Nozomu Hatakeyama

A Computational Chemistry Study on Friction of h-MoS2. Part I. Mechanism of Single Sheet Lubrication

In this work, we theoretically investigated the friction mechanism of hexagonal MoS(2) (a well-known lamellar compound) using a computational chemistry method. First, we determined several parameters for molecular dynamics simulations via accurate quantum chemistry calculations and MoS(2) and MoS(2-x)O(x) structures were successfully reproduced. We also show that the simulated Raman spectrum and peak shift on X-ray diffraction patterns were in good agreement with those of experiment. The atomic interactions between MoS(2) sheets were studied by using a hybrid quantum chemical/classical molecular dynamics method. We found that the predominant interaction between two sulfur layers in different MoS(2) sheets was Coulombic repulsion, which directly affects the MoS(2) lubrication. MoS(2) sheets adsorbed on a nascent iron substrate reduced friction further due to much larger Coulombic repulsive interactions. Friction for the oxygen-containing MoS(2) sheets was influenced by not only the Coulomb repulsive interaction but also the atomic-scale roughness of the MoS(2)/MoS(2) sliding interface.

A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

Applying ultra-accelerated quantum chemical molecular dynamics technique for the evaluation of ligand protein interactions

Ligand–protein interactions have been studied using several chemical information techniques including quantum chemical methods that are applied to truncated systems composed of the ligand molecule and the surrounding amino acids of the receptor. Fragmented quantum molecular chemical studies are also a choice to study the enzyme–ligand system holistically, however there are still restrictions on the number of water molecules that can be included in a study of this nature. In this work we adopt a completely different approach to study ligand–protein interactions accounting explicitly for as many solvent molecules as possible and without the need for a fragmented calculation. Furthermore, we embed our quantum chemical calculations within a molecular dynamics framework that enables a fundamentally fast system for quantum chemical molecular dynamic simulations (QCMD). Central to this new system for QCMD is the tight binding QC system, newly developed in our laboratories, which combined with the MD paradigm results in an ultra-accelerated QCMD method for protein–ligand interaction evaluations. We have applied our newly developed system to the dihydrofolate reductase (DHFR)–methotrexate (MTX) system. We show how the proposed method leads us to new insights into the main interactions that bind MTX to the enzyme, mainly the interaction between the amino group of MTX and Asp27 of DHFR, as well as MTX amino group with Thr113 of DHFR, which have been only elucidated experimentally to date.

Aeolian tones radiated from flow past two square cylinders in a side-by-side arrangement

The sound generated by two square cylinders placed in a side-by-side arrangement in a uniform flow at low Mach numbers is studied by direct solution of the two-dimensional unsteady compressible Navier-Stokes equations. Special attention is paid to the effect of the spacing between the two cylinders on the generation mechanism of the sound. Results show that sound pressure fields as well as flow fields show different features depending on the spacing; six different wake patterns (nonsynchronized, antiphase and in-phase synchronized, flip-flopping, single bluff-body, and steady patterns) are observed and, depending on the wake pattern, sound pressure fields also show different features. Two types of bifurcation phenomenon are observed where two different wake patterns exist for the same spacing and either of the two patterns appears depending on an initial condition; different sound pressure fields appear in response to the wake patterns. Results also show that Curles dipole solution does not predict well ...

Aeolian tones radiated from flow past two square cylinders in tandem

The sound generated by two square cylinders placed in a tandem arrangement in a uniform flow at low Mach numbers is studied by direct solution of the two-dimensional unsteady compressible Navier-Stokes equations. Special attention is paid to the effect of the spacing between the two cylinders on the generation mechanism of the sound. Results show that the magnitude of the generated sound varies drastically depending on the spacing. When the spacing is small, roll-up of the shear layer separated from the upstream cylinder is suppressed and the sound generation is also suppressed; near a critical value of the spacing, the magnitude of the generated sound is about 10−2 as small as that generated by a single square cylinder. With increased spacing beyond a critical value, the shear layer separated from the upstream cylinder rolls up to form vortices in front of the downstream cylinder; the rolled-up vortices interact with the downstream cylinder, leading to a drastic increase in the magnitude of the generated...

Development of a quantum chemical molecular dynamics tribochemical simulator and its application to tribochemical reaction dynamics of lubricant additives

Tribology at the atomistic and molecular levels has been theoretically studied by a classical molecular dynamics (MD) method. However, this method inherently cannot simulate the tribochemical reaction dynamics because it does not consider the electrons in nature. Although the first-principles based MD method has recently been used for understanding the chemical reaction dynamics of several molecules in the tribology field, the method cannot simulate the tribochemical reaction dynamics of a large complex system including solid surfaces and interfaces due to its huge computation costs. On the other hand, we have developed a quantum chemical MD tribochemical simulator on the basis of a hybrid tight-binding quantum chemical/classical MD method. In the simulator, the central part of the chemical reaction dynamics is calculated by the tight-binding quantum chemical MD method, and the remaining part is calculated by the classical MD method. Therefore, the developed tribochemical simulator realizes the study on tribochemical reaction dynamics of a large complex system, which cannot be treated by using the conventional classical MD or the first-principles MD methods. In this paper, we review our developed quantum chemical MD tribochemical simulator and its application to the tribochemical reaction dynamics of a few lubricant additives.

Effect of Surface Termination on Superlow Friction of Diamond Film: A Theoretical Study

We have applied molecular dynamics simulation and density functional theory calculations to analyze the effects of H and OH terminations on the frictional properties of diamond films at the atomistic and electronic levels. Molecular dynamics simulations were carried out for H-, OH-, and non-terminated diamond surfaces against an iron surface. Results of molecular dynamics simulations show that the frictional force is clearly decreased by the H or OH termination on the diamond surfaces. Moreover, results of density functional calculations show that a covalent bond is formed between Fe and C, while H- or OH-terminated diamond surfaces interact repulsively with an iron surface owing to antibonding interactions. We concluded that this interaction change between iron and diamond surfaces is the major contributing factor for achieving a low friction in H- or OH-terminated diamond.

Theoretical Study on Electronic and Electrical Properties of Nanostructural ZnO

The electronic and electrical properties of ZnO semiconductor single wall nanotube were investigated using periodic supercell approach within density functional theory combined with tight-binding quantum chemistry method. Armchair (10,10) and zigzag (10,0) nanotubes were considered. The lower strain energies required to roll up a ZnO graphitic sheet into a tube and the negative cohesive energies implied the possibility for the formation of ZnO single wall nanotubes. It was shown that the band gaps between the valence band maximum (VBM) and conduction band minimum (CBM) of nanotubes calculated by means of the two methods are similar and are larger than that of the bulk ZnO. It was found that the band gaps of ZnO nanotube are relatively insensitive to the chirality and diameter. According to the estimated electrical conductivities, the non-defect bulk and nanotube ZnO exhibited insulator properties, while they exhibited semiconductor properties when oxygen vacancies are introduced in the structures. The relative stability and band gap of fullerene-like ZnO clusters were also analyzed.

Statistical laws and vortex structures in fully developed turbulence

Fully developed turbulence is structured with a number of intense elongated vortices. Recognizing that statistical laws are related to such structures, the flow field is modeled by an ensemble of strained vortices (i.e. Burgers vortices) distributing randomly in space, from which probability density functions (pdfs) for longitudinal and transversal components of velocity difference are derived by taking statistical averages ensuring isotropy and homogeneity for the velocity field. It is found that the pdfs tend to close-to-exponential forms at small scales, and that there exist two scaling ranges in the structure function of every order, which are identified as the viscous range and inertial range, respectively, with a transition scale between the two ranges being at the order of mean size of Burgers vortices. Velocity structure functions show scaling behaviors in the second interval corresponding to the inertial range with the scaling exponents close to those known in the experiments and direct numerical simulations. It is remarkable that the Kolmogorovs four-fifths law is observed to be valid in a small-scale range. The scaling exponents of higher order structure functions are numerically estimated up to the 25th order. It is found that asymptotic scaling exponents, as the order increases, are in good agreement with the behavior of a recent experiment. The above model analysis is considered to represent successfully the statistical behaviors at small scales (possibly less than the Taylor microscale) and higher orders. The present statistical analysis leads to scale-dependent probability density functions.

Anti-wear Chemistry of ZDDP and Calcium Borate Nano-additive. Coupling Experiments, Chemical Hardness Predictions, and MD Calculations

Zinc dialkyl dithiophosphate (ZDDP) is an anti-wear additive for steel surfaces currently used in engine oils. Its anti-wear performance (against abrasion) is due to tribochemical reaction of zinc polyphosphate with abrasive metal oxides nanoparticles, under the combined effect of pressure and shear. However, phosphorous and sulfur are nocuous for environmental issues. Borates are possible candidates to replace phosphates in engine oils. Friction reduction with borates is found to be better than ZDDP but the anti-wear efficiency of borates is lower. In this work, we show how chemical hardness model and computer simulation can explain these different behaviors. Also we show that molecular dynamics is able to predict accurately behavior of mixtures of phosphates and borate. Results show that mixtures of additives with a P:B ratio slightly above unity can be a good compromise to have both good tribological performance and low content of phosphorous and sulfur in the lubricant.