Keiko M. Aoki

Experimental study on vibration-induced convection and heaping in granular beds

Abstract This paper presents an experimental study on the role of interstitial gas in the vibration-induced granular convection and heaping. Two kinds of experiments were made: (i) flow visualization of granular convection, and (ii) investigation of the effects of opening of the top of a vessel wherein granules are contained. It has been observed from the first experiment that uniform mobility of granular materials in the container results in a symmetric pair of convection rolls covering the entire range of the bed, which leads to the heap formation. The second experiment has revealed that, due to the difference in the vessel top opening, a large difference in the shape of the free surface occurs for some range of the granular bed height, suggesting that not only the magnitude of gas pressure, but also how the gas percolates through the granular bed play important roles in the heaping process.

Symplectic Integrators Designed for Simulating Soft Matter

Symplectic integrators designed for simulating soft matter at constant temperature and constant pressure (or constant surface tension) are presented. In addition to the well-known merits of symplectic integrators, such as long time stability, these methods allow the estimation of thermal heat production/absorption and thus entropy differences through a simple function of the time scaling factor ν. The relative entropy estimation around crystal–liquid phase transition is given for systems with soft core repulsive potential.

A fractal analysis of solids mixing in two-dimensional vibrating particle beds

Using flow visualization and digital image processing we have shown experimentally that convective motions and mixing of particles within vertically vibrating two-dimensional particle beds are amenable to characterization by fractal properties. The time profile of a fractal dimension presents quantitative information on the rate and degree of mixing. Thus, the fractal dimension has been used to show the variation in the rate of mixing along the bed height. Numerical studies based on molecular dynamics simulations confirmed the fractal properties of the system. The effect of bed height on the regional variations in the rate of mixing is explained in terms of kinetic energy per particle.

Investigation of Liquid Crystalline Phases by Means of Constant-Pressure Molecular-Dynamics Simulation

Abstract Constant pressure molecular-dynamics (MD) simulations of systems consisting of soft spherocylinders with both translational and orientational freedom are performed to study the behavior of liquid crystalline phases and its transitions. In order to investigate to what extent can the properties known to real liquid crystalline phases be explained by the anisotropy of the shape of the molecules alone, the MD simulation uses purely repulsive short-range pair potentials representing soft spherocylinders. Both the nematic and smectic phases are simulated and the properties of each phase are investigated.

Molecular dynamic simulation methods for anisotropic liquids

Methods of molecular dynamics simulations for anisotropic molecules are presented. The new methods, with an anisotropic factor in the cell dynamics, dramatically reduce the artifacts related to cell shapes and overcome the difficulties of simulating anisotropic molecules under constant hydrostatic pressure or constant volume. The methods are especially effective for anisotropic liquids, such as smectic liquid crystals and membranes, of which the stacks of layers are compressible (elastic in direction perpendicular to the layers) while the layer itself is liquid and only elastic under uniform compressive force. The methods can also be used for crystals and isotropic liquids as well.

Percolation in Penrose tiling and its dual: in Comparison with Analysis for Kagomé, Dice and Square lattices

Abstract We combine the idea of finite-size scaling and Monte Carlo simulations to study percolation in Penrose tiling (nonperiodic with mixed-valued coordination), its dual (non periodic with coordination number z = 4), dice (periodic with mixed-valued coordination), kagome (periodic with z = 4), and square (periodic with z = 4). We estimate percolation thresholds and some critical exponents of these lattices both for bond and site percolation 1 . Our analysis suggests that ‘ universality ’ concerning critical exponents holds even in lattices without periodicity and/or without single-valued coordination.

A fractal property of vertically vibrated beds of granules

We have shown experimentally and using particle dynamics simulations that convective motions and mixing of particles within vertically vibrating particle beds are amenable to characterization by a fractal property

Order Parameter Discretization in Metastable States of Hexatic Smectic B Liquid Crystal

Hexatic smectic B liquid crystal (HexB) is studied by molecular dynamics simulations for a wide range of temperatures. In addition to the thermodynamic equilibrium HexB phase, there exist multiple metastable states with a smaller sixfold bond orientational order C 6 than the thermodynamic equilibrium phase. The values of C 6 are discrete, with each metastable state having different thermodynamic characteristics. 12- and 18-fold bond orientational orders C 6 n ( n = 2, 3) satisfy the scaling relation C 6 n = C 6 σ n for a wide range. The metastable HexB states are nested and each layer also has a discrete value of bond order forming these discrete states as a whole.

Molecular Dynamics Simulations of Liquid Crystal Phase Transitions

Abstract Phase transitions in a sequence of crystal(C)-smecticA(SmA)-nematic(N)-isotropic(I) phases are investigated by constant pressure molecular-dynamics (MD) simulations, in which the hydrostatic pressure is preserved. The present MD simulation uses a simple model that represents soft spherocylinders with both translational and orientational freedoms. This model enables us to investigate to what extent the properties known to real liquid crystal phases can be explained by the anisotropy of the molecular shape alone. The phase transitions are manifested not only in the macroscopic properties, such as enthalpy, but also in the microscopic properties as well. Anisotropic diffusion of molecules in liquid crystal phases is also investigated.

Extended methods of molecular dynamic simulations under hydrostatic pressure and/or isostress

By introducing new cell dynamics to the Parrinello–Rahman method, we propose extended methods for molecular dynamics simulations under hydrostatic pressure and/or isostress. The new method leads straightforwardly to the virial theorem. The obtained equations of motions are invariant under transformations between spatially periodic simulation cells. Molecular dynamics simulations of the crystalline phases of spheres and elongated molecules are conducted.