Shuji Shimamura

Fast convergence to equilibrium for long-chain polymer melts using a MD/continuum hybrid method

Effective and fast convergence toward an equilibrium state for long-chain polymer melts is realized by a hybrid method coupling molecular dynamics and the elastic continuum. The required simulation time to achieve the equilibrium state is reduced compared with conventional equilibration methods. The polymers move on a wide range phase space due to large-scale fluctuation generated by the elastic continuum. A variety of chain structures is generated in the polymer melt which results in the fast convergence to the equilibrium state.

Atomic force microscopy simulation by MD/continuum coupling method

We have performed atomic force microscopy (AFM) simulations to understand the microscopic mechanism of an AFM experiment, especially the observed energy dissipation. The oscillation of a cantilever in AFM is described by spring motion, and the atomic interaction between the tip attached on cantilever and surface is calculated by the molecular dynamics (MD) method. In order to couple the spring motion with the atomic dynamics, we use the MD/continuum coupling method which was developed by our group. We propose a simple computational model using Lennard Jones interatomic potential. As the spring approaches the surface, the atomic interaction between the tip and surface increases and it perturbs harmonic oscillation of the spring with the frequency shifted and the amplitude damped. The kinetic energy of the spring is transferred to the atoms on the surface. It is shown that this energy dissipation comes from two atomic processes: irreversible atomic dynamics and atomic thermal fluctuation.

Model calculation of the pore-size and porosity dependences of bulk moduli in nanoporous materials

The bulk moduli of nanoporous materials are studied theoretically on the basis of an interatomic potential. We consider model nanoporous materials in which nanometer-sized cubic-shaped pores are periodically arranged in the fcc lattice. The pore-size and porosity dependences of bulk modulus are calculated exactly for the model nanoporous materials. It is shown that the bulk moduli of model nanoporous materials decrease moderately with increasing porosity. If nanoporous materials of the same porosity are compared, the bulk modulus increases and becomes constant with increasing pore-size. It is suggested from the calculation results that nanoporous materials without defects have higher bulk moduli than materials having micrometer-sized pores.

Segregation Diagram of a Mixture of Particles with Different Sizes and Densities

Segregation in granular materials has been studied by means of Monte Carlo simulation. The mechanism of segregation in a binary mixture of particles with different sizes and densities has been investigated for shaking and rotating processes in two-dimensional space. From the simulations we have obtained the segregation diagrams for shaking and rotating. These diagrams show what type of segregation occurs in a binary mixture with given values of the size ratio and the density ratio. From the diagrams we can also know what conditions are required to avoid segregation and mix uniformly a mixture of particles with different sizes and densities by the shaking and fast rotating processes. It is also shown that in the shaking process there is a critical size-ratio above which large-sized particles always segregate upward irrespective of the density ratio. Some implications of the segregation diagrams for shaking and rotating obtained by our simulations are discussed with consideration for real granular materials.