鷲津 仁志

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.

Bulk copolymerization of alkyl acrylates and cyclodextrin (CD) host monomers produced a single movable cross-network (SC). The CD units acted as movable crosslinking points in the obtained SC elastomer. Introducing movable crosslinks into a poly(ethyl acrylate/butyl acrylate) copolymer resulted in good toughness (G(f)) and stress dispersion. Here, to improve the Young's modulus (E) and G(f) of movable cross-network elastomers, the bulk copolymerization of liquid alkyl acrylate monomer swelling in SC gave another type of movable cross-network elastomer with penetrating polymers (SCPs). Moreover, the bulk copolymerization of alkyl acrylate and the CD monomer in the presence of SC resulted in dual cross-network (DC) elastomers. The G(f) of the DC elastomer with a suitable weight % (wt%) of the secondary movable cross-network polymer was higher than those of the SCP or SC elastomers. The combination of suitable hydrophobicity and glass transition of the secondary network was important for improving G(f). Small-angle X-ray scattering (SAXS) indicated that the DC elastomers exhibited heterogeneity at the nanoscale. The DC elastomers showed a significantly broader relaxation time distribution than the SC and SCP elastomers. Thus, the nanoscale heterogeneity and broader relaxation time distribution were important to increase G(f). This method to fabricate SCP and DC elastomers with penetrating polymers would be applicable to improve the G(f) of conventional polymeric materials.

The nematic-isotropic (NI) phase transition of 4-cyano-4'-pentylbiphenyl was simulated using the generalized replica-exchange method (gREM) based on molecular dynamics simulations. The effective temperature is introduced in the gREM, allowing for 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.

The paper uses molecular dynamics simulations to investigate lubrication of graphene for the iron contacts. The graphene sheets are vertically buried into the iron surface by the annealing of melting and cooling it. The friction detection is carried out in various conditions as dependence of friction on the number of the graphene sheets buried on the surface, graphene combined with only the substrate or both of the contacting surfaces, pressure, temperature and sliding velocity. We find that by using this annealing the graphene sheets are tightly held on the contacting surfaces during the sliding. This makes graphene inside the contacts to stably maintain its lubricity. The friction coefficient has the superlow values or the superlubricity one of 0.006.

Self-assembled ionic liquid crystals can transport water and ions via the periodic nanochannels, and these materials are promising candidates as water treatment membranes. Molecular insights on the water transport process are, however, less investigated because of computational difficulties of ionic soft matters and the self-assembly. Here we report specific behavior of water molecules in the nanochannels by using the self-consistent modeling combining density functional theory and molecular dynamics and the large-scale molecular dynamics calculation. The simulations clearly provide the one-dimensional (1D) and 3D-interconnected nanochannels of self-assembled columnar and bicontinuous structures, respectively, with the precise mesoscale order observed by x-ray diffraction measurement. Water molecules are then confined inside the nanochannels with the formation of hydrogen bonding network. The quantitative analyses of free energetics and anisotropic diffusivity reveal that, the mesoscale geometry of 1D nanodomain profits the nature of water transport via advantages of dissolution and diffusion mechanisms inside the ionic nanochannels.

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.

<p>The paper focuses on examining agreement of the adaptive smoothed particle hydrodynamics (ASPH) in the investigation of the sliding friction of silica at micronscale throughout observation of several friction characteristics. It is found that the ASPH approach well presents the friction of micronscale silica due to agreement of the friction coefficient and the applied load-friction coefficient relationship between the present results and the previously experimental reports. The shape of the particle modeled in the ASPH almost does not effect on the detected results for the hard system due to the very slight variation of the particles during the sliding. However, the variation of the particles can explain for the discrepancy between the stick time and the slip time and the unsharp change between the stick state and the slip one. The study is also extended for the contacts of the two sinusoidal rough surfaces and finds that the friction coefficient is almost independent of the wavelength while it linearly increases with the amplitude.</p>

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.

As a model system of the adsorption process of anticorrosion 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.

The paper presents the construction of 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 = 864x10(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 the study of the friction of the Fe system, and the anisotropic behavior presents better friction of the system than the isotropic one. Copyright (C) EPLA, 2018

We investigate the structure of polyelectrolyte brushes to determine the effects of the charge fraction of the polymers, grafting density, chain length, and salt concentration. A hybrid coarse-grained model is employed, where a soft potential is applied to coarse-grained particles representing the solvent, while a hard potential is used for the polymer beads, and co- and counterions. A steep increase in brush height with charge fraction is observed in the low-to-moderate charge fraction regime, whereas the brush approaches the contour height in the high charge fraction regime. The effects of graft density and chain length on brush height are well explained by the scaling theory based on the balance between the osmotic pressure and chain elasticity, properly taking into account the polymer stiffness. In addition, Pincus's power law for varying added salt concentration is also reproduced by the simulation.

The study investigates sliding friction and heat transfer of the coarse-grained iron micron-scale system by smoothed particle hydrodynamics simulation. The body-centered-cubic iron system of 2187x10(12) atoms is coarse-grained into the simple-cubic crystal of 40500 particles. Heat transfer of particles is monitored in the sliding time. Stick-slip motion is observed during the sliding. The detected results also demonstrate that a combination of the coarse-grained model and the spring friction force reasonably presents these quantities of the iron systems.

A molecular dynamics simulation is used to investigate the occurrence of thermal escape motion of a graphene transfer layer in all atom levels. In the simulation, the substrate is modelled as a 3-layer graphene slab, and the transfer layer as layered circle graphene sheets. The top graphene sheet is force to move in a constant velocity. After the sliding motion, the dynamics of the transfer layers showed different dependences on the sliding velocity and the size of the graphene sheet. Only when the sliding motion is low enough and the size is large enough, is the thermal escape motion found. When the sliding speed is too high, the lower layers cannot follow the top sheet. When the graphene sheet is too small, the lower layered structure is broken due to an internal motion. The latter motion is not found during the study using the previous coarse-grained simulation. The size of the layers experimentally observed is the same as this simulation, and when the sliding motion is low enough, a low friction is observed. Thus, a low friction is indicated as a result of the thermal escape motion.

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. (C) 2017 Elsevier B.V. All rights reserved.

Using a coarse-grained model that accurately reproduces the structures and pressures of the parent all-atom simulations, we performed molecular dynamics simulations to gain insight into the high-speed shear behavior of nanometer-thick perfluoropolyether (PFPE) films confined between two corrugated solid surfaces. The PFPE films exhibit shear thinning behavior (i.e., decrease of viscosity with increasing shear rate) following a power law. The degree of shear thinning (i.e., the exponent of the power law) is largely determined by the degree of geometric confinement rather than layering and molecular orientation of the confined films. Severe geometric confinement at a small solid-solid spacing gives a large degree of shear thinning. (C) 2015 Elsevier Ltd. All rights reserved.

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 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 inhomogeneously, one-way flow occurs even under the restriction of a zero net surface charge along the channel. (C) 2015 Elsevier B.V. All rights reserved.

Numerous researchers have focused on the relation between kinetic friction and the surface properties of materials, such as their surface roughness and chemical state, because kinetic friction is the loss of kinetic energy at a sliding interface. Contrary to conventional wisdom, recent theory has predicated that, given the fact of phonon energy dissipation, kinetic friction depends on the properties of bulk atoms in a solid, not only on the surface properties. However, this expectation has not been proven. Here we show evidence of this idea via atomic-scale experiments and simulations. We compared the kinetic frictions of isotopically distinct single-crystal diamonds, which differ only in atomic mass, using atomic force microscopy and observed that the friction of &lt sup&gt 13&lt /sup&gt C diamond is lower than that of &lt sup&gt 12&lt /sup&gt C diamond by approximately 3%. Simulations and theoretical analysis reproduce this result well, suggesting that the lower friction of &lt sup&gt 13&lt /sup&gt C diamond originates from the inhibition of the energy-dissipative phonon by a heavier atom mass. This discovery provides a design concept of low-friction materials by tuning the energy-dissipation process with modification of the inner-solid properties i.e. phonon properties.

Physical parameters characterizing electrokinetic transport in a confined electrolyte solution are reconstructed from the generic transport coefficients obtained within the classical nonequilibrium 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 nonequilibrium 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.

We present a coupled lattice Boltzmann method (LBM) to solve a set of model equations for electrokinetic flows in micro-/nano-channels. The model consists of the Poisson equation for the electrical potential, the Nernst-Planck equation for the ion concentration, and the Navier-Stokes equation for the flows of the electrolyte solution. In the proposed LBM, the electrochemical migration and the convection of the electrolyte solution contributing to the ion flux are incorporated into the collision operator, which maintains the locality of the algorithm inherent to the original LBM. Furthermore, the Neumann-type boundary condition at the solid/liquid interface is then correctly imposed. In order to validate the present LBM, we consider an electro-osmotic flow in a slit between two charged infinite parallel plates, and the results of LBM computation are compared to the analytical solutions. Good agreement is obtained in the parameter range considered herein, including the case in which the nonlinearity of the Poisson equation due to the large potential variation manifests itself. We also apply the method to a two-dimensional problem of a finite-length microchannel with an entry and an exit. The steady state, as well as the transient behavior, of the electro-osmotic flow induced in the microchannel is investigated. It is shown that, although no external pressure difference is imposed, the presence of the entry and exit results in the occurrence of the local pressure gradient that causes a flow resistance reducing the magnitude of the electro-osmotic flow. (C) 2014 Elsevier B.V. All rights reserved.

A boundary scheme in the lattice Boltzmann method (LBM) for the convection-diffusion equation, which correctly realizes the internal boundary condition at the interface between two phases with different transport properties, is presented. The difficulty in satisfying the continuity of flux at the interface in a transient analysis, which is inherent in the conventional LBM, is overcome by modifying the collision operator and the streaming process of the LBM. An asymptotic analysis of the scheme is carried out in order to clarify the role played by the adjustable parameters involved in the scheme. As a result, the internal boundary condition is shown to be satisfied with second-order accuracy with respect to the lattice interval, if we assign appropriate values to the adjustable parameters. In addition, two specific problems are numerically analyzed, and comparison with the analytical solutions of the problems numerically validates the proposed scheme.

Electrokinetic flows of an aqueous NaCl solution in nanochannels with negatively charged surfaces are studied using molecular dynamics simulations. The four transport coefficients that characterize 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. (C) 2014 AIP Publishing LLC.