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In order to examine the tribological behaviour of the polymer brush, we constructed a mesoscale particle model of a polymer brush system, based on a Brownian dynamics scheme. The polymer model consists of coarse-grained beads connected... more
In order to examine the tribological behaviour of the polymer brush, we constructed a mesoscale particle model of a polymer brush system, based on a Brownian dynamics scheme. The polymer model consists of coarse-grained beads connected with harmonic springs. The Lennard-Jones type interaction is assumed between beads. The flow velocity is affected by the local packing fraction of beads. With this model, we executed a series of molecular dynamics simulation to investigate the mechanical responses of brush to shear flow.
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Research Interests: Materials Engineering, Condensed Matter Physics, Chemistry, Computational Materials Science, Molecular Dynamics Simulation, and 12 moreMolecular Dynamics, Computer Simulation, Cavitation, Nucleation, Bubble, Phase Change, Large Scale, MD Simulation, Molecular Dynamic Simulation, Homogeneous, Supersaturation, and Molecular dynamic
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Analysis of phonon dynamics based on a linearized Boltzmann transport equation is widely used for thermal analysis of solid thin films, but couplings among various phonon modes appear in some situations. We propose a direct simulation... more
Analysis of phonon dynamics based on a linearized Boltzmann transport equation is widely used for thermal analysis of solid thin films, but couplings among various phonon modes appear in some situations. We propose a direct simulation Monte Carlo (DSMC) scheme to simulate the phonon gas starting without the conventional linearization approximation. This requires no relaxation time as an input parameter, and we can investigate the couplings among phonons with different modes. A prototype code based on a simple phonon model was developed, and energy flux was evaluated for thin films of various thickness as a test calculation.
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Molecular dynamics computer simulation was carried out for three Lennard-Jones systems to investigate the dynamics of vapor phase homogeneous nucleation under super saturation ratio 6.6, 6.8, and 7.1. The observed nucleation rates are... more
Molecular dynamics computer simulation was carried out for three Lennard-Jones systems to investigate the dynamics of vapor phase homogeneous nucleation under super saturation ratio 6.6, 6.8, and 7.1. The observed nucleation rates are seven orders of magnitude larger than prediction of a classical nucleation theory.
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We have developed hybrid numerical simulation codes to investigate the dynamics of nanobubbles. The idea is based on a combination of a molecular dynamics (MD) technique and a continuum dynamics with the CIP method. The MD technique... more
We have developed hybrid numerical simulation codes to investigate the dynamics of nanobubbles. The idea is based on a combination of a molecular dynamics (MD) technique and a continuum dynamics with the CIP method. The MD technique enables us to examine rapid change of the bubble surface and the inside region of nanobubbles at molecular scale. The CIP method enables us to trace the mass and energy transfer processes far from the bubble surface at continuum scale. In the hybrid simulation, the simulation cell is divided into two parts. The inner region containing a bubble (or bubbles) consists of sufficiently large number of particles and is treated with the MD method. The outer region is treated with a computational fluid dynamics (CFD) scheme. The boundary between the inner and outer regions is movable and driven with the pressure difference between the two regions. To deal with the moving boundary, we adopt the CIP scheme. Two different codes have been developed, i.e., one-dimensional CFD with assuming a spherical symmetry, and full three-dimensional CFD. Examples will be given in the paper to demonstrate each code. The first example is the oscillating dynamics of a spherical bubble with one-dimensional CFD. The bubble is initially located at the center of the MD region with no translational motion. The outer (continuum) region is treated one dimensionally, with the assumption of spherical symmetry. Under the equilibrium condition, where there is no pressure difference between the two regions, we give a sudden pressure increase in the continuum region far from the bubble. The spherical pressure wave propagates through the continuum region, and the pressure difference drives the boundary to shrink the MD region. As the MD region shrinks, the bubble inside the region starts to collapse. The collapsed bubble bounces back. We have analyzed the oscillation dynamics under several different conditions, such as different initial pressures and the state of gas inside the bubble. The second one is the deformation of a non-spherical bubble. For that purpose, the continuum region is treated with full three-dimensional CIP scheme. Furthermore, we use the level set method in order to capture the interfaces (boundary) between the MD region and the continuum region.
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Research Interests: Chemistry, Carbon Dioxide, Molecular Dynamics, Molecular Simulation, Methanol, and 15 morePhysical sciences, Solubility, CHEMICAL SCIENCES, Supercritical carbon dioxide, Mixture Model, Neutron Scattering, Coordination number, Radial Distribution Function, Small Angle Neutron Scattering, Critical Point, Supercritical Fluid, Solvent, Thermodynamic Properties, Experimental Analysis, and Chemical Potential
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Research Interests: Chemical Engineering, Thermodynamics, Chemistry, Evaporation, Molecular Dynamics, and 13 moreAdsorption, Computer Simulation, Classical Physics, Molecular Simulation, Phase Change, Desorption, Temperature Dependence, Group, Fluid phase equilibria, Condensation, Molecular Dynamic Simulation, Capillary Condensation, and Molecular dynamic
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Abstract Surfaces with a wettability gradient, which require no external energy input, play a significant role in controlling and manipulating the self-transport of liquid drops. This work numerically investigates the directional rebound... more
Abstract Surfaces with a wettability gradient, which require no external energy input, play a significant role in controlling and manipulating the self-transport of liquid drops. This work numerically investigates the directional rebound behavior of water droplets on surfaces with wettability gradients obtained by gradually altering the groove width. The results show that three types of droplet rebound behavior (vertical bouncing, following, or against the roughness gradient) and the rotating motion (counterclockwise), which are dominated by the combined effects of the unbalanced Young's force and the wetting state, are affected by the Weber number (We), groove width, and groove depth. Droplets remain in the Cassie state and rebound following the roughness gradient for a small We, small groove width, and larger groove depth. By contrast, when the Cassie and Wenzel states are both present, droplets are partially arrested by the ridges and grooves and rebound against the roughness gradient. Additionally, vertical rebounding behaviors are observed at extremely small W e owing to the low contact-angle hysteresis, and at critical W e because the Young's force and capillary emptying process are balanced. Therefore, both the unbalanced Young's force and wetting state should be considered when investigating the bounce behavior of droplets impinging surfaces with a roughness gradient. The results provide important insight into the design of wettability gradient surfaces for controlling droplet transport.
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Abstract Thermoelectric devices are devices that allow direct conversion between thermal and electric energy. That is, a voltage is generated when there is a temperature gradient between two different points on the device, and,... more
Abstract Thermoelectric devices are devices that allow direct conversion between thermal and electric energy. That is, a voltage is generated when there is a temperature gradient between two different points on the device, and, conversely, given a voltage difference between two different points, a temperature gradient is generated. The miniaturization of thermoelectric devices has driven the need to improve the heat conduction performance in these devices. In our research, we focus on the level set-based topology optimization of heat conduction at the nanoscale, to obtain improved designs for thermoelectric devices. Macroscale and nanoscale heat conduction phenomena are quite different. According to Fourier’s law, which is used when analyzing macroscale heat conduction, heat flux is proportional to the temperature gradient of a system and the heat conduction is an isotropic conduction. On the other hand, nanoscale heat conduction is ballistic, and analysis of this type of heat conduction is required to obtain improved designs for thermoelectric devices that depend on nanoscale phenomena. In 1993, A. A. Joshi and A. Majumder pioneered the use of the Boltzmann transport equation to numerically analyze nanoscale heat conduction. However, until now, high performance thermoelectric devices have been designed through trial and error, and research on design methods for constructing thermoelectric devices focused on nanoscale behavior has been scarce. Topology optimization, which can dramatically change both structural shape profiles and the number of holes in a structure during optimization procedures, is widely used in many structural design problems. A level set-based topology optimization method, proposed by Yamada et al [1], has been applied to a variety of optimization problems involving different physical phenomena. In this design method, the iso-surface of a scalar function, the level set function, represents structural boundaries and structural profiles are optimized by updating the level set function, based on design sensitivity information. This method therefore always provides optimal configurations that have clear boundaries. When dealing with design problems at the nanoscale, scattering effects and the penetration of phonons at the material boundaries must be considered. We extend a level set-based topology optimization method to enable accurate analysis of nanoscale heat conduction in a structural design problem. For this extension, we derive the topological derivative as the design sensitivity. This derivative can precisely evaluate differences in the value of the objective functional when a hole is generated, i.e., when a new boundary is generated, during the optimization procedure. In this manner, we obtain optimal configurations that include consideration of scattering effects and the penetration of phonons, based on the topological derivative. In this work, we present the following four items: (1) design requirements and an objective functional for the optimization problems; (2) derivation of the topological derivative as the design sensitivity; (3) development of the optimization algorithm for the design problem; and (4) numerical examples of topology optimization for a thin film structure of a thermoelectric device, to confirm the effectiveness and utility of the proposed method. References [1] Yamada, Takayuki, et al. "A topology optimization method based on the level set method incorporating a fictitious interface energy." Computer Methods in Applied Mechanics and Engineering 199.45 (2010): 2876-2891.
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Molecular dynamics (MD) simulations were executed to investigate the molecular mechanism of nucleation around a seed in Lennard-Jones vapor. It was found that the seed-induced nucleation takes place at a supersaturation ratio that is too... more
Molecular dynamics (MD) simulations were executed to investigate the molecular mechanism of nucleation around a seed in Lennard-Jones vapor. It was found that the seed-induced nucleation takes place at a supersaturation ratio that is too low to cause homogeneous nucleation. At low supersaturation ratio, no clusters larger than a certain size (15-20) appear around the seed. We also executed grand canonical Monte Carlo (GCMC) simulations to estimate the cluster formation free energy under the same conditions. The free energy curves estimated from GCMC simulations are consistent with the results of the MD simulations. .
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Molecular dynamics computer simulation of a water system was carried out to investigate the dynamics of vapor phase homogeneous nucleation at 350 K under super saturation ratio 14.6. To control the system temperature, 5000 target... more
Molecular dynamics computer simulation of a water system was carried out to investigate the dynamics of vapor phase homogeneous nucleation at 350 K under super saturation ratio 14.6. To control the system temperature, 5000 target particles were mixed with 5000 carrier gas particles. The observed nucleation rate is three orders of magnitude smaller than prediction of a classical nucleation theory.
Research Interests: Chemical Engineering, Thermodynamics, Chemistry, Water, Molecular Dynamics, and 14 moreComputer Simulation, Classical Physics, Molecular Simulation, Nucleation, Free Energy, Steady state, Application, Size Distribution, Fluid phase equilibria, Water Vapor, Molecular Dynamic Simulation, Homogeneous, Supersaturation, and Molecular dynamic
The liquid–vapor nucleation phenomenon is important for understanding the initial stage of a process such as cavitation, boiling and phreatic explosion. The recent development in molecular simulation has enabled us to study the bubble... more
The liquid–vapor nucleation phenomenon is important for understanding the initial stage of a process such as cavitation, boiling and phreatic explosion. The recent development in molecular simulation has enabled us to study the bubble nucleation phenomena from a molecular point of view. The molecular dynamics simulations of the homogeneous liquid–vapor nucleation for a Lennard-Jones fluid were carried out to microscopically