Plastic flow in polycrystal states in a binary mixture.

Using molecular dynamics simulation, we examine the dynamics of sheared polycrystal states in a binary mixture composed of small and large particles in two dimensions. We vary the composition c of the large particles and the shear rate ._gamma to realize changeovers among crystal, polycrystal, and glass. We find large stress fluctuations arising from sliding motions of the particles at the grain boundaries, which occur cooperatively to release the elastic energy stored. These stress fluctuations decrease as the system crosses over from polycrystal to glass. The dynamic processes are visualized with the aid of a sixfold angle alpha(j)(t) representing the local crystal orientation and a disorder variable D(j)(t) representing a deviation from the hexagonal order for particle j.

Heterogeneous dynamics in polycrystal and glass in a binary mixture with changing size dispersity and composition.

Using molecular dynamics simulation we investigate the dynamics in a two-dimensional binary mixture at a low temperature and at high densities. We increase the size ratio of the diameters of the two components or the fraction of the larger particles. Then changeovers occur from polycrystal to glass with proliferation of defects. The relationship between the degree of disorder and the slow dynamics is studied by simultaneous visualization of a disorder variable D{j} introduced in our previous paper T. Hamanaka and A. Onuki[Phys. Rev. E 74, 011506 (2006)] and the particle displacement Deltar{j} in a long time interval. In polycrystal, the particles in the grain boundary regions have large D{j} and a relatively large mobility, producing significant dynamic heterogeneity on long time scales. In the crossover from polycrystal to glass, the crystalline regions become narrow, but the particles with relatively large D{j} still trigger the formation of chainlike particle motions, leading to the dynamic heterogeneity.

Transitions among crystal, glass, and liquid in a binary mixture with changing particle-size ratio and temperature.

Using molecular dynamics simulation we examine changeovers among crystal, glass, and liquid at high density in a two dimensional binary mixture. We change the ratio between the diameters of the two components and the temperature. The transitions from crystal to glass or liquid occur with proliferation of defects. We visualize the defects in terms of a disorder variable Dj(t) representing a deviation from the hexagonal order for particle j. The defect structures are heterogeneous and are particularly extended in polycrystal states. They look similar at the crystal-glass crossover and at the melting. Taking the average of Dj(t) over the particles, we define a disorder parameter D(t), which measures the degree of overall disorder. The relaxation of D(t) after quenching becomes slow at low temperature in the presence of size dispersity. Its steady state average is small in crystal and large in glass and liquid. We also find that grain boundaries tend to be pinned with increasing size dispersity in polycrystal states.

Molecular dynamics simulation of heat conduction in near-critical fluids.

Using molecular dynamics simulations we study supercritical fluids near the gas-liquid critical point under heat flow in two dimensions. We calculate the steady-state temperature and density profiles. The resultant thermal conductivity exhibits critical singularity in agreement with the mode-coupling theory in two dimensions. We also calculate distributions of the momentum and heat fluxes at fixed density. They indicate that liquidlike (entropy-poor) clusters move toward the warmer boundary and gaslike (entropy-rich) regions move toward the cooler boundary in a temperature gradient. This counterflow results in critical enhancement of the thermal conductivity.