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Motohiko Tanaka

Publications and source records attributed to Motohiko Tanaka.

13 recordsLinked to original sources
arXiv2024

Molecular dynamics simulations of ice and methane hydrate by means of the rotation coordinate TIP5P-Ewald model

Molecular dynamics simulations are utilized to study the microwave heating of methane hydrate by the five-body rotation coordinate system with the TIP5P-Ewald model. The structure I of methane hydrate is constructed, and the ice and free methane or methane hydrate are exposed to microwave electric fields of 10 GHz. Provisional methane hydrate of the normal density and a temperature of 273 K is dynamically unstable and collapses after some periods of irradiation. The period of a collapse time is $1.7 \times 10^{6} τ$ and the temperature increase is $ΔT \cong 61$ deg, with the external electric field $3 \times 10^{7} \rm{V/cm}$ (i.e. 0.3 V/Å) and $τ= 1 \times 10^{-14}$ s. For the ice and free methane of the temperature 193 K and the pressure 1 atm, the system is stable while it is heated under microwave irradiation. About the temperature of 273 K, high density methane hydrate becomes stable, whereas the density of 0.93 g/cm$^{3}$ is marginally stable but is heated when microwaves are present. In the microwave device of 1,000 V/cm and the pressure 1 atm, the simulation of the ice and methane points to 1 s in the 100\% microwave efficiency.

physics.chem-ph↗
arXiv2023

Microwave heating and collapse of methane hydrate by molecular dynamics simulations

Microwave heating of methane hydrate is investigated with electrostatic molecular dynamics simulations by the SPC/E water model. The structure I of methane hydrate is constructed. When the methane hydrate with a density of 0.91 $\rm{g/cm}^3$ and temperature of 273 K is exposed to microwave electric fields, it suddenly collapses in liquid after a certain period of irradiation. However, a hydrate with a five percent higher density of 0.95 $\rm{g/cm}^3$ and the same initial temperature shows no collapse as a crystal caused by the microwave. A hydrate with CO$_{2}$ guest molecules has increased kinetic energy but rapidly collapses due to the Lennard-Jones potentials. The methane hydrate with variable density and temperature is presented and exhibits slow heating as a crystal and an unchanging volume.

physics.chem-ph↗
arXiv2019

Relativistic and Electromagnetic Molecular Dynamics Simulations for a Carbon-Gold Nanotube Accelerator

Relativistic molecular dynamics are described in ultra-high temperature and MeV energy behaviors. In strongly-coupled systems, the Coulomb electrostatic field is collected in the infinite space, and the electromagnetic fields are added in the coordinate space. Separation of the electromagnetic and electrostatic electric fields is a good approximation for short time periods. For a numerical application, a nanotube accelerator under an $E \times B$ pulse is studied. Positive ions are accelerated in the parallel direction, whereas the electrons proceed in the perpendicular direction. Rapid expansion to infinite space and short-range electromagnetic radiation cooperate for large intensities. At $10^{22}\rm{W/cm}^{2}$, pulsation oscillations for gold and carbon ions flare up and electrons acquire the relativistic velocities. They are observed in relativistic molecular dynamics simulation.

physics.app-ph↗
arXiv2009

Selective Heating Mechanism of Magnetic Metal Oxides by Alternating Magnetic Field in Microwave Sintering Process

The mechanism of rapid and selective heating of magnetic metal oxides under the magnetic field of microwaves which continues beyond the Curie temperature $ T_{c} $ is identified by using the Heisenberg model. Monte Carlo calculations based on the energy principle show that such heating is caused by non-resonant response of electron spins in the unfilled 3d shell to the wave magnetic field. Small spin reorientation thus generated leads to a large internal energy change through the exchange interactions between spins, which becomes maximal around $ T_{c} $ for magnetite $ {\rm Fe}_{3}{\rm O}_{4} $. The dissipative spin dynamics simulation yields the imaginary part of the magnetic susceptibility, which becomes largest around $ T_{c} $ and for the microwave frequency around 2 GHz. Hematite $ {\rm Fe}_{2}{\rm O}_{3} $ with weak spontaneous magnetization responds much less to microwaves as observed in experiments. The heating of titanium oxide by microwave magnetic field only when oxygen defects are present is also explained by our theory in terms of the absence of spontaneous magnetization.

cond-mat.mtrl-sci↗
arXiv2008

Enhanced heating of salty water and ice under microwaves: Molecular dynamics study

By molecular dynamics simulations, we have studied the enhanced heating process of salty ice and water by the electric field of applied microwaves at 2.5 GHz, and those in the range 2.5-10 GHz for the frequency dependence. We show that water molecules in salty ice are allowed to rotate in response to the microwave electric field to the extent comparable to those in pure water, because the molecules in salty ice are loosely tied by hydrogen bonds with adjacent molecules unlike the case of rigidly bonded pure ice. The weakening of hydrogen-bonded network of molecules in salty ice is mainly caused by the electrostatic effect of salt ions rather than the short-range geometrical (atom size) effect of salt ions since the presence of salt ions with small radii results in similar enhanced heating.

cond-mat.mtrl-sci↗
arXiv2006

Microwave Heating of Water, Ice and Saline Solution: Molecular Dynamics Study

In order to study the heating process of water by the microwaves of 2.5-20GHz frequencies, we have performed molecular dynamics simulations by adopting a non-polarized water model that have fixed point charges on rigid-body molecules. All runs are started from the equilibrated states derived from the I$_{c}$ ice with given density and temperature. In the presence of microwaves, the molecules of liquid water exhibit rotational motion whose average phase is delayed from the microwave electric field. Microwave energy is transferred to the kinetic and inter-molecular energies of water, where one third of the absorbed microwave energy is stored as the latter energy. The water in ice phase is scarcely heated by microwaves because of the tight hydrogen-bonded network of water molecules. Addition of small amount of salt to pure water substantially increases the heating rate because of the weakening by defects in the water network due to sloshing large-size negative ions.

cond-mat.mtrl-sci↗
arXiv2004

DNA in nanopore-counterion condensation and coion depletion

Molecular dynamics simulations are used to study the equilibrium distribution of monovalent ions in a nanopore connecting two water reservoirs separated by a membrane, both for the empty pore and that with a single stranded DNA molecule inside. In the presence of DNA, the counterions condense on the stretched macromolecule effectively neutralizing it, and nearly complete depletion of coions from the pore is observed. The implications of our results for experiments on DNA translocation through alpha-hemolysin nanopores are discussed.

cond-mat.soft↗
arXiv2004

PC Cluster Machine Equipped with Low-Latency Communication Software

A high performance Beowulf (PC cluster) machine installed with Linux operating system and MPI (Message Passing Interface) for interprocessor communications has been constructed using Gigabit Ethernet and the communication software GAMMA (Genoa Active Message Machine), instead of the standard TCP/IP protocol. Fast C/Fortran compilers have been exploited with the GAMMA communication libraries. This method has eliminated large communication overhead of TCP/IP and resulted in significant increase in the computational performance of real application programs including the first-principle molecular dynamics simulation code. Scalability on the number of processors and reliability over a long period of time (days) have also been confirmed for the GAMMA communications. Keywords: non TCP/IP, active messages, small latency, fast C/Fortran compilers, first-principle (quantum mechanical) molecular dynamics, advanced materials

physics.comp-ph↗
arXiv2004

Electrophoresis of a rod macroion under polyelectrolyte salt: Is mobility reversed for DNA?

By molecular dynamics simulation, we study the charge inversion phenomenon of a rod macroion in the presence of polyelectrolyte counterions. We simulate electrophoresis of the macroion under an applied electric field. When both counterions and coions are polyelectrolytes, charge inversion occurs if the line charge density of the counterions is larger than that of the coions. For the macroion of surface charge density equal to that of the DNA, the reversed mobility is realized either with adsorption of the multivalent counterion polyelectrolyte or the combination of electrostatics and other mechanisms including the short-range attraction potential or the mechanical twining of polyelectrolyte around the rod axis.

cond-mat.soft↗
arXiv2003

The Effects of Asymmetric Salt and a Cylindrical Macroion on Charge Inversion: Electrophoresis by Molecular Dynamics Simulations

The charge inversion phenomenon is studied by molecular dynamics simulations, focusing on size and valence asymmetric salts, and a threshold of surface charge density for charge inversion. The charge inversion criteria by the electrophoretic mobility and the radial distribution functions of ions coincide except around the charge inversion threshold. The reversed electrophoretic mobility increases with the ratio of coion to counterion radii, while it decreases with the ratio of coion to counterion valences. The monovalent salt enhances charge inversion of a strongly charged macroion at small ionic strength, but it reduces reversed mobility otherwise. A cylindrical macroion is more persistent to monovalent salt than a spherical macroion of the same radius and surface charge density.

cond-mat.soft↗
arXiv2002

Electrophoresis of a charge inverted macroion complex: Molecular dynamics study

We have performed molecular dynamics simulations to study the effect of an external electric field on a macroion in the solution of multivalent Z:1 salt. To obtain plausible hydrodynamics of the medium, we explicitly make the simulation of many neutral particles along with ions. In a weak electric field, the macroion drifts together with the strongly adsorbed multivalent counterions along the electric field, in the direction proving inversion of the charge sign. The reversed mobility of the macroion is insensitive to the external field, and increases with salt ionic strength. The reversed mobility takes a maximal value at intermediate counterion valence. The motion of the macroion complex does not induce any flow of the neutral solvent away from the macroion, which reveals screening of the hydrodynamic interactions at short distances in electrolyte solutions. A very large electric field, comparable to the macroion unscreened field, disrupts charge inversion by stripping the adsorbed counterions off the macroion.

cond-mat.soft↗
arXiv2001

Giant Charge Inversion of a Macroion Due to Multivalent Counterions and Monovalent Coions: Molecular Dynamics Study

We report molecular dynamics simulation of the (overall neutral) system consisting of an immobile macroion surrounded by the electrolyte of multivalent counterions and monovalent coions. As expected theoretically, counterions adsorb on the macroion surface in the amount much exceeding neutralization requirement, thus effectively inverting the sign of the macroion charge. We find two conditions necessary for charge inversion, namely, counterions must be multivalently charged and Coulomb interactions must be strong enough compared to thermal energy. On the other hand, coion condensation on the multivalent counterions similar to Bjerrum pairing is the major factor restricting the amount of charge inversion. Depending on parameters, we observe inverted charge up to about 200% the original charge of the macroion in absolute value. Our findings are consistent with the theory of "giant charge inversion" [Phys.Rev.Lett., v. 85, 1568 (2000)].

cond-mat.soft↗