Velocity fluctuations and hydrodynamic diffusion in sedimentation

We study non-equilibrium velocity fluctuations in a model for the sedimentation of non-Brownian particles experiencing long-range hydrodynamic interactions. The complex behavior of these fluctuations, the outcome of the collective dynamics of the particles, exhibits many of the features observed in sedimentation experiments. In addition, our model predicts a final relaxation to an anisotropic (hydrodynamic) diffusive state that could be observed in experiments performed over longer time ranges.Comment: 7 pages, 5 EPS figures, EPL styl

Free Thermal Convection Driven by Nonlocal Effects

We report and explain a convective phenomenon observed in molecular dynamics simulations that cannot be classified either as a hydrodynamics instability nor as a macroscopically forced convection. Two complementary arguments show that the velocity field by a thermalizing wall is proportional to the ratio between the heat flux and the pressure. This prediction is quantitatively corroborated by our simulations.Comment: RevTex, figures is eps, submited for publicatio

Stratified horizontal flow in vertically vibrated granular layers

A layer of granular material on a vertically vibrating sawtooth-shaped base exhibits horizontal flow whose speed and direction depend on the parameters specifying the system in a complex manner. Discrete-particle simulations reveal that the induced flow rate varies with height within the granular layer and oppositely directed flows can occur at different levels. The behavior of the overall flow is readily understood once this novel feature is taken into account.Comment: 4 pages, 6 figures, submitte

Numerical model of the temperature dependence of the up-conversion efficiency of fluoride crystals codoped with ytterbium and thulium

We review the role of temperature on the up-conversion efficiency of materials that are good candidates to be used as the three primary-color emitters in optically written displays. A rate equation model is given for the blue emitting material that includes temperature-dependent coefficients for energy transfer and cross relaxation. Increased temperature leads to decreased up-conversion efficiency. The effect of pumping conditions on heating the emitting material is illustrated and explained. Steps to reduce the temperature rise when pumping up-conversion materials are described

Magnetic friction due to vortex fluctuation

We use Monte Carlo and molecular dynamics simulation to study a magnetic tip-sample interaction. Our interest is to understand the mechanism of heat dissipation when the forces involved in the system are magnetic in essence. We consider a magnetic crystalline substrate composed of several layers interacting magnetically with a tip. The set is put thermally in equilibrium at temperature T by using a numerical Monte Carlo technique. By using that configuration we study its dynamical evolution by integrating numerically the equations of motion. Our results suggests that the heat dissipation in this system is closed related to the appearing of vortices in the sample.Comment: 6 pages, 41 figure

Polyelectrolyte stars in planar confinement

We employ monomer-resolved Molecular Dynamics simulations and theoretical considerations to analyze the conformations of multiarm polyelectrolyte stars close to planar, uncharged walls. We identify three mechanisms that contribute to the emergence of a repulsive star-wall force, namely: the confinement of the counterions that are trapped in the star interior, the increase in electrostatic energy due to confinement as well as a novel mechanism arising from the compression of the stiff polyelectrolyte rods approaching the wall. The latter is not present in the case of interaction between two polyelectrolyte stars and is a direct consequence of the impenetrable character of the planar wall.Comment: 34 pages, 8 figures. Revised version of the manuscript. To appear in J. Chem. Phys. May, 200

Soft disks in a narrow channel

The pressure components of "soft" disks in a two dimensional narrow channel are analyzed in the dilute gas regime using the Mayer cluster expansion and molecular dynamics. Channels with either periodic or reflecting boundaries are considered. It is found that when the two-body potential, u(r), is singular at some distance r_0, the dependence of the pressure components on the channel width exhibits a singularity at one or more channel widths which are simply related to r_0. In channels with periodic boundary conditions and for potentials which are discontinuous at r_0, the transverse and longitudinal pressure components exhibit a 1/2 and 3/2 singularity, respectively. Continuous potentials with a power law singularity result in weaker singularities of the pressure components. In channels with reflecting boundary conditions the singularities are found to be weaker than those corresponding to periodic boundaries

Using the fractional interaction law to model the impact dynamics in arbitrary form of multiparticle collisions

Using the molecular dynamics method, we examine a discrete deterministic model for the motion of spherical particles in three-dimensional space. The model takes into account multiparticle collisions in arbitrary forms. Using fractional calculus we proposed an expression for the repulsive force, which is the so called fractional interaction law. We then illustrate and discuss how to control (correlate) the energy dissipation and the collisional time for an individual article within multiparticle collisions. In the multiparticle collisions we included the friction mechanism needed for the transition from coupled torsion-sliding friction through rolling friction to static friction. Analysing simple simulations we found that in the strong repulsive state binary collisions dominate. However, within multiparticle collisions weak repulsion is observed to be much stronger. The presented numerical results can be used to realistically model the impact dynamics of an individual particle in a group of colliding particles.Comment: 17 pages, 8 figures, 1 table; In review process of Physical Review