arXiv subjects

Hitoshi Washizu

Publications and source records attributed to Hitoshi Washizu.

15 recordsLinked to original sources

arXiv2025

First passage time properties of diffusion with a broad class of stochastic diffusion coefficients

This study investigates the first passage time (FPT) properties of particles with a broad class of positive stochastic diffusion coefficients (DCs), representing diffusion in heterogeneous environments or of particles with conformational fluctuations. We demonstrate that for diffusion in a one-dimensional semi-infinite domain with an absorbing boundary, particles will eventually reach the absorbing boundary with probability one. We also show that a stochastic DC provides higher transport efficiency in an early arrival of particles at the boundary than would be expected under diffusion whose DC is the ensemble average of the stochastic DC. Furthermore, a stochastic DC with a larger supremum exhibits a more efficient transport even if ensemble averages are the same. For ergodic DCs, we show three more properties: the mean FPT diverges, the enhancement of early-arrival efficiency diminishes over long times, and the FPT distribution converges to a L\'evy-Smirnov distribution in the long-time limit. These properties are shown to arise from the convergence of the time-averaged DC to the ensemble average, with the convergence speed determined by the DC's fluctuation time scale. We finally discuss the similarities and differences of FPT properties between three-dimensional diffusion outside a spherical absorbing boundary and the one-dimensional diffusion. Our results indicate that fluctuations in DCs may need to be non-Markov and/or non-ergodic to allow efficient transport of particles to distant targets. Our results also suggest that fluctuations in a DC play an important role, for example, in diffusion-limited reactions triggered by single molecules in physics, chemistry, or biology.

cond-mat.stat-mech↗

arXiv2024

Atomistic analysis of nematic phase transition in 4-cyano-4$^{\prime}$-$n$-alkyl biphenyl liquid crystals: Sampling for the first-order phase transition and the free-energy decomposition

Molecular dynamics (MD) simulations were conducted using the generalized replica exchange method (gREM) on the 4-cyano-4$^{\prime}$-$n$-alkylbiphenyl ($n$CB) system with $n=5$, 6, 7, and 8, which exhibits a nematic-isotropic (NI) phase transition. Sampling near the phase transition temperature in systems undergoing first-order phase transitions, such as the NI phase transition, is demanding due to the substantial energy gap between the two phases. To address this, gREM, specifically designed for first-order phase transitions, was utilized to enhance sampling near the NI phase transition temperature. Free-energy calculations based on the energy representation (ER) theory were employed to characterize the NI phase transition. ER evaluates the insertion free energy of $n$CB molecule for both nematic and isotropic phases, revealing a change in the temperature dependence across the NI phase transition. Further decomposition into intermolecular interaction energetic and entropic terms shows quantitatively the balance between these contributions at the NI phase transition temperature.

cond-mat.soft↗

arXiv2023

Diffusion with a broad class of stochastic diffusion coefficients

In many physical or biological systems, diffusion can be described by Brownian motions with stochastic diffusion coefficients (DCs). In the present study, we investigate properties of the diffusion with a broad class of stochastic DCs with a novel approach. We show that for a finite time, the propagator is non-Gaussian and heavy-tailed. This means that when the mean square displacements are the same, for a finite time, some of the diffusing particles with stochastic DCs diffuse farther than the particles with deterministic DCs or exhibiting a fractional Brownian motion. We also show that when a stochastic DC is ergodic, the propagator converges to a Gaussian distribution in the long time limit. The speed of convergence is determined by the autocovariance function of the DC.

cond-mat.stat-mech↗

arXiv2021

Simulating the nematic-isotropic phase transition of liquid crystal model via generalized replica-exchange method

The nematic-isotropic (NI) phase transition of 4-cyano-4'-pentylbiphenyl (5CB) was simulated using the generalized replica-exchange method (gREM) based on molecular dynamics simulations. The effective temperature is introduced in gREM, allowing the enhanced sampling of configurations in the unstable region, which is intrinsic to the first-order phase transition. The sampling performance was analyzed with different system sizes and compared with that of the temperature replica-exchange method (tREM). It was observed that gREM is capable of sampling configurations at sufficient replica-exchange acceptance ratios even around the NI transition temperature. A bimodal distribution of the order parameter at the transition region was found, which is in agreement with the mean-field theory. In contrast, tREM is ineffective around the transition temperature owing to the potential energy gap between the nematic and isotropic phases.

cond-mat.soft↗

arXiv2019

Numerical simulation method for Brownian particles dispersed in incompressible fluids

We present a numerical scheme for simulating the dynamics of Brownian particles suspended in a fluid. The motion of the particles is tracked by the Langevin equation, whereas the host fluid flow is analyzed by using the lattice Boltzmann method. The friction force between a particle and the fluid is evaluated correctly based on the velocity difference at the position of the particle. The coupling method accurately reproduces the long-time tail observed in the velocity auto-correlation function. We also show that the fluctuation-dissipation relation holds between the relaxation of a single particle and the velocity autocorrelation function of fluctuating particles.

cond-mat.mes-hall↗

arXiv2018

Molecular dynamics analysis of adsorption process of anti-copper-corrosion additives to the copper surface

As a model system of the adsorption process of anti-corrosion additives on a metal surface, molecular dynamics simulations of benzotriazole (BTA) molecules with copper slabs were completed. As a force field, ReaxFF was used to simulate both adsorption dynamics and the charge transfer on the solid surface. Two simulations are presented. In order to investigate the physical adsorption on the surface, the simulation was done for the adsorption process of BTA molecules on the copper (II) oxide slab. BTA molecules formed on adsorbed layer in parallel to the surface, and aggregation of the molecules due to the surface diffusion was found. In order to investigate the selective and chemical adsorption on the surface, a hybrid slab, which has both a copper (Cu) area and a copper (I) oxide (Cu2O) area, was used. A selective adsorption phenomenon was found. The number of BTA molecules adsorbed on the Cu area is 5 times greater than that on the Cu2O area. Detailed dynamics focused on charge transfer showed a surface diffusion and enhancement of polarization due to charge transfer from the metal surface caused the selective adsorption. In a real phenomenon, the reason why a few anti-corrosion additives are able to protect a metal surface is postulated to be due to this selective nature of adsorption onto a newly formed metal surface.

physics.chem-ph↗

arXiv2018

Smoothed particle hydrodynamics study of friction of the coarse-grained $\alpha$-Al2O3/$\alpha$-Al2O3 and $\alpha$-Fe2O3/$\alpha$-Fe2O3 contacts in behavior of the spring interfacial potential

The paper uses the spring potential to present interaction between the coarse-grained interfacial particles of the $\alpha$-Al2O3/$\alpha$-Al2O3 and $\alpha$-Fe2O3/$\alpha$-Fe2O3 contacts in the sliding friction study of these micron-scale oxides by smoothed particle hydrodynamics simulations. The spring constants of the potential for the particle systems are converted from those of the atomic oxide systems that are yielded by the second order polynomial fits of the probed surface potentials in molecular dynamics simulations, and are dependent on the particle coarse-graining. It is founded that at micron-scale the friction properties of the oxides are almost independent of the coarse-graining and are the same in the different sliding directions. Even the hardness contacts friction coefficient shows a decrease with increasing intensity of normal component of the interfacial interaction, originating from stability of friction force and growth of normal force. This result is as an implementation for the previous observations of sliding friction of various materials that showed that a drop of friction coefficient with increasing externally applied normal load has originated from deformation of interfaces or occurrence of debris at contact, indicating an unsteady contact.

cond-mat.soft↗

arXiv2018

Friction of the micron-scale silica under various behaviors of the shape and the orientation of the coarse-grained particle in adaptive smoothed particle hydrodynamics

The paper investigates dry sliding friction of the coarse-grained micronscale {\alpha}-SiO2 oxide. Adaptive smoothed particle approach is used to consider various shapes and orientations of the particles. It is found that because of the stable system the friction characteristics almost do not depend on the shape and the orientation of the particle. The friction coefficient of 0.1376 observed in the present work is in accordance with that found in previously experimental reports. The friction coefficient steady maintains in the applied load range of 5-80 {\mu}N, showing a very slightly linear drop from 0.1379 to 0.1341 in this load range. This observation is also consistent with the applied load-friction coefficient relationship mentioned in previously experimental studies.

cond-mat.soft↗

arXiv2018

Understanding the effect of the base oil on the physical adsorption process of organic additives using molecular dynamics

Organic friction modifiers (OFMs) are widely added to oil to reduce the boundary friction in many kinds of lubricants such as vehicle engine oils. At the contact area in machine elements, the OFMs form a self-assembled organic monolayer. Although the friction properties of the monolayer are widely studied on a molecular level, the formation process is not well-known. In this study, the initial adsorbing process of additive molecules in explicit base oil molecules are calculated using molecular dynamics. The adsorption time depends on the structure of the base oils. Another effect of the base oil other than "chain matching" is found.

cond-mat.soft↗

arXiv2018

Friction and friction heat of micronscale iron

The paper investigates friction and friction heat of the micronscale iron under influences of velocity of the slider and temperature of the substrate by using smoothed particle hydrodynamics simulations. In the velocity range of 10 - 100 m/s, change of friction coefficient via velocity well complies with exponent or hyperbolic tangent form and friction coefficient begins to approach a stable value of 0.3 at around a velocity of 50 m/s after a rapidly increasing situation. Friction coefficient steady maintains over the temperature range of 200 - 400 K at each velocity of 10, 50 or 100 m/s. Friction heat is detailed analyzed via sliding time. Change of temperature of the system via sliding time well complies with sigmoidal functions, an exception of that of the particle layer directly causing friction. The layer causing friction has the highest steady temperature and its temperature rise is the largest one. The temperature rise is found to be dependent on increment of the initial temperatures of the substrate and the slider while the incnt does not efect on configration of the slid time - temperature curve

cond-mat.soft↗

arXiv2018

Coarse-grained model for spring friction study of micron-scale iron by smoothed particle hydrodynamics

The paper constructs a coarse-grained model to investigate dry sliding friction of the body-centered-cubic Fe micron-scale system by smoothed particle hydrodynamics simulations and examines influences of the spring force on the characters of friction. The N_atom = 864 \times 10^12 atoms Fe system is coarse-grained into the two different simple-cubic particle systems, one of 432000 and the other of 16000 particles. From the detection of stick-slip motion, friction coefficient, dependence of friction coefficient on isotropy or anisotropy of the spring force and externally applied normal load, we find that the coarse-grained model is a reasonable modeling process for study of friction of the Fe system and the anisotropic behavior presents better friction of the system than the isotropic one.

cond-mat.soft↗

arXiv2017

Molecular Origin of Limiting Shear Stress of Elastohydrodynamic Lubrication Oil Film Studied by Molecular Dynamics

All-atom molecular dynamics simulations of an elastohydrodynamic lubrication oil film are performed to study the effect of pressure. Fluid molecules of n-hexane are confined between two solid plates under a constant normal force of 0.1--8.0 GPa. Traction simulations are performed by applying relative sliding motion to the solid plates. A transition in the traction behavior is observed around 0.5--2.0 GPa, which corresponds to the viscoelastic region to the plastic--elastic region, which are experimentally observed. This phase transition is related to the suppression of the fluctuation in molecular motion.

cond-mat.soft↗

arXiv2016

Analysis of electro-osmotic flow in a microchannel with undulated surfaces

The electro-osmotic flow through a channel between two undulated surfaces induced by an external electric field is investigated. The gap of the channel is very small and comparable to the thickness of the electrical double layers. A lattice Boltzmann simulation is carried out on the model consisting of the Poisson equation for electrical potential, the Nernst--Planck equation for ion concentration, and the Navier--Stokes {\color{black}equations} for flows of the electrolyte solution. An analytical model that predicts the flow rate is also derived under the assumption that the channel width is very small compared with the characteristic length of the variation along the channel. The analytical results are compared with the numerical results obtained by using the lattice Boltzmann method. In the case of a constant surface charge density along the channel, the variation of the channel width reduces the electro-osmotic flow, and the flow rate is smaller than that of a straight channel. In the case of a surface charge density distributed in homogeneously, one-way flow occurs even under the restriction of a zero net surface charge along the channel.

cond-mat.soft↗

arXiv2014

Generic transport coefficients of a confined electrolyte solution

Physical parameters characterising electrokinetic transport in a confined electrolyte solution are reconstructed from the generic transport coefficients obtained within the classical non-equilibrium statistical thermodynamic framework. The electro-osmotic flow, the diffusio-osmotic flow, the osmotic current, as well as the pressure-driven Poiseuille-type flow, the electric conduction, and the ion diffusion, are described by this set of transport coefficients. The reconstruction is demonstrated for an aqueous NaCl solution between two parallel charged surfaces with a nanoscale gap, by using the molecular dynamic (MD) simulations. A Green-Kubo approach is employed to evaluate the transport coefficients in the linear-response regime, and the fluxes induced by the pressure, electric, and chemical potential fields are compared with the results of non-equilibrium MD simulations. Using this numerical scheme, the influence of the salt concentration on the transport coefficients is investigated. Anomalous reversal of diffusio-osmotic current, as well as that of electro-osmotic flow, is observed at high surface charge densities and high added-salt concentrations.

cond-mat.soft↗

arXiv2014

Molecular dynamics simulation of electrokinetic flow of an aqueous electrolyte solution in nanochannels

Electrokinetic flows of an aqueous NaCl solution in nanochannels with negatively charged surfaces are studied using molecular dynamics (MD) simulations. The four transport coefficients that characterise the response to weak electric and pressure fields, namely the coefficients for the electrical current in response to the electric field ($M^{jj}$) and the pressure field ($M^{jm}$), and those for the mass flow in response to the same fields ($M^{mj}$ and $M^{mm}$), are obtained in the linear regime using a Green--Kubo approach. Nonequilibrium simulations with explicit external fields are also carried out, and the current and mass flows are directly obtained. The two methods exhibit good agreement even for large external field strengths, and Onsager's reciprocal relation ($M^{jm} = M^{mj}$) is numerically confirmed in both approaches. The influence of the surface charge density on the flow is also considered. The values of the transport coefficients are found to be smaller for larger surface charge density, because the counter-ions strongly bound near the channel surface interfere with the charge and mass flows. A reversal of the streaming current and of the reciprocal electro-osmotic flow, with a change of sign of $M^{mj}$ due to the excess co-ions, takes places for very high surface charge density.

physics.chem-ph↗