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P Viot

Publications and source records attributed to P Viot.

At least 19 recordsLinked to original sources
PubMed2005

Boltzmann equation for a granular capped rectangle in a thermalized bath of hard disks.

By using the Boltzmann approach, we study the steady-state dynamics of a granular capped rectangle placed in a two-dimensional bath of thermalized hard disks. Hard core collisions are assumed elastic between disks and inelastic between the capped rectangle and the disks, with a normal coefficient of restitution alpha < 1. Assuming a Gaussian ansatz for the probability distribution functions, we obtain analytical expressions for the granular temperatures. We show the absence of equipartition and investigate both the role of the anisotropy of the capped rectangle and of the relative ratio of the bath particles to the linear sizes of the capped rectangle. In addition, we investigate a model of a capped rectangle with two normal coefficients of restitution for collisions along the straight and curved surfaces of the capped rectangle. In this case one observes equipartition for a nontrivial ratio of the normal coefficient of restitutions.

Journal Article↗
PubMed2004

Thermalization of an anisotropic granular particle.

We investigate the dynamics of a needle in a two-dimensional bath composed of thermalized point particles. Collisions between the needle and points are inelastic and characterized by a normal restitution coefficient alpha<1. By using the Enskog-Boltzmann equation, we obtain analytical expressions for the translational and rotational granular temperatures of the needle and show that these are, in general, different from the bath temperature. The translational temperature always exceeds the rotational one, though the difference decreases with increasing moment of inertia. The predictions of the theory are in very good agreement with numerical simulations of the model.

Anisotropy↗
PubMed2004

Statistical mechanical description of the parking-lot model for vibrated granular materials.

We apply the statistical mechanical approach proposed by Edwards and co-workers to the parking-lot model, a model that reproduces the main features of the phenomenology of vibrated granular materials. We first build the compactivity-based measure for the case of vanishingly small tapping strength and then generalize the approach to finite tapping strengths by introducing a "thermodynamic" parameter, the available volume for particle insertion, in addition to the particle density. This description is able to take into account the various memory effects observed in vibrated granular media. Although not exact, the approach gives a good description of the behavior of the parking-lot model in the regime of slow compaction.

Journal Article↗
PubMed2002

Langevin dynamics of the Coulomb frustrated ferromagnet: A mode-coupling analysis.

We study the Langevin dynamics of the soft-spin, continuum version of the Coulomb-frustrated Ising ferromagnet. By using the dynamical mode-coupling approximation, supplemented by reasonable approximations for describing the equilibrium static correlation function, and the somewhat improved dynamical self-consistent screening approximation, we find that the system displays a transition from an ergodic to a nonergodic behavior. This transition is similar to that obtained in the idealized mode-coupling theory of glass-forming liquids and in the mean-field generalized spin glasses with one-step replica symmetry breaking. The significance of this result and the relation to the appearance of a complex free-energy landscape are also discussed.

Journal Article↗
PubMed2002

Wall-enhanced convection in vibrofluidized granular systems.

An event-driven molecular dynamics simulation of inelastic hard spheres contained in a cylinder and subject to strong vibration reproduces accurately experimental results [R. D. Wildman et al., Phys. Rev. Lett. 86, 3304 (2001)] for a system of vibrofluidized glass beads. In particular, we are able to obtain the velocity field and the density and temperature profiles observed experimentally. In addition, we show that the appearance of convection rolls is strongly influenced by the value of the sidewall-particle restitution coefficient. Suggestions for observing more complex convection patterns are proposed.

Journal Article↗
PubMed2002

Evidence for "fragile" glass-forming behavior in the relaxation of Coulomb frustrated three-dimensional systems.

We show by means of a Monte Carlo simulation study that three-dimensional models with long-range frustration display the generic phenomena seen in fragile glass-forming liquids. Due to their properties (absence of quenched disorder, physical motivation in terms of structural frustration, and tunable fragility), these systems appear as promising minimal theoretical models for describing the glass transition of supercooled liquids.

Journal Article↗
PubMed2001

Monte Carlo study of the three-dimensional Coulomb frustrated Ising ferromagnet.

We have investigated, by Monte Carlo simulation, the phase diagram of a three-dimensional Ising model with nearest-neighbor ferromagnetic interactions and small, but long-range (Coulombic) antiferromagnetic interactions. We have developed an efficient cluster algorithm and used different lattice sizes and geometries, which allows us to obtain the main characteristics of the temperature-frustration phase diagram. Our finite-size scaling analysis confirms that the melting of the lamellar phases into the paramagnetic phase is driven first order by the fluctuations. Transitions between ordered phases with different modulation patterns are observed in some regions of the diagram, in agreement with a recent mean-field analysis.

Journal Article↗
PubMed2000

Adsorption-desorption model and its application to vibrated granular materials.

We investigate both analytically and by numerical simulation the kinetics of a microscopic model of hard rods adsorbing on a linear substrate, a model that is relevant for compaction of granular materials. The computer simulations use an event-driven algorithm that is particularly efficient at very long times. For a small, but finite desorption rate, the system reaches an equilibrium state very slowly, and the long-time kinetics display three successive regimes: an algebraic one where the density varies as 1/t, a logarithmic one where the density varies as 1/ln(t), followed by a terminal exponential approach. The characteristic relaxation time of the final regime, though incorrectly predicted by mean field arguments, can be obtained with a systematic gap-distribution approach. The density fluctuations at equilibrium are also investigated, and the associated time-dependent correlation function exhibits a power law regime followed by a final exponential decay. Finally, we show that denser particle packings can be obtained by varying the desorption rate during the process.

Adsorption↗
PubMed2000

Phase diagram of an ising model with long-range frustrating interactions: A theoretical analysis

We present a theoretical study of the phase diagram of a frustrated Ising model with nearest-neighbor ferromagnetic interactions and long-range (Coulombic) antiferromagnetic interactions. For nonzero frustration, long-range ferromagnetic order is forbidden, and the ground state of the system consists of phases characterized by periodically modulated structures. At finite temperatures, the phase diagram is calculated within the mean-field approximation. Below the transition line that separates the disordered and the ordered phases, the frustration-temperature phase diagram displays an infinite number of "flowers," each flower being made by an infinite number of modulated phases generated by structure combination branching processes. The specificities introduced by the long-range nature of the frustrating interaction and the limitation of the mean-field approach are finally discussed.

Journal Article↗
PubMed1998

A Particle-Level Model of Irreversible Protein Adsorption with a Postadsorption Transition.

Modeling the kinetics of protein adsorption at solid surfaces is needed to predict protein separations, design biosensors, and determine the body's initial response to foreign objects. We develop, at the particle level, a kinetic model that accounts geometrically for the surface blockage due to adsorption and postadsorption conformational (or orientational) transitions. Proteins are modeled as disk-shaped particles of diameter final sigmaalpha that adsorb irreversibly at random positions onto a surface at a rate kac (c is the concentration of protein in the bulk solution). Adsorption occurs only where the surface is empty. Following adsorption, a particle attempts to spread (symmetrically) to a larger diameter final sigmabeta at a rate ks. Spreading only occurs if no overlap with any previously placed particle would result. A set of equations is developed for determining the time evolution of the adsorbed protein density. These predictions are compared to new experimental data for fibronectin onto silica-titania obtained using optical waveguide lightmode spectroscopy (OWLS). We also discuss the general application of this model to experimental data. Copyright 1998 Academic Press.

Journal Article↗