Biomedical subjects

S Joon Kwon

Publications and source records attributed to S Joon Kwon.

6 recordsLinked to original sources

PubMed2006

Dewetting of a sol-gel-derived thin film.

We report on the dewetting of a thin film produced by the sol-gel method. In the early stages of dynamic morphological instability, the drying stress in the capillary wave model determines the linearly scaling behaviors of the characteristic wavelength with the initial film thickness and the square law dependence of the number density of the dewetted holes on the film thickness. These power law dependences are weaker than those observed in the case of the spinodal dewetting of a polymer thin film. The wavelength determined in the early stages also dominates the scaling behaviors of the average length of the sides and number density of the polygons and the diameter of the droplets of the dewetting pattern with the film thickness in the final stages of the dynamic instability. We also observed that further drying eventually induces wrinkles in the droplets, rim, and film, which have a characteristic wavelength that can be theoretically predicted.

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PubMed2006

Theoretical analysis of non-catalytic growth of nanorods on a substrate.

A theoretical analysis explaining the whole process of the growth of nanorods on a substrate without a catalyst is presented. Prior to the growth of the nanorods, the reaction precursors form nuclei on the substrate. The nuclei undergo cluster migration caused by the surface diffusion of adatoms on the substrate, and this migration continues until the mean free time of the adatoms is larger than surface diffusion time. The most probable mechanism by which cluster migration takes place is the one that leads to the minimization of the cluster free-energy, namely the migration of six adatoms into one fixed adatom. This cluster migration continues during several (typically smaller than 6) consecutive nuclei growth steps. After the process of cluster migration comes to an end, the nuclei grow in an isotropic manner by collection of the adatoms, until the nucleus reaches the thermodynamic size limit. The one-dimensional growth of nanorods on the nuclei, which is associated with the critical radius, begins when the reactant dose is smaller than a certain value, which is determined by the thermodynamic size limit and the mass transport parameter. The mass transport of the reaction precursors leads to the expansion of the radius and elongation of the height of the nanorods, and the growth rate of the height is greater than that of the radius. This difference in the growth rate causes the aspect ratio to increase with increasing growth time. By comparing the experimental data in the literature (ZnO nanorods), the presented analysis explains well the noncatalytic growth of nanorods on a substrate.

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PubMed2005

Theoretical analysis of growth of ZnO nanorods on the amorphous surfaces.

Semiconductor nanorod arrays on a substrate have a preferential alignment orientation that minimizes the excessive free energy of the system. In the case of wet chemically synthesized zinc oxide (ZnO) nanorod on the amorphous surfaces, the thermodynamic driving force determines the orientation to be normal to the surface. Among the various kinds of amorphous surfaces, the spherical seed layer composed of ZnO precursors gives isotropic radially aligned arrays. For other surfaces, such as wrinkled and planar ZnO precursor thin film, nanorod arrays are aligned to be perpendicular to the tangential line of the surface. The maximum value of the aspect ratio of the nanorod is determined by the thermodynamic relationship. The number density of nanorods per unit precursor particles decreases with increasing contact angle of the seed particles.

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PubMed2005

Morphological dynamics of swelling-induced surface patterns in metal-capped polymer bilayer.

We report on the morphological dynamics of surface patterns induced by swelling of metal-capped polymer bilayer on a substrate. When the bilayer is subject to solvent vapor, the strain is generated in the polymer layer that is confined by the substrate and the metal capping layer. An increase in the strain induces the development of the stress in the bilayer to deform the lower polymer layer perpendicularly to the surface of the bilayer. Isotropic surface wave patterns results from the stress relaxation, the wave number of the patterns shows a characteristic temporal dependency on the swelling time, such that km(t) approximately t(-1/8). This temporal evolution accompanied by the morphological dynamics gives smaller value of the growth rate of the characteristic wavelength than that of the case of swelling of gel.

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PubMed2005

Wrinkling of a sol-gel-derived thin film.

We report on the wrinkle formation in a thin film produced by the sol-gel method. Through the relaxation of stress, which results from the removal of the solvent during the drying process, an isotropic wavy pattern is generated in the form of skeletal branches. The patterns have a dominant wavelength satisfying a relationship of three-fourths order of thickness. Densification of the gelated film is enhanced by an increase in the volumetric strain caused by the evaporation of the remaining solvent from the film. The number of skeletal branches and surface roughness increases as the annealing time progresses, without any change in the skeletal wavelength.

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PubMed2004

Solvent-driven dewetting and rim instability.

An experimental method suitable for reproducible results has been used to investigate dewetting behavior of thin films of solvent-laden polymer. This solvent-driven dewetting enables one to change spreading coefficient by an order of magnitude that is not readily realizable in thermal dewetting and to study polar interactions that have not been fully exploited experimentally. While the film instability is similar to that found in thermal dewetting, the rim instability is quite different. Two different types of the rim instability have been found. With a polar solvent, the rim instability changes from one type to another with increasing film thickness whereas the unstable rim becomes stable for an apolar solvent.

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