Ki-Taek Byun

A Model of Laser-Induced Cavitation

Evaporation and plasma formation due to laser irradiation and subsequent expansion of the evaporated volume accompanying shock emission were studied theoretically as a phenomenon of laser-induced thermal breakdown rather than an optic breakdown. In recent years, this problem has become particularly important in the field of laser-mediated surgery. The Gilmore model of bubble dynamics based on the Kirkwood–Bethe hypothesis with proper initial conditions was used to calculate the shock pressure. This pressure depends on the radius and expansion velocity of the plasma formed by the laser irradiation. Subsequent bubble evolution after shock departure including the light emission at the collapse point was studied using the Keller–Miksis equation for the bubble wall motion and analytical solutions for the vapor inside the bubble. Calculation results show that the maximum size of the bubble and the half-width of the emitted power of the light pulse at the collapse point depend on the initial bubble radius and the bubble wall velocity. The light emission from the laser-induced bubble at the collapse point turns out to be black body radiation.

Preparation of Li4Ti5O12 nanoparticles by a simple sonochemical method

Homogeneous Li4Ti5O12 nanoparticles were prepared: First, LiOH was coated onto TiO2 nanoparticles via a sonochemical method, operated at 20 kHz and 220 W for 20 min. These particles have about 2–5 nm coating depth of LiOH on the TiO2 nanoparticle surface. Second, the resulting nanoparticles were thermally treated at 500 °C for 1 h, resulting in the formation of Li4Ti5O12 (LTO) nanoparticles. These LTO nanoparticles had an average grain size of about 30–40 nm with excellent phase purity in good stoichiometric ratios of Li4Ti5O12. These nanoparticles with a uniform size distribution were characterized by infrared spectroscopy, X-ray diffraction, and high resolution-transmission electron microscopy analyses.

Characteristics of Sonoluminescing Bubbles in Aqueous Solutions of Sulfuric Acid

A new paradigm of sonoluminescence phenomena which displays alternating pattern of on/off luminescence pulse was observed recently in aqueous sulfuric acid solutions. A set of solutions of the Navier–Stokes equation for the gas inside a spherical bubble with heat transfer through the bubble wall permits to predict correctly characteristics of the sonoluminescing phenomena in the solutions. Calculation results of the minimum velocity of bubble wall, the peak temperature and pressure are in excellent agreement with the observed ones. Further, the calculated bubble radius–time curve displays alternating pattern of bubble motion as observed in experiment. The origin of sonoluminescence from gas bubble in sulfuric acid turns out to be blackbody emission with finite absorption.

Bubble Evolution and Radiation Mechanism for Laser-Induced Collapsing Bubble in Water

Bubble behavior and the radiation mechanism from a laser-induced collapsing bubble were investigated theoretically using the Keller–Miksis equation for the bubble wall motion and analytical solutions for the vapor inside the bubble. The calculated time-dependent bubble radius is in good agreement with observed ones. Also, the calculated half width and the total emission of the luminescence pulse from the vapor bubble at the collapse point agreed well with the observed value of 8 nanoseconds and 2.3 mW, respectively. The inert gas content inside the laser-induced vapor bubble was too small to produce light emission due to bremsstrahlung.

The Mesoscopic Simulation on the Structures of the Surfactant Solution Using Dissipative Particle Dynamics

With a simple model of surfactant which consists of hydrophilic head group and hydrophobic tail groups connected by harmonic springs, structural change of the association structures of surfactant in an aqueous solution was studied using the dissipative particle dynamics (DPD) simulation. The effect of the hydrophilic interaction between the head and water molecules and the hydrophobic interaction between the tail and water molecules and the head and tail on the structural change of the association structures was studied. Simulations show that proper value of these interaction parameters could yield desirable change of the association structure depending on the concentration of the surfactant. For example, a hexagonal structure appears when the volume fraction of surfactant of SDS (sodium dodecyl sulfate) becomes 25% in aqueous solution, which is in good agreement with observation.Copyright

Proteinaceous Bubbles and Nano Particle Flows in Microchannel

Proteinaceous bubbles of 185 nm in average diameter were synthesized by a sonochemical treatment of bovine serum albumin in aqueous solution and the nanoparticles (TiO2 ) solution was made by ultrasonic irradiation. To study the macroscopic flow behavior associated with the changes in the state of microparticles, a flow test of these solutions in microchannels was done. Also the size distributions of the proteinaceous bubbles in solution before and after the flow test were measured by a light scattering method. Test results show that the air-filled proteinaceous bubbles in solution adjust their size to reduce the shear stress encountered in the flow through the microchannel. On the other hand, the flow rate of the solution with nanoparticles suspensions becomes smaller than that of deionized water above the flow rate of 6 cm3 /min in the microchannel with a dimension of 100×150 μm2 .Copyright