending on the shape of electrode arrays. For the diagonal shape, repeated and staggered patterns of the electrode arrays were studied. For the herringbone shape, diverging and converging patterns were examined. These microelectrode patterns forced fluid flows in the lateral direction leading to mixing and in the channel direction leading to pumping. Three-dimensional numerical simulations were carried out using the linear theories of ac electro-osmosis. The performances of the mixing and pumping were assessed in terms of the mixing efficiency and the pumping flow rate. The results indicated that...