Biomedical subjects

Junhyung Kim

Publications and source records attributed to Junhyung Kim.

5 recordsLinked to original sources

PubMed2006

Electron hopping dynamics in Au38 nanoparticle langmuir monolayers at the air/water interface.

The electron-transfer dynamics between nanoparticles has been studied as a function of interparticle distance by in situ voltammetry of well-defined monolayers of a metal quantum dot nanoparticle Au38(hexanethiolate)24. The interparticle distance is precisely controlled by the Langmuir technique and addition of various lengths of dithiol linkers (HS(CH2)nSH; n = 5, 6, 8, and 9). Voltammograms of Au38 monolayers display a well-defined single-electron charging peak that increases remarkably with decreasing the interparticle distance. The diffusion coefficient and rate constant calculated from the peak current for core-core electron hopping reaction both exponentially increase respectively, from 3.3 x 10-10 to 5.2 x 10-9 cm2/s and 2.2 x 104 to 5.0 x 105 s-1 as the distance decreases from 13.3 to 9.5 A and then levels off at 8.0 A. These rate constants are in good agreement with the literature values, demonstrating that the present experimental approach provides a powerful way to investigate the correlation between the electron-transfer dynamics and nanoparticle assembly structure.

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PubMed2005

Movement of colloidal particles in two-dimensional electric fields.

We characterize the movement of carbon black particles in inhomogeneous, two-dimensional dc electric fields. Motivated by display applications, the particles are suspended in a nonpolar solvent doped with a charge control agent. The two-dimensional fields are generated between strip electrodes on a glass slide spaced 120 microm apart with field strengths up to 10(4) V/m. Such fields are insufficient to drive either electrohydrodynamic instabilities or natural convection due to ohmic heating, but they move the particles between the electrodes in about 30 s. In the center region between the strip electrodes, the particles move by electrophoresis; that is, the particle velocity is proportional to the electric field. However, when imposing a constant-potential or constant-current boundary condition at the electrodes to derive the electrical field, the electrophoretic mobility calculated from the measured particle velocities is outside the range of mobilities predicted from the theory of O'Brien and White. Near the electrodes the particles either speed up or slow down, depending on the polarity of the electrode, and these changes in velocity cannot be explained simply by electrophoresis in a spatially varying electric field. We suggest that this anomalous motion arises from electrohydrodynamic flows originating from the interaction between the space charge of the polarized layers above the electrodes and the electric field. Approximate calculations indicate such flows could be sufficiently strong to explain the anomalous trajectories near the edges of the electrodes.

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PubMed2005

Ionic conduction and electrode polarization in a doped nonpolar liquid.

Electrical current versus potential relationships were measured for solutions of dodecane containing the charge control agent poly(isobutylene succinimide) (PIBS) at various concentrations. Both one-dimensional (parallel planar electrodes) and two-dimensional (strip electrodes) fields were studied. The initial current was proportional to the applied voltage for both electrode configurations. Using the initial decay rate of the current (t < 0.5 s) in the planar electrode cell and the Gouy-Chapman model for electrode polarization, we determined the diffusion coefficient of the charge carriers (micelles) in the solution, from which we calculated their effective radius to be 10 nm. The constancy of the carrier radius over a 7-fold change in PIBS concentration, along with the proportionality between conductivity and concentration, supports the hypothesis that the charged species result from the interactions between two micelles. The experimentally determined geometric factor (cell constant) relating current to applied potential at time zero for the strip electrode cell agrees with the value predicted from the solution of Laplace's equation for the electrical potential in this system. The intermediate-time (0.5-3.0 s) decay rate of current was faster than predicted from the classical Gouy-Chapman theory of the double layer, possibly because of volume fraction effects in the double layer. The very long-time (minutes to hours) residual current that we observed is not explained, but we suspect that some charge transfer across the electrode must have occurred because there was insufficient ion capacity (i.e., amount of PIBS) in the solution to account for the total charge transferred through the cell.

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PubMed2005

Anatomic correction of polysyndactyly of the fifth toe fused with the fourth toe.

A new surgical technique for correction of polysyndactyly with the fifth toe fused with the fourth toe was devised. The technique consists of the creation of a dorsal rectangular flap on the dorsum of the syndactyly web for the new fourth web, making a medially based plantar rectangular flap on the plantar surface of the fifth toe, and a sixth toe-tip-based rectangular hinge flap to reconstruct the lateral side of the fourth toe and medial side of the new fifth toe, respectively. All suture lines are intended to be located in the transition of the plantar and dorsal surfaces of the web and digit. This technique is characterised by accurate anatomic reconstruction, which produces better aesthetic results with no wasting of any skin components.

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PubMed2002

Two-particle dynamics on an electrode in ac electric fields.

The relative motion between pairs of negatively charged latex particles 9.7 microm in diameter and deposited on an electrode was measured by optical microscopy and image analysis. At an rms field of approximately 30 V cm(-1), the two particles moved toward each other at frequencies below 500 Hz, but they separated at 1000 Hz. In the cases of aggregation, there are several interesting characteristics. First, when the center-to-center separation of a pair was initially 6 particle radii or more apart, a transient 'incubation' period of tens of seconds was observed before the particles began to move toward each other. Second, the two particles never came into contact, rather at long times the pair maintained a stationary gap between them equal to approximately one-half the particle radius. This stationary gap between particles was also observed for the aggregation of clusters of three or more particles. Finally, the rate of approach for a pair of particles decreased as the frequency increased. Larger fields are required to move particles together in ac compared to dc fields; at 30 Hz the ac field must be 130 times greater than the dc field to achieve the same rate of approach. Taking advantage of the qualitative and quantitative differences of the cooperative motion of particles in dc vs. ac fields, one should be able to re-position particles by alternating between these two modes. We demonstrated that the same pair of particles can be brought together at low frequency (100 or 200 Hz) and then separated at high frequency (1000 Hz).

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