鷲津 仁志

As an example of a very low friction system, Monte Carlo Brownian dynamics simulations have been used to calculate equilibrium structures of a polyelectrolyte brush grafted onto planes. The polymers were calculated in a semi-flexible coarse-grain model that is appropriate to treat the charge density of the polyion. The effect of linear charge density on the polyion xi, the surface negative charge, and added salts were studied. In salt-free solution, scaling theories predicted the structure well in the low xi region. In the high xi region, additional shrinkage was found from the theories due to counterion condensation. The effect of surface charge showed not only the repulsion of the polyion from the surface but also the shrinkage in the high xi region due to the additional counterions required for electrical neutrality. The addition of salts led to the shrinkage of the brush heights, and in the high xi region, additional extension was found. The computational strategy for calculating the friction dynamics of the system is also discussed.

The no-slip boundary condition widely used in the macroscopic fluid mechanics has not been explained from the molecular level. This letter describes all atom molecular dynamics simulation to study boundary slip of hydrocarbon oil film under shear of a submicron thickness confined between solid walls. The large time-space scale simulation under the realistic interactions of fluid atoms, solid-fluid interaction and sliding speed has shown the no-slip of the oil film. The difference between the nanoscale film and the submicron thick film is explained from the viewpoint of the anisotropic viscosity rise in the vicinity of the solid wall and the correlation length of the momentum up to several tenths of nanometers. The results of the present simulation coincide with the experiments of fluid flow through nanometer-scale channels. © 2014 Japanese Society of Tribologists.

We investigate the dependence of solid friction on the number of surface atoms in a unit cell under periodic boundary conditions. A two-dimensional friction model between a single asperity and an elastic solid surface is examined. The solid surface is modeled using a coupled-oscillator lattice that consists of an infinite number of inner solid atoms the dynamics are simulated by the recently proposed semi-infinite dynamic lattice Green's function method. A significant dependence of the friction on the number of surface atoms is observed. The dependence is attributed to the requirement for a large number of surface atoms for the excitation of the energy-dissipative surface phonons with nonzero wave numbers. In order to eliminate the problematic dependence, a correction method combined with the continuum contact theory is developed to evaluate friction in the limit of the infinite surface atoms. In addition, we found a relationship between the friction and the power spectrum of the temporal fluctuation in the force the latter quantity does not significantly depend on the number of surface atoms. © 2012 American Physical Society.

We investigate the dependence of solid friction on the number of surface atoms in a unit cell under periodic boundary conditions. A two-dimensional friction model between a single asperity and an elastic solid surface is examined. The solid surface is modeled using a coupled-oscillator lattice that consists of an infinite number of inner solid atoms; the dynamics are simulated by the recently proposed semi-infinite dynamic lattice Green's function method. A significant dependence of the friction on the number of surface atoms is observed. The dependence is attributed to the requirement for a large number of surface atoms for the excitation of the energy-dissipative surface phonons with nonzero wave numbers. In order to eliminate the problematic dependence, a correction method combined with the continuum contact theory is developed to evaluate friction in the limit of the infinite surface atoms. In addition, we found a relationship between the friction and the power spectrum of the temporal fluctuation in the force; the latter quantity does not significantly depend on the number of surface atoms.

Coarse-grained Metropolis Monte Carlo Brownian Dynamics simulations are used to clarify the ultralow friction mechanism of a transfer film of multilayered graphene sheets. Each circular graphene sheet consists of 400 to 1,000,000 atoms confined between the upper and lower sliders and are allowed to move in 3 translational and 1 rotational directions due to thermal motion at 300 K. The sheet-sheet interaction energy is calculated by the sum of the pair potential of the sp2 carbons. The sliding simulations are done by moving the upper slider at a constant velocity. In the monolayer case, the friction force shows a stick-slip like curve and the average of the force is high. In the multilayer case, the friction force does not show any oscillation and the average of the force is very low. This is because the entire transfer film has an internal degree of freedom in the multilayer case and the lowest sheet of the layer is able to follow the equipotential surface of the lower slider.

To investigate the relationship between solid friction and energy dissipation due to phonon, we developed a coupled-oscillator surface model that consists of an infinitely large number of bulk atoms in a solid. This method is formulated using a dynamic lattice Green's function. A self-consistent scheme used for achieving a steady state and a fast convolution method that reduces the high computational overhead are also presented. Furthermore, a methodology to decompose the friction coefficient with the surface phonon modes is obtained. The energy absorption band corresponding to the wave number of the surface phonon is found. These approaches clarify the role of the energy-dissipation mechanism in sliding friction. Two-dimensional friction models in which both surfaces have same lattice constant, i.e., commensurate surfaces, are used to demonstrate these methods. In the analysis of a friction system between flat surfaces, energy transfer from the kinetic energy of a sliding solid to low-frequency surface phonons in the counter solid occurs in the presence of bulk atoms. The energy dissipation into the bulk system leads to friction. We also investigate a friction system between periodically contacting surfaces. It is found that surface phonons with nonzero wave number act as channels for energy dissipation and alter the friction profile depending on the size of the contact area. When the contact size is so large that a sufficient number of the nonzero wave number modes act as the energy-dissipation channels, the profile of the friction decomposition with the nonzero wave number modes exhibits good agreement with that estimated by a simple continuum model. © 2010 The American Physical Society.

This paper reviews recent research in molecular dynamics studies of the traction properties of hydrocarbon fluids under elastohydrodynamic lubrication, focusing on the technical problems that arise on making predictions of the traction properties of an oil film with a submicron thickness at the actual sliding contacts of the machine elements by at a nanoscale molecular simulation. The effect of the oil film thickness and shear rate are examined including the result of a submicron thickness simulation of the oil film using a tera-flops computer. The mechanism of the phase transition of the fluids under high pressure, the boundary slip, and the momentum transfer related to the molecular structure of the fluids are also presented. Copyright (c) 2010 John Wiley & Sons, Ltd.

To investigate the relationship between solid friction and energy dissipation due to phonon, we developed a coupled-oscillator surface model that consists of an infinitely large number of bulk atoms in a solid. This method is formulated using a dynamic lattice Green's function. A self-consistent scheme used for achieving a steady state and a fast convolution method that reduces the high computational overhead are also presented. Furthermore, a methodology to decompose the friction coefficient with the surface phonon modes is obtained. The energy absorption band corresponding to the wave number of the surface phonon is found. These approaches clarify the role of the energy-dissipation mechanism in sliding friction. Two-dimensional friction models in which both surfaces have same lattice constant, i.e., commensurate surfaces, are used to demonstrate these methods. In the analysis of a friction system between flat surfaces, energy transfer from the kinetic energy of a sliding solid to low-frequency surface phonons in the counter solid occurs in the presence of bulk atoms. The energy dissipation into the bulk system leads to friction. We also investigate a friction system between periodically contacting surfaces. It is found that surface phonons with nonzero wave number act as channels for energy dissipation and alter the friction profile depending on the size of the contact area. When the contact size is so large that a sufficient number of the nonzero wave number modes act as the energy-dissipation channels, the profile of the friction decomposition with the nonzero wave number modes exhibits good agreement with that estimated by a simple continuum model.

To analyze kinetic friction between solids on the atomic scale, a coupled-oscillator surface model including an infinite number of atomic layers is developed by a self-consistent scheme using a Green's function. The numerical approach shows that friction involves not only surface atoms and their interaction with an opposite surface but also bulk atoms in a solid. Energy transfer from kinetic energy of a sliding solid to low-frequency lattice vibration occurs in the presence of bulk atoms, and energy dissipation into the bulk system leads to friction. Copyright (C) EPLA, 2009

All-atom molecular dynamics simulations of an elastohydrodynamic lubricating oil film have been performed to study the effect of the oil film thickness (large spatial scale; thickness: 430 nm, MD time: 25 ns) and the effect of pressure (long time scale; thickness: 10 nm, NM time: 50 ns, external pressure: 0.1 to 8.0 GPa). Fluid layers of n-hexane are confined between two solid Fe plates by a constant normal force. Traction simulations are performed by applying a relative sliding motion to the Fe plates. In a long spatial scale simulation, the mean traction coefficient was 0.03, which is comparable to the experimental value of 0.02. In a long time scale simulation, a transition of the traction behavior is observed around 0.5 GPa to 1.0 GPa which corresponds to a change from the viscoelastic region to the plastic-elastic region which have been experimentally observed. This phase transition is related to a suppressed fluctuation of the molecular motion.

Riction control of machine elements on a molecular level is a challenging subject in vehicle technology. We describe the molecular dynamics studies of friction in two significant lubrication regimes. As a case of elastohydrodynamic lubrication, we introduce the mechanism of momentum transfer related to the molecular structure of the hydrocarbon fluids, phase transition of the fluids under high pressure, and a submicron thickness simulation of the oil film using a tera-flops computer. For boundary lubrication, the dynamic behavior of water molecules on hydrophilic and hydrophobic silicon surfaces under a shear condition is studied. The dynamic structure of the hydrogen bond network on the hydrophilic surface is related to the low friction of the diamond-like carbon containing silicon (DLC-Si) coating.

Monte Carlo simulations are performed to determine the anisotropy of the electric polarizability of a model DNA fragment in aqueous salt solution. By taking into consideration the participation of coions in the electroneutrality condition, at every simulation step, we obtain a list of counterions constituting the net charge arranged in increasing order of their distance from the DNA and calculate the contribution to the dipole moment from the first n counterions in the list. We define a partial polarizability tensor due to these n counterions to understand the origin of the polarizability in close relation to the solution structure. The ionic distributions are described by the counterion condensation theory. Characteristic features of the electric properties of polyelectrolytes are reproduced. The anisotropy of the electric polarizability Delta alpha of DNA decreases with the addition of salt, yielding values comparable to experiment. The effect of electrophoretic motion of the polyion is examined by estimating its upper limit.

Non-equilibrium all-atom MD simulations are used to study traction properties of hydrocarbon fluids. Fluid layer is confined between two solid Fe walls under constant normal force of 1.0 GPa. Traction simulations are performed by applying relative sliding motion on the Fe walls. Shear behaviors of eight hydrocarbon fluids are simulated on sufficiently large film thickness of 6.7 nm, and succeeded to reproduce in the order of experimental traction coefficients. The dynamical mechanism of momentum transfer on layers of fluid molecules are analyzed focusing on intermolecular interactions (density profile, orientation factor, pair-correlation function) and intramolecular interactions (intramolecular interaction energy, conformation change of cyclohexyl ring). In contrast with the case of n-hexane which shows low traction due to fragile chain like interaction, other mechanisms are obtained in high traction molecules of cyclohexane and dicyclohexyl. In cyclohexane, cyclohexyl rings face each other in high ordered molecular layer, and motion of conformational changes cooperates. In dicyclohexyl, cyclohexyl rings which distributes across low ordered molecular layers behaves as stiff bulky mass, and momentum transfers on the end of molecular axis. Traction mechanism of eight hydrocarbon fluids are also obtained in the course of analysis.

Traction properties of hydrocarbon fluids are studied by molecular dynamics simulations. Fluid layer is confined between two solid Fe walls under constant normal force of 1.0 GPa. Traction simulations axe performed by applying relative sliding motion on the Fe walls. Interfacial slip is suppressed by applying large Fe-fluid interaction. In order to determine the condition which can simulate qualitatively equivalent with experiments, film thickness (1-10 nm) and shear rate (107-10(9) s(-1)) dependence of traction coefficient are calculated of n-hexane. Although our calculated traction coefficients show higher value than experimental one over whole simulation range, traction coefficients decreased with both decreasing shear rate and increasing film thickness, which made qualitative agreement with experiment. In ultra thin film less then 2.5 nm, large stick-slip like motion caused by slip between fluid layers are observed. These results suggest that film thickness of at least 5.0 nm is needed to simulate experimentally observed viscoelastic traction behavior.

Anisotropy of the electrical polarizability Deltaalpha of model DNA fragments in salt-free aqueous solutions is determined by Monte Carlo simulation. According to the fluctuation-dissipation theorem, the electrical polarizability of polyelectrolytes is related to the fluctuations of the dipole moment generated in the counterion atmosphere around the polyion in the absence of an applied electric field. At every simulation step we numerically sort counterions in increasing order of the sum of their distances from both ends of the polyion. Two kinds of counterions are recognized in distinct spatial distributions, allowing us to give a definition of condensed counterions for charged oligomers based on the simulation. The fraction of condensed counterions so determined approaches Manning's theoretical value as the molecular weight of polyelectrolytes increases. We calculate the contribution to the dipole moment from the first n counterions in the sorting list and define a partial polarizability tensor due to these n counterions. Its introduction enables us to distinguish between contributions to the polarizability from the two kinds of ions. Contribution from condensed counterions to the radial components of the polarizability tensor is very small as has hitherto often been postulated in various theories. However, contribution from the diffuse ion atmosphere is very large and cannot be neglected in the calculation of the anisotropy. Although in our simulations solvent convection is suppressed, characteristic features of the. electric properties of polyelectrolytes in aqueous solutions with no added salt are reproduced. The anisotropy of the electrical polarizability Deltaalpha of DNA in salt-free aqueous solution increases on dilution of the polymer concentration and is proportional to the second or higher power of the molecular weight. The similar to1 nm thick apparently stable ionic sheath in the immediate vicinity of the polyion should be distinguished from condensed counterions. It is the latter that behaves as a physical entity having characteristic electrical properties.

Metropolis Monte Carlo (MC) Brownian dynamics simulations are performed to determine the anisotropy of the electrical polarizability Delta alpha of a model DNA fragment in aqueous salt solutions. A 64 base-pair fragment of the double-stranded DNA is modeled as an impenetrable cylinder and placed in a spherical MC cell. Taking into consideration the contribution of colons to the electroneutrality condition, at every simulation step we obtain a list of counterions which constitute the net charge that compensates the polyion charge. According to the fluctuation-dissipation theorem, polarizability is calculated from fluctuations of the dipole moment created in the distribution of the net charge. Electric properties of polyelectrolyte solutions are reproduced: Delta alpha decreases with the addition of salt yielding a steady value comparable to the experiment. (C) 2000 Elsevier Science B.V. All rights reserved.

Metropolis Monte Carlo (MC) Brownian dynamics simulations are performed to determine values of the anisotropy of the electrical polarizability of a model DNA fragment in salt-free aqueous solutions. A 64 base-pair fragment of the double-stranded DNA is modeled as an impenetrable cylinder and placed in a spherical MC cell. At every simulation step we numerically sort counterions in increasing order of the sum of their distances from both ends of the polyion and calculate the contribution to the dipole moment of the first it counterions in the sorting list. According to the fluctuation-dissipation theorem, partial polarizability is calculated from the fluctuation of this quantity. The anisotropy of the electrical polarizability de! is determined by cutting off the contribution from the Debye-Huckel ion atmosphere which shows large statistical uncertainties assuming that this contribution to the polarizability tensor is isotropic as a first approximation. (C) 1999 Elsevier Science B.V. All rights reserved.

Metropolis Monte Carlo Brownian dynamics simulations are performed on model DNA solutions. It is shown that the induced dipole moment responsible for the orientation of a DNA fragment in an applied electric field may have its origin in the displacement of Manning's condensed counterions along the DNA rod axis.