Hitoshi Washizu

Organophosphates are well-known as the canonical additives for lubricants. Thus, understanding of the additive behaviour is a key aspect in the design of films on metal surfaces. Different types of phosphates are added to improve their antiwear properties, but the contributions of individual esters to these properties has not been studied using a combination of practical and theoretical approaches. In this study, organophosphates were isolated with high purity and their tribological characteristics were evaluated by using a Bowden-type reciprocating friction tester and a four-ball wear tester. Mono-oleylphosphate had a lower friction than di-oleylphosphate and exhibited excellent antiwear characteristics. Analysis of the sliding surfaces using desorption electrospray ionization-mass spectrometry (DESI-MS) and X-ray photoelectron spectroscopy (XPS) indicated that the film structure could predict the occurrence factor of the tribological characteristics of the oleylphosphates. Then the adsorption energies of the monoester on iron and iron oxide surfaces were higher than those of the diester, as assessed using density functional theory (DFT) calculations, owing to the difference in their chemisorption processes, as confirmed by further DFT analysis. Studies on the reactivity of additives and their interactions with surfaces are important for understanding the tribochemistry of additives.

In recent years, substantial advancements have been achieved in augmenting the energy efficiency of hydraulic fluids through the integration of polymers. This study employs a multiscale approach, encompassing an analysis of polymer coil size evolution and flow field characteristics, with the aim of investigating the behavior of both dipole and non-dipole polymers within the host solvent. The ultimate goal is to establish a comprehensive understanding of the correlation between atomic properties of additives and the macroscopic properties of the polymer solution. To achieve this, the study employs an atomistic-continuum hybrid model that combines Brownian Dynamics and Lattice Boltzmann techniques within the bead-spring concept framework. Four chains, each comprising 64 particles, are subjected to varying shear rates. The primary focus centers on the examination of alterations in the radius of gyration and the velocity field within the polymer solution. Notably, the inclusion of dipole-dipole interactions exerts a profound influence on the configuration of the polymers. The results illuminate that non-dipole polymers display a more pronounced coupling with bulk flow hydrodynamics, leading to the confinement of particle motion in directions perpendicular to the primary stream. In contrast, dipole polymers experience a slower increase in coil size when compared to their non-dipole counterparts. These findings furnish valuable insights for the enhancement of energy-efficient hydraulic fluids and contribute to a fundamental comprehension of polymer behavior in lubricants, charting the course for the development of advanced hydraulic fluids in the future.

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.

Reentrant phenomena in soft matter and biosystems have attracted considerable attention because their properties are closely related to high functionality. Here, we report a combined experimental and computational study on the self-assembly and reentrant behavior of a single-component thermotropic smectic liquid crystal toward the realization of dynamically functional materials. We have designed and synthesized a mesogenic molecule consisting of an alicyclic trans,trans -bicyclohexyl mesogen and a polar cyclic carbonate group connected by a flexible tetra(oxyethylene) spacer. The molecule exhibits an unprecedented sequence of layered smectic phases, in the order: smectic A-smectic B-reentrant smectic A. Electron density profiles and large-scale molecular dynamics simulations indicate that competition between the stacking of bicyclohexyl mesogens and the conformational flexibility of tetra(oxyethylene) chains induces this unusual reentrant behavior. Ion-conductive reentrant liquid-crystalline materials have been developed, which undergo the multistep conductivity changes in response to temperature. The reentrant liquid crystals have potential as new mesogenic materials exhibiting switching functions.

In this paper, we classified the types of water in the vicinity of the chitosan polymer and gold plate by applying an electric field of magnitude 1 V Å-1 in various directions at varying temperatures by using molecular dynamics simulation. The three types of water were categorized by analyzing the data through the tetrahedral order method with four water regions separated in the distance from 1 to 6 Å around polymers. The interaction between water molecules and functional groups, such as hydroxyl, ether, and ester, leads to the formation of intermediate and nonfreezing water. Under an electric field, this formation appeared more clearly due to the transformation of liquid water to crystal cubic ice with two structural formations depending on gold plates at a temperature of 300 K. The enhancement of the tetrahedral order of water in cubic ice is related to the existence of a four-fold H-bonded structure and lower ones in the XES experiment.

Polyethylene (PE) and branched PE are common materials inserted inside contacts for medical and industrial applications. Consequently, their tribological properties have been extensively investigated in the past. This paper aims to determine the lubrication behavior of polyethylethylene (PEE) for iron contact and the two main factors of temperature and carbon chain length influencing the lubrication in the high-pressure lubrication regime by molecular dynamics simulations. Additionally, the influences of pressure and sliding velocity on the friction are also considered. These factors are found to be significantly influencing the lubricity. The obtained results mainly originated from shear and stretching of the PEE molecules, condensed situation of the lubricants, and adhesion between the lubricants and the iron surfaces. These observations are explained in detail.

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.

A key to achieve the accuracy of molecular dynamics (MD) simulation is the set of force fields used to express the atomistic interactions. In particular, the electrostatic interaction remains the main issue for the precise simulation of various ionic soft materials from ionic liquids to their supramolecular compounds. In this study, we test the nonpolarizable force fields of ionic liquids (ILs) and self-assembled ionic liquid crystals (ILCs) for which the intermolecular charge transfer and intramolecular polarization are significant. The self-consistent modeling scheme is adopted to refine the atomic charges of ionic species in a condensed state through the use of density functional theory (DFT) under the periodic boundary condition. The atomic charges of the generalized amber force field (GAFF) are effectively updated to express the electrostatic properties of ionic molecules obtained by the DFT calculation in condensed phase, which improves the prediction accuracy of ionic conductivity with the obtained force field (GAFF-DFT). The derived DFT charges then suggest that the substitution of a hydrophobic liquid-crystalline moiety into IL-based cations enhances the charge localization of ionic groups in the amphiphilic molecules, leading to the amplification of the electrostatic interactions among the hydrophilic/ionic groups in the presence of hydrophobic moieties. In addition, we focus on an ion-conductive pathway hidden in the self-assembled nanostructure. The MD results indicate that the ionic groups of cation and anion interact strongly for keeping the bicontinuous nanosegregation of ionic nanochannel. The partial fractions of hydrophilic/ionic and hydrophobic nanodomains are then quantified with the volume difference from referenced IL systems, while the calculated ionic conductivity decreases in the self-assembled ILCs more than the occupied volume of ionic nanodomains. These analyses suggest that the mobility of ions in the self-assembled ILCs remains quite restricted even with small tetrafluoroborate anions because of strong attractive interaction among ionic moieties.

The paper carries out the smoothed particle hydrodynamics simulations of lubrication for the micronscale iron contact by the 20 types of the spherical solid oxide particles. The iron slabs and oxide particles of micrometer sizes are modeled by the elastic particle lattice coarse-graining and discrete element method, respectively. Motion of the iron particles is presented by the governing equations of the smoothed particle hydrodynamics approach. The interactions between the particle pairs such as iron-oxide and oxide-oxide ones are presented by the Hertz repulsive contact and van der Waals attractive noncontact forces. Furthermore, the oxide particle dissipates its kinetic energy into the environment containing it through the Stockes damping force. It is found that stability and shear of the tribofilm are the main factors influencing friction reduction in all the behaviors. The interactions between the oxide-oxide particles and between the oxide-asperity iron particles mainly contribute to formation and maintainability of the tribofilm. The oxide with the higher density or the heavier oxide particle results in the lower friction coefficient. The friction reduction also strongly depends on the water/diesel environment containing the particles; however, the relative lubrication ability among the oxides is almost independent of the environment. It is also found that in the contacts with the highly condensed situation of the oxide lubricant particles the van der Waals force can be neglected due to its very slight contribution to the detected forces and the rolling of the lubricant particles almost has no contribution to friction, in contrast to the strong influence of temperature of the environment.

Elucidating the state of interfacial water, especially the hydrogen-bond configurations, is considered to be key for a better understanding of the functions of polymers that are exhibited in the presence of water. Here, an analysis in this direction is conducted for two water-insoluble biocompatible polymers, poly(2-methoxyethyl acrylate) and cyclic(poly(2-methoxyethyl acrylate)), and a non-biocompatible polymer, poly(n-butyl acrylate), by measuring their IR spectra under humidified conditions and by carrying out theoretical calculations on model complex systems. It is found that the OH stretching bands of water are decomposed into four components, and while the higher-frequency components (with peaks at ∼3610 and ∼3540 cm-1) behave in parallel with the C═O and C-O-C stretching and CH deformation bands of the polymers, the lower-frequency components (with peaks at ∼3430 and ∼3260 cm-1) become pronounced to a greater extent with increasing humidity. From the theoretical calculations, it is shown that the OH stretching frequency that is distributed from ∼3650 to ∼3200 cm-1 is correlated to the hydrogen-bond configurations and is mainly controlled by the electric field that is sensed by the vibrating H atom. By combining these observed and calculated results, the configurations of water at the interface of the polymers are discussed.

This study focuses on designing solid lubricant particles by combining graphene and iron nanoparticles (namely, graphene-iron (GI) particles) and carrying out studies for behaviors of their lubrication for the iron contact by molecular dynamics simulations. By the annealing process of melting and cooling iron, we can create the lubricant particle, where the iron nanoparticle tightly holds the graphene sheet. In the sliding friction investigations, it is found that the influences of orientation of the graphene sheets inside the contact, size and configuration of the GI particles, and lubrication with the bare iron nanoparticles on friction are strong at low pressure and very slight at high pressure. The GI particles provide stability of the friction coefficient over a wide range of pressure; however, it strongly increases with pressure in the lubrication behaviors by the bare iron particles due to the deformation of the particles. The iron contact in the presence of the GI particles can achieve the ultralow values of the friction coefficient from 0.009 to 0.042. The contact surfaces are not nearly damaged (slightly elastic deformation) with the pressure up to 2.0 GPa. From the comparisons between the results in this study and previous reports, the GI particles have better lubrication than graphene coated on a surface and well stabilize under pressure compared to the different lubricant nanoparticles. The main reason for this is due to the contributions of graphene, besides reduction of the contact area resulted from the configuration of the nanoparticle, which promotes sliding and sharing of the pressure, preventing collision between the lubricant particles.

In materials science, water plays an important part, especially at the molecular level. It shows various properties when sorbed onto surfaces of polymers. The structure of the molecular water ensemble in the vicinity of the polymers is under discussion. In this study, we used molecular dynamics methods to analyze the structure of water in the vicinity of the polymer polyrotaxane (PR), composed of α-cyclodextrins (α-CDs), a poly(ethylene glycol) (PEG) axial chain, and α-lipoic acid linkers, at various temperatures. The distribution of water around the functional groups, hydrogen bond network, and tetrahedral order were analyzed to classify the various types of water around the polymer. We found that the tetrahedral order of water had a strained relationship from the XES experiment. Four water regions were separated from each other in the vicinity of 1 to 5 Å around PR. The intermediate and non-freezing water were formed due to the interaction between water molecules and the functional groups, such as hydroxyl, ether, and ester.

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.

<title>Abstract</title> The present study uses the smoothed particle hydrodynamics (SPH) and discrete element method (DEM) coupling to investigate influence of the hexagonal boron nitride (hBN) particles on friction of the elastic coarse-grained micronscale iron. The hBN lubricant particles significantly improve the friction performance of iron in various simulation behaviors. The particle size, the air/water background containing the particles, and its temperature result in reduction of the friction coefficient. The surface mending, the protective film, and the energy dissipation are the main mechanisms related to the friction reduction. Additionally, it is worthy to note that the static friction and the kinetic friction can be clearly observed by this elastic coarse-graining.

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.

<title>Abstract</title>The optimal method of the polymer Materials Informatics (MI) has not been developed because the amorphous nature of the higher-order structure affects these properties. We have now tried to develop the polymer MI’s descriptor of the higher-order structure using persistent homology as the topological method. We have experimentally studied the influence of the MD simulation cell size as the higher-order structure of the polymer on its electrical properties important for a soft material sensor or actuator device. The all-atom MD simulation of the polymer has been calculated and the obtained atomic coordinate has been analyzed by the persistent homology. The change in the higher-order structure by different cell size simulations affects the dielectric constant, although these changes are not described by a radial distribution function (RDF). On the other hand, using the 2nd order persistent diagram (PD), it was found that when the cell size is small, the island-shaped distribution become smoother as the cell size increased. There is the same tendency for the condition of change in the monomer ratio, the polymer chain length or temperature. As a result, the persistent homology may express the higher-order structure generated by the MD simulation as a descriptor of the polymer MI.

<title>Abstract</title> The study focuses on monitoring influence of the alumina coatings on friction and stability of the microscale iron contacts by the smoothed particle hydrodynamics. The obtained results show a better stability and a higher value of the friction coefficient of the coated surface compared to the uncoated one. This study also supposes that in concern of stability of the surface the coating should be done with only the substrate surface accompanied with the roughness of the coating layer. The proportion of the coated particles is found to be strongly resulting in the friction properties and the roughness of the coating layer also slightly results in those. The surface reaches the most stability at the proportion of around 70%.

<title>Abstract</title> This paper investigates the friction and friction heat of the micronscale iron under the influences such as the velocity of the slider and temperature of the substrate by using the smoothed particle hydrodynamics simulations. It is found that in the velocity range of 10–100 m/s, the sliding velocity–friction coefficient relationship well complies with the fitted exponent or hyperbolic tangent function, and the friction coefficient approaches a stable value of 0.3 at around the velocity of 50 m/s after a rapidly increasing situation. The steady friction coefficient maintains over the temperature range of 200–400 K. The friction heat is detailed analyzed versus the sliding time. The sliding time–system temperature relationship is well fitted by the sigmoidal functions, except the interfacial particle layers. The layer causing friction shows the highest steady temperature and largest temperature rise. The increment between the initial temperatures of the slider and the substrate strongly results in the temperature rise while it does not affect the configuration of the sliding time–system temperature curves.

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.

The paper investigates sliding friction of the alpha-Al2O3/alpha-Al2O3 and alpha-Fe2O3/alpha-Fe2O3 contacts by using the spring interfacial potential. It is found that at micronscale 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 the normal component of the interfacial interaction 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.

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.

A large-scale parallel computing model based on the SPH (Smoothed Particle Hydrodynamics) method was developed for dry shear friction between elastic-plastic solids. Our main purpose is to elucidate the mechanism of the frictional wear and heat generation between the asperities on the interface in meso-scale. In our model, the elastic-plastic motion is expressed by the general SPH method for the solid. The frictional interaction between the asperities is also added as a two-body interaction between SPH particles. In thus general SPH model, the isotropic weighting function is used and the SPH particle is the isotropic sphere. However, the surface roughness of the real metal interfaces is very small with respect to the size of the frictional surface area. The aspect ratio between the sliding direction and the thickness direction of the asperities is very large. For the modified model, we propose a method using coordinate transformation, in which the unit length between the shearing direction and the direction perpendicular to the shear plane is different. We call it the “Disk-like SPH method”. We show the validity of thus model and think it as the effective coarsening scheme to clear the wear and heat generation at the real 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.