Kyung Heon Lee

Three-dimensional microfluidic liquid-core/liquid-cladding waveguide

This letter describes a three-dimensional liquid-core/liquid-cladding optical waveguide system. The core fluid was focused in the vertical direction by a transverse secondary flow (produced by a Dean vortex) and focused in the horizontal direction by two parallel sheath flows. The waveguide introduced less optical loss between the core fluid and the channel wall. Diffusion between the core fluid and the cladding fluid was reduced by high fluid velocities. The present system can be considered as a graded-index waveguide due to the diffusion effect. The width of the core fluid was manipulated by adjusting the sheath flow rates. Numerical simulations were conducted to support and interpret the experimental results.

Optical levitation of a non-spherical particle in a loosely focused Gaussian beam

The optical force on a non-spherical particle subjected to a loosely focused laser beam was calculated using the dynamic ray tracing method. Ellipsoidal particles with different aspect ratios, inclination angles, and positions were modeled, and the effects of these parameters on the optical force were examined. The vertical component of the optical force parallel to the laser beam axis decreased as the aspect ratio decreased, whereas the ellipsoid with a small aspect ratio and a large inclination angle experienced a large vertical optical force. The ellipsoids were pulled toward or repelled away from the laser beam axis, depending on the inclination angle, and they experienced a torque near the focal point. The behavior of the ellipsoids in a viscous fluid was examined by analyzing a dynamic simulation based on the penalty immersed boundary method. As the ellipsoids levitated along the direction of the laser beam propagation, they moved horizontally with rotation. Except for the ellipsoid with a small aspect ratio and a zero inclination angle near the focal point, the ellipsoids rotated until the major axis aligned with the laser beam axis.

Optical mobility of blood cells for label-free cell separation applications

This paper describes the optical mobilities of blood cell components. Blood cells are heterogeneous, and their optical behaviors depend on size, morphology, and other optical properties. In a step toward the label-free separation of blood cells, the optical mobility resulting from the optical scattering and cell properties was derived and evaluated for each cell component. The optical mobilities of red blood cells, lymphocytes, granulocytes, and monocytes were measured under various flow conditions of a cross-type optical particle separator.

Characterization of a Microscale Cascade Impactor

Although a cascade impactor is widely used for bioaerosol sampling and separation, the conventional system has some shortcomings when it is applied for the portable device. In the present study, a microscale cascade impactor system was developed using a soft lithography process to provide good portability and cost efficiency. The system described here included three impaction stages, and a rectangular-shaped jet nozzle was employed with a 0.5 L/min airflow rate. The collection efficiency of each stage was evaluated using polystyrene latex (PSL) (0.2–2.5 μm) and ammonium sulfate (10–700 nm) particles. The 50% cutpoints of each stage were numerically calculated to be 1.06, 0.55, and 0.26 μm, and the experimental values were found to be 1.19, 0.51, and 0.27 μm, respectively. The bounce and re-entrainment in the impaction zone were reduced by spin-coating of silicon oil onto the microchannel surface. The overall particle losses were less than 10% for the 0.2–2.5 μm particles and 9%–12% for the 0.05–0.20 μm particles. Copyright 2012 American Association for Aerosol Research

Nonlinear particle behavior during cross-type optical particle separation

The effects of varying the ratio of the optical force to the viscous drag force, termed S, on cross-type optical particle separation were investigated experimentally to test previous theoretical predictions. The experiments were performed for various flow velocities, powers of the laser beam, and radii of the laser beam waist and the particles. The behaviors of the particles during optical separation were examined by measuring the retention distances and analyzing the particle trajectories. For small values of S, the particles move with constant velocity in the flow direction and the retention distance increases linearly with S. However, the particles accelerate and decelerate within the laser beam and the retention distance increases nonlinearly with S when S increases further.

Inhibition of Reticulon-1A–Mediated Endoplasmic Reticulum Stress in Early AKI Attenuates Renal Fibrosis Development

Several animal studies have shown an important role for endoplasmic reticulum (ER) stress in AKI, whereas human studies are lacking. We recently reported that Reticulon-1A (RTN1A) is a key mediator of ER stress and kidney cell injury. Here, we investigated whether modulation of RTN1A expression during AKI contributes to the progression to CKD. In a retrospective study of 51 patients with AKI, increased expression of RTN1A and other ER stress markers were associated with the severity of kidney injury and with progression to CKD. In an inducible tubular cell-specific RTN1A-knockdown mouse model subjected to folic acid nephropathy (FAN) or aristolochic acid nephropathy, reduction of RTN1A expression during the initial stage of AKI attenuated ER stress and kidney cell injury in early stages and renal fibrosis development in later stages. Treatment of wild-type mice with tauroursodeoxycholic acid, an inhibitor of ER stress, after the induction of kidney injury with FA facilitated renoprotection similar to that observed in RTN1A-knockdown mice. Conversely, in transgenic mice with inducible tubular cell-specific overexpression of RTN1A subjected to FAN, induction of RTN1A overexpression aggravated ER stress and renal injury at the early stage and renal fibrosis at the late stage of FAN. Together, our human and mouse data suggest that the RTN1A-mediated ER stress response may be an important determinant in the severity of AKI and maladaptive repair that may promote progression to CKD.

Radiation forces on a microsphere in an arbitrary refractive index profile

The force equations describing the radiation forces on a microsphere in an arbitrary refractive index profile are derived here by using the photon-stream method in a ray-optics regime. A loosely focused Gaussian beam was employed as the radiating illumination beam. The radiation forces on a spherical microsphere were calculated in a time-varying refractive index profile. The refractive index profile of the surrounding medium was evaluated according to the concentration distribution obtained from the diffusion equation. The scattering and gradient forces on a microsphere were calculated for different refractive indices (1.22, 1.33, 1.43, and 1.59), and the radiation forces on a perfectly reflecting microsphere were calculated. The results were compared with previous results to validate the derivation.

Optical separation of ellipsoidal particles in a uniform flow

The behavior of an ellipsoidal particle subjected to a vertical optical force by a loosely focused laser beam in a uniform flow was studied numerically. The fluid flow and the particle motion were separately solved and coupled using the penalty immersed boundary method, and the optical force was calculated using the dynamic ray tracing method. The optical force and optically induced torque on the ellipsoidal particle varied according to the aspect ratio and initial inclination angle. The ellipsoidal particle, whose major axis was initially aligned with the laser beam axis, was more migrated as the aspect ratio increased. The migration distance also depended on the initial inclination angle, even for a given ellipsoidal particle shape. As the laser beam power increased and the flow velocity decreased, the effect of the initial inclination angle increased. The ellipsoidal particles with different aspect ratios could be effectively separated if the rotation along the spanwise direction was suppressed. Moreov...

Optofluidic particle manipulation in a liquid-core/liquid-cladding waveguide

This paper describes a method for particle manipulation in a liquid-core/liquid-cladding optical waveguide system. Step-index and graded-index waveguides were modeled with consideration for, respectively, miscible and immiscible core and cladding fluids. The characteristic motions of four different particles with refractive indices of 1.59, 1.48, 1.37, and 1.22 were examined. The guided beam was assumed to be Gaussian in shape. Our results showed that high-refractive-index particles converged at the center of the core fluid due to a positive gradient force, whereas low-refractive-index particles converged at the flow periphery. The nonlinearity of the particle motion increased as the flow velocity and the guided beam waist decreased and the laser power and the particle size increased. The initial beam waist of the guided beam in the graded-index waveguide did not significantly affect the characteristics of the particle motion due to the effects of diffusion.

Behavior of double emulsions in a cross-type optical separation system.

The behavior of double emulsions in a cross-type optical particle separation system was studied for different combinations of refractive indices and different inner and outer layer radii. The radii and refractive indices of the double emulsions were easily adjusted by taking advantage of the coflowing geometry of a cross-type optical particle separation device. An analytical expression of the optical forces on a pair of concentric spheres was derived using the photon stream method in the ray optics regime. The predicted trajectories of the double emulsions by the optical force agreed well with the experimental data. This work has potential uses in cell separation by morphometry, drug delivery vehicle, and emulsion-based biomedical applications.