Hitoshi Washizu

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 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 Onsagers 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.

Deep bulk atoms in a solid cause friction

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 Greens 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.

Mechanism of ultra low friction of multilayer graphene studied by coarse-grained molecular simulation

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.

Molecular dynamics simulations of elasto-hydrodynamic lubrication and boundary lubrication for automotive tribology

Friction 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.

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

Abstract 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.

All-Atom Molecular Dynamics Simulation of Submicron Thickness EHL Oil Film

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, MD 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.© 2007 ASME

Generic transport coefficients of a confined electrolyte solution

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.

Polarizable Dissipative Particle Dynamics Simulation of Electrolyte Solutions

Solvent polarizability is introduced into a coarse-grained (CG) particle model to represent polar solvents on a mesoscopic level. In our method, the polarization of coarse-grained particles is represented by an oscillator which consists of two oppositely charged particles connected to each other by a spring. The charges and the spring constant are chosen so that the constant of proportionality relating the electric field to the induced dielectric polarization density corresponds to the macroscopic susceptibility. Dissipative particle dynamics (DPD) simulations of polar and the non-polar solvents are carried out to model electrolyte solutions. The cation-anion radial distribution functions (RDFs) for the polar solvent show an oscillatory character, as well as a sharp peak at a distance which corresponds to contact ion pair formation. In contrast, the RDFs for the non-polar solvent shows a monotonic change and a broad peak. This suggests that the introduction of the oscillator model for polar solvents improves the adequacy of the coarse-grained model of electrolyte solutions.

Coarse-grained simulations of polyelectrolyte brushes using a hybrid model

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.