Papers by Akinori Fukushima
Bulletin of the American Physical Society, Mar 18, 2013
ku, Japan-Molecular dynamics simulation was conducted in order to investigate the vapor-liquid co... more ku, Japan-Molecular dynamics simulation was conducted in order to investigate the vapor-liquid coexistence of the water molecules in nanopore. In this research, the Lennard-Jones energy parameter between a water molecule and an atom of nanopore was optimized so as to model the contact angle between a water droplet and the carbon material in the fuel cell. The TIP4P/2005 as the model of a water molecule was used; this model produces well the vapor-liquid coexistence line. All of the systems were equilibrated by Nosé-Hoover thermostat. The electrostatic interaction between water molecules was calculated through smooth particle mesh Ewald method. First, we equilibrated a water plug in the single-wall atomistic nanocylinder as a model of nanopore in the fuel cell with radius 1.3nm. Water molecules burst from an interface of the water plug in equilibration. Then, the equilibrium densities both in dense and dilute region ware sampled over 1 ns. The vapor-liquid coexistence line, density profile, free energy profile will be presented in the session.
ECS transactions, Aug 31, 2013
Research Square (Research Square), Jun 16, 2022
We calculate the free energies of adsorption between a solid wall and various arginine derivative... more We calculate the free energies of adsorption between a solid wall and various arginine derivatives in water using molecular dynamics simulations. We vary the hydrophobic group size of the amino acids in two ways and the hydrophilicity of the solid wall in ve ways. The free energy of adsorption decreases by increasing the hydrophilicity of the solid wall, regardless of the size of the hydrophobic group. Amino acids with small hydrophobic groups are adsorbed on the solid wall with the strongest hydrophilicity, but the free energy of the amino acids with large hydrophobic groups is relatively small. The free energy of adsorption in benzene solvent is increased by increasing the hydrophilicity of the solid wall; however, analogous to results in water solvent, it is independent of the size of the hydrophobic group. We attribute these differences in free energy to the effect of solvent molecules localized on the solid wall. Thus, these results may modulate peptide adsorption on a solid surface by the size of the peptide's hydrophobic groups.
Proceedings. International Conference on Flow Dynamics (CD-ROM), 2019
Journal of Molecular Modeling, May 9, 2020
We carried out quantum chemical calculations to analyze the effects of fluorination on the activa... more We carried out quantum chemical calculations to analyze the effects of fluorination on the activation energy (Ea) of sulfonic group deprotonation by water molecules. The model molecule was 2,3,4,5,6-pentafluorobenzenesulfonic acid (5FBSA), which was obtained by substituting all aromatic hydrogen atoms of benzenesulfonic acid (BSA) for fluorine atoms. The target hydration level was three. Our analysis indicated that the Ea of deprotonation in 5FBSA was lower than that of BSA, suggesting that the cation of 5FBSA was stabilized. Previous studies have reported that fluorinated molecules have a lower Ea to deprotonation and a stabilized deprotonated state even at a hydration level of three. This effect is attributed to the strong electron withdrawing ability of fluorine. However, compared with non-aromatic molecules, the Ea of deprotonation of aromatic molecules is slightly higher, and the overall energy change (ΔE) is lower, even if the molecule is fluorinated.
HAL (Le Centre pour la Communication Scientifique Directe), Sep 7, 2015
Considering a nanometer-thick droplet shared by two solid walls, shear forces were evaluated by m... more Considering a nanometer-thick droplet shared by two solid walls, shear forces were evaluated by molecular dynamics simulation. Expressly, shear forces around the contact line regions have been focused and evaluated the velocity dependence of forces. As a result, it has been clarified that no difference is observed between the velocity dependence of force around two contact lines. Recently, due to the development in materials science, new functional materials can be used. These materials realize useful mechanical and energy devices that have small structures in nm-order. Important issues to determine the efficiency of these small devices, mass transport properties are greatly different from those in microscopic states. We have studied this phenomenon through a liquid-vapor interface in a water plug by molecular dynamics simulations [1]. In this study, we have focused on the shearing of a nanometer-thick droplet that can be seen between solids that do not have enough lubricants between them. When a channel size is in nm-order scale, a ratio of a liquid-vapor interface domain among a volume of a droplet is much larger than that of a macroscopic droplet. Therefore the momentum transport through a liquid-vapor interface highly affects the total momentum transport, and cannot be ignored. Thus we clarify the effects of the liquid-vapor interface in microscopic droplet. Figure 1 shows a micro channel model of this simulation. This model is composed of two parallel slabs that have a FCC (001) surface. Two solid walls move along y direction and in an opposite manner. The velocity of the upper wall was set at Vw, and the velocity of the lower wall was set at –Vw. Figure 2 shows a velocity dependence of a friction force on each region. At both liquid-vapor interface regions, the slope of the force against the wall velocity can be calculated as 4.4×10-13 N/(m/s). This means that there is no difference between the dynamics along the advancing direction and that along the receding direction in terms of the increasing rate of the force on the liquid-vapor interface. [1]T. Tokumasu, M. Meurisse, N. Fillot and P. Vergne, Tribology International Journal, 59 (2013), 10-16
Journal of Nanoscience and Nanotechnology, Apr 1, 2015
In this study, the channel size dependence of the shear stress between water droplets and solid w... more In this study, the channel size dependence of the shear stress between water droplets and solid walls in nm-order channel was analyzed. We considered a several different-sized and highly hydrophobic channel whose macroscopic contact angle was about 150 degrees. We have evaluated the shear stress and the normal pressure by molecular dynamics simulation. Analyzing shear stress and normal pressure based on the macroscopic model, we have discussed the difference between the macroscopic model based on hydrodynamics and the microscopic model. As a result, in the high hydrophobic case, it became clear that the shear stress depends on the channel size due to the large Laplace pressure. Furthermore, in the case that the channel size was less than 50 Å, the normal pressure by the molecular simulation didn't agree with the expected value from the Young-Laplace equation. From this study it was clear that molecular simulation is needed when the channel size is less than 40 Å.
ECS transactions, Mar 15, 2013
In this study, the transport property of a water droplet in the nano hydrophobic pore was clarifi... more In this study, the transport property of a water droplet in the nano hydrophobic pore was clarified by molecular dynamics simulation. The size dependence of the transport property of the water droplet in a nano slit pore was focused. Considering several models of the nano pore, the differences of the structures of water droplets were shown and these differences were discussed from the viewpoint of the interaction between hydrophobic walls. Furthermore, forces by the hydrophobic walls were evaluated. From these results, the dependence of the width of the pore and the size of the droplet were clarified. Moreover, the effect of the liquid-vapor interface was considered.
Journal of Chemical Physics, Jul 6, 2015
Articles you may be interested in Effect of bound state of water on hydronium ion mobility in hyd... more Articles you may be interested in Effect of bound state of water on hydronium ion mobility in hydrated Nafion using molecular dynamics simulations
Physical Review E, Oct 18, 2013
We derive the equation of motion for non-Markovian dissipative particle dynamics (NMDPD) by intro... more We derive the equation of motion for non-Markovian dissipative particle dynamics (NMDPD) by introducing the history effects on the time evolution of the system. Our formulation is based on the generalized Langevin equation, which describes the motions of the centers of mass of clusters comprising microscopic particles. The mean, friction, and fluctuating forces in the NMDPD model are directly constructed from an underlying molecular dynamics (MD) system without any scaling procedure. For the validation of our formulation, we construct NMDPD models from high-density Lennard-Jones systems, in which the typical time scales of the coarse-grained particle motions and the fluctuating forces are not fully separable. The NMDPD models reproduce the temperatures, diffusion coefficients, and viscosities of the corresponding MD systems more accurately than the dissipative particle dynamics models based on a Markovian approximation. Our results suggest that the NMDPD method is a promising alternative for simulating mesoscale flows where a Markovian approximation is not valid.
Research Square (Research Square), Feb 21, 2022
We calculated the free energies of adsorption between a solid wall and various arginine derivativ... more We calculated the free energies of adsorption between a solid wall and various arginine derivatives in water using molecular dynamics simulations. We varied the hydrophobic group size of the amino acids in two ways and the hydrophilicity of the solid wall in five ways. The free energy of adsorption decreased by increasing the hydrophilicity of the solid wall, regardless of the size of the hydrophobic group. Amino acids with small hydrophobic groups adsorbed on the solid wall with the strongest hydrophilicity, but the free energy of the amino acids with large hydrophobic groups was relatively small. The free energy of adsorption in benzene solvent increased by increasing hydrophilicity of the solid wall, yet analogously to results in water solvent was independent of the size of the hydrophobic group. We attribute these differences in free energy to the effect of solvent molecules localized on the solid wall. Thus, these results may modulate peptide adsorption on a solid surface by the size of the peptide’s hydrophobic groups.
Hokuriku Shin'etsu Shibu Sokai, koenkai koen ronbunshu, 2018
Maikuro, Nano Kogaku Shinpojiumu, 2011
Journal of Computer Chemistry, Japan, 2013
Computer simulation is a very powerful tool to analyze transport phenomena in the membrane electr... more Computer simulation is a very powerful tool to analyze transport phenomena in the membrane electrode assembly (MEA) of polymer electrolyte fuel cells (PEFCs). In particular, there are many nanoscale structures in this flow field, and therefore the phenomena should be analyzed from the microscopic point of view rather than computational fluid dynamics. In this paper, we report large-scale molecular dynamics (MD) simulations to analyze these flows. In particular, dissociation phenomena of a hydrogen molecule on a Pt catalyst, transport phenomena of proton and water in a polymer electrolyte membrane (PEM), oxygen permeability of ionomers in a catalyst layer (CL), and transport phenomena of a water droplet in a nanopore were simulated, and their characteristics are discussed. In the analysis of the dissociation phenomena of the hydrogen molecule, it was found that the trend of dissociation probability as a function of impinging energy considering the motion of the molecule differs from that without considering the motion of the molecule. In the analysis of proton transfer in a PEM, the diffusion coefficients obtained by this simulation were consistent with the experimental data. In the analysis of oxygen permeability of ionomers, the dependence of water content on the permeability was estimated and the difference between ionomer on catalyst layer and that in bulk state was clarified. In the analysis of transport phenomena of a water droplet in a nanopore, we compared the results of our simulation with the macroscopic governing equation.
Composites Part A-applied Science and Manufacturing, Feb 1, 2023
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Papers by Akinori Fukushima