Seungho Park

A molecular dynamics study on surface tension of microbubbles

Abstract This work is the first molecular dynamics (MD) study on the surface tension of bubbles and their related characteristics. Compared to thin films or droplets, bubbles have not been investigated using the MD simulation method for their properties owing to their inherent difficulties. To confirm their existences, a stable bubble regime with respect to simulation domain sizes is defined for the Lennard–Jones molecules. As well as the local densities, normal and tangential pressure components are calculated and used for the estimation of bubble surface tensions. While the surface tension of droplets varies as predicted by Tolmans equation, that of bubbles changes slightly and is greater than the value for the planar interface by 15% or less. In addition, effects of solute molecules on the surface tension of bubbles in a binary molecule system are investigated for the cases of less and more attractive interactions between solute and solvent molecules.

NUMERICAL ANALYSIS ON HEAT TRANSFER CHARACTERISTICS OF A SILICON FILM IRRADIATED BY PICO-TO FEMTOSECOND PULSE LASERS

This work aims to investigate the heat transfer characteristics of a silicon microstructure irradiated by picosecond-to-femtosecond ultrashort laser pulses from a microscopic point of view. Carrier-lattice nonequilibrium interactions are simulated with a set of governing equations for the carrier and lattice temperatures to obtain the time evolutions of the lattice temperatures, the carrier number densities, and carrier temperatures. In particular, the relaxation time for approaching the thermal equilibrium between carriers and lattices is introduced to estimate duration of the nonequilibrium state. An appropriate regime map to make a distinction between one-peak and two-peak structures is also established for picosecond laser pulses. It is noted that a substantial increase in carrier temperature is observed for pulse lasers of a few picoseconds duration, whereas the lattice temperature rise is relatively small with decreasing laser pulse widths. It is also found that the laser fluence significantly affects the N 3 decaying rate of the Auger recombination, the carrier temperature distribution exhibits two peaks as a function of time due to the Auger heating as well as the direct laser heating of carriers, and finally, both laser fluence and pulse width play an important role in controlling the nonequilibrium between carriers and lattices.

Cavitation and bubble nucleation using molecular dynamics simulation

This article reports the first systematic study on cavitation and bubble nucleation using the molecular dynamics simulation method. It successfully simulates the hysteretic process of bubble collapse and nucleation as a numerical counterpart of the Berthelot tube cavitation experiment. For a unary molecule system, a stable bubble regime and minimum equimolar dividing radii of bubbles are obtained with respect to computational domain sizes. For a binary molecule system, the addition of foreign molecules to the solvent molecules stimulates the nucleation more effectively in comparison to that in the unary system. The affinity between the solute and the solvent molecules controls the inception of nucleation and results in different nucleation characteristics according to its value. For an attraction coefficient greater than unity, the solute molecules spread uniformly and attract the solvent molecules, which induces bubble nucleation readily. For the coefficient less than unity, the solvent molecules segrega...This article reports the first systematic study on cavitation and bubble nucleation using the molecular dynamics simulation method. It successfully simulates the hysteretic process of bubble collapse and nucleation as a numerical counterpart of the Berthelot tube cavitation experiment. For a unary molecule system, a stable bubble regime and minimum equimolar dividing radii of bubbles are obtained with respect to computational domain sizes. For a binary molecule system, the addition of foreign molecules to the solvent molecules stimulates the nucleation more effectively in comparison to that in the unary system. The affinity between the solute and the solvent molecules controls the inception of nucleation and results in different nucleation characteristics according to its value. For an attraction coefficient greater than unity, the solute molecules spread uniformly and attract the solvent molecules, which induces bubble nucleation readily. For the coefficient less than unity, the solvent molecules segregate themselves from the solvent molecules, which results in a void shell between the solute and the solvent molecules.

Supergrains produced by lateral growth using Joule-heating induced crystallization without artificial control

In Joule-heating induced crystallization, phase transformation can occur through solid-to-solid or liquid-to-solid phases, according to the input conditions of the pulsed power. It was observed that during a Joule-heating period of several tens of microseconds, randomly nucleated liquid seeds followed by rapid solidification in an amorphous matrix play an important role, especially for liquid-to-solid transformation. Meanwhile, under high-power input processing conditions, supergrains of greater than 5 μm in size were produced by lateral growth from the initial seeds without artificial control.

Experimental and molecular dynamics study on crystallization of amorphous silicon under external fields

Solid-phase crystallization (SPC) of amorphous silicon (a-Si) under an external force field is investigated experimentally and numerically. Experimental results show that the kinetics of crystallization can be greatly enhanced by applying induction fields without the heating problems of a-Si film and its substrate, since temperature rises during the crystallization process are negligibly small. To explore the underlying acceleration mechanisms for the SPC process under the external fields, molecular dynamics simulations are carried out using the Tersoff potential. The numerical amorphous structure is obtained by the liquid quenching method and is utilized to simulate the crystallization processes at various process temperatures with and without external force fields. While homogeneous crystallization of a-Si could not be achieved readily, it is shown that the heterogeneous crystallization can be significantly accelerated by external force fields. This enhancement is due to increased molecular jumping frequencies associated with the molecular potential energies being increased by external excitations, rather than due to thermal mechanisms dominant in conventional SPC processes.

Behaviour of water molecules in Nafion 117 for polymer electrolyte membrane fuel cell by molecular dynamics simulation

Proton exchange membranes play a critical role as electrolytes for proton transports in polymer electrolyte membrane fuel cells. A membrane, such as Nafion 117, consists of a polytetrafluoroethylene backbone and side chains that terminate with sulfonate groups ( ). During operation of fuel cells, membranes become preferentially hydrated by absorbing water needed for effective proton conduction. Water management and movement, therefore, are extremely important for the efficient operation of the fuel cells. In this paper, we set up the molecular models for hydrated Nafion 117 and perform molecular simulations for various temperatures and monomer numbers to analyse the motion of water and hydronium molecules. Diffusion coefficients estimated from the mean-square displacements agree well with the experimental estimation. The distribution and structure of water molecules in Nafion 117 are analysed using radial distribution functions and Voronoi tessellation. The result shows that the distribution of water molecules in the Nafion membrane is quite close to that of hexagonal ices but quite deviated from that of pure water molecules.

INTERFACIAL AMBIGUITIES IN MICRODROPLETS AND MICROBUBBLES

In the past two centuries , interfacial phenomena have been a subject of ( ) considerable research interest. The liquid-vapor including liquid-gas interface of microdrople ts and microbubble s is of particular importance in many industrial applications. One article in this issue introduces some applications of phase-change w x phenomena in a microsystem 1 . It emphasizes the actuation mechanism controlled by the microbubble formation in a metastable liquid. This mechanism is strongly dependent on interfacial phenomena through the macroscopic parameters such as superheat limit and nucleation rate. Interfacial phenomena undergo some major changes from the macroscale regime to the micro r nano-scale regime, which causes some well-defined macroscopic parameters to lose their physical significance. In the case of homogeneous nucleation, the radius of a critical embryo is usually 1 ; 10 nm, and the thickness ( ) of the density transition layer from one phase to the other interface thickness is w x about 1 nm 2 . The embryo shape usually has significant deviation from a sphere. ( ) ( ) Thus, the embryo size or radius is no longer well-defined i.e., size ambiguity . Similarly, if the whole embryo is under the influence of tensile stress, surface ( ) tension is no longer a surface property, its action location the surface of tension ( cannot be easily specified, and its magnitude differs from the bulk value. i.e., ) surface tension ambiguity . Most classical approaches have difficulties in dealing with nanoscale embryos w x 3 . They, including Gibbs thermodynamic theory and density functional theory ( ) DFT , either assume the core of the embryo to be at the bulk phase , or the embryo shape to be perfectly spherical, both of which may not be valid for ( ) nanoscale embryos. Molecular dynamics MD simulation does not have these assumptions , but its accuracy is limited by computational power. This perspective brings forth some ambiguous issues of size and surface tension and discusses their practical implication.

A Molecular Dynamics Study on Stability and Thermophysical Properties of Nanoscale Liquid Threads

The thermophysical properties and stability characteristics of liquid threads are investigated by molecular dynamics (MD) simulations. Density and pressure profiles are obtained, and properties such as equimolar dividing radius, radius of surface of tension, and surface tension are determined. The relationship between the surface tension and thread radius based on the Gibbs theory of capillarity is derived for the liquid thread, and compared with MD simulation results. It shows that the surface tension rapidly deceases as the radius approaches the molecular scale. The surface tension of a simple binary mixture is also considered. The surface tension relative to that of one-component fluid decreases with increasing the binary ratio of the solute with a strong affinity. For a given binary ratio, the surface tension increases gradually as the affinity coefficient reaches 1.6 and decreases rapidly as it gets larger. The critical wavelength of perturbation for the instability is shown to be smaller than is predicted by the classical theory.

Thermopower profiling of a silicon p-n junction

An ac type thermopower measurement technique was suggested and demonstrated with a simple experimental setup. The thermopower distribution across a silicon p-n junction was measured point by point at every 10nm, so that it was free from the noise due to the built-in potential and photoionization effects, and it was compared with the theoretical result. Although this ac type thermopower measurement technique could not follow the sharp variation of the theoretical thermopower near the p-n junction, it could identify a smooth peak of the thermopower distribution in the depletion layer of the p-n junction.

Molecular Dynamics Simulation of Elastic Properties of Silicon Nanocantilevers

The molecular dynamics simulation of nanoscale cantilevers made of pure crystalline silicon with different lattice conditions is presented. Youngs moduli for various sized specimen is obtained by simulating clamped-free cantilever beam vibrations and static tensile responses. Youngs modulus decreases monotonically as the thickness of the specimen decreases. Although significant discrepancies exist between the simulated and experimentally determined Youngs modulus, incorporating a minute amount of voids in the specimen during simulation offers a partial account of this discrepancy. The dependence of the Youngs modulus on dimensional scaling is then applied to estimate thermal fluctuations of the cantilever under various temperatures, sizes, and lattice conditions and shows excellent agreement with the theoretical estimate based on the equipartition theorem. Finally, the applicability of the nanocantilevers as molecular mass sensors is demonstrated by simulating the change in the first flexural mode frequency as the number of silicon molecules placed at the tip of the cantilever is varied. The results show good agreement with the theoretical predictions of the Euler-Bernoulli beam vibration model.