Yasuhiro Senda

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.

A methodology for coupling an atomic model with a continuum model using an extended Lagrange function

We propose a hybrid method combining an atomic model and a continuum model, in which the displacement field of the continuum is introduced as a new degree of freedom by extending Andersens Lagrange function for constant-pressure molecular dynamics. We applied our method to a one-dimensional hybrid model which is composed of an atomic chain and springs. Large-scale fluctuation of the atomic system is found in the hybrid model. The density of states of the phonon is derived, and the large-scale fluctuation induces the generation of a variety of states of phonons. It is shown that the hybrid model proposed by our methodology enables us to perform large-scale simulations without intensive computations.

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.

Stability of Tip in Adhesion Process on Atomic Force Microscopy Studied by Coupling Computational Model

We investigated the stability of ionic configurations of the tip of the cantilever in non-contact AFM.; For this, we used a computational model that couples the ionic motion of the MgO surface and the oscillating cantilever. The motion of ions was connected to the oscillating cantilever using a coupling method that had been recently developed. The adhesive process on the ionic MgO surface leads to energy dissipation of the cantilever. It is shown that limited types of ionic configurations of the tip are stable during the adhesive process. Based on the present computational model, we discuss the adhesive mechanism leading to energy dissipation.

Computational model for noncontact atomic force microscopy: Energy dissipation of cantilever

We propose a computational model for noncontact atomic force microscopy (AFM) in which the atomic force between the cantilever tip and the surface is calculated using a molecular dynamics method, and the macroscopic motion of the cantilever is modeled by an oscillating spring. The movement of atoms in the tip and surface is connected with the oscillating spring using a recently developed coupling method. In this computational model, the oscillation energy is dissipated, as observed in AFM experiments. We attribute this dissipation to the hysteresis and nonconservative properties of the interatomic force that acts between the atoms in the tip and sample surface. The dissipation rate strongly depends on the parameters used in the computational model.

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.