A self-similar model for shear flows in dense granular materials

We propose a model to describe the quasistatic shearing of dry granular materials, which notably captures the differences in velocity profiles recently observed in 2 and 3-D Couette flow experiments. In our scheme, the steady-state flow is due to the intermittent motion of particle clusters moving together with the wall. The motion of a cluster is associated with the transient formation of a fracture inside the sheared pack. The model is based on the existence of a persistence length for the fractures, which imposes a self-similar structure on the clusters. Through a probabilistic approach, we can evaluate the rate of appearance of a cluster of a given size and obtain a prediction for the average velocity profiles. We also predict the existence of large stress fluctuations at the moving wall, which characteristics are in good agreement with experimental data.Comment: 7 pages, 2 figures, correction of the tex

A 2-D asymmetric exclusion model for granular flows

A 2-D version of the asymmetric exclusion model for granular sheared flows is presented. The velocity profile exhibits two qualitatively different behaviors, dependent on control parameters. For low friction, the velocity profile follows an exponential decay while for large friction the profile is more accurately represented by a Gaussian law. The phase transition occurring between these two behavior is identified by the appearance of correlations in the cluster size distribution. Finally, a mean--field theory gives qualitative and quantitative good agreement with the numerical results.Comment: 13 pages, 5 figures; typos added, one definition change

Anomalous diffusion mediated by atom deposition into a porous substrate

Constant flux atom deposition into a porous medium is shown to generate a dense overlayer and a diffusion profile. Scaling analysis shows that the overlayer acts as a dynamic control for atomic diffusion in the porous substrate. This is modeled by generalizing the porous diffusion equation with a time-dependent diffusion coefficient equivalent to a nonlinear rescaling of timeComment: 4 page

Stability and individual variability of social attachment in imprinting

Filial imprinting has become a model for understanding memory, learning and social behaviour in neonate animals. This mechanism allows the youngs of precocial bird species to learn the characteristics of conspicuous visual stimuli and display affiliative response to them. Although longer exposures to an object produce stronger preferences for it afterwards, this relation is not linear. Sometimes, chicks even prefer to approach novel rather than familiar objects. To date, little is known about how filial preferences develop across time. This study aimed to investigate filial preferences for familiar and novel imprinting objects over time. After hatching, chicks were individually placed in an arena where stimuli were displayed on two opposite screens. Using an automated setup, the duration of exposure and the type of stimuli were manipulated while the time spent at the imprinting stimulus was monitored across 6 days. We showed that prolonged exposure (3 days vs 1 day) to a stimulus produced robust filial imprinting preferences. Interestingly, with a shorter exposure (1 day), animals re-evaluated their filial preferences in functions of their spontaneous preferences and past experiences. Our study suggests that predispositions influence learning when the imprinting memories are not fully consolidated, driving animal preferences toward more predisposed stimuli

Memory effects in classical and quantum mean-field disordered models

We apply the Kovacs experimental protocol to classical and quantum p-spin models. We show that these models have memory effects as those observed experimentally in super-cooled polymer melts. We discuss our results in connection to other classical models that capture memory effects. We propose that a similar protocol applied to quantum glassy systems might be useful to understand their dynamics.Comment: 24 pages, 12 figure

A continuous non-linear shadowing model of columnar growth

We propose the first continuous model with long range screening (shadowing) that described columnar growth in one space dimension, as observed in plasma sputter deposition. It is based on a new continuous partial derivative equation with non-linear diffusion and where the shadowing effects apply on all the different processes.Comment: Fast Track Communicatio

Long range correlations in the non-equilibrium quantum relaxation of a spin chain

We consider the non-stationary quantum relaxation of the Ising spin chain in a transverse field of strength h. Starting from a homogeneously magnetized initial state the system approaches a stationary state by a process possessing quasi long range correlations in time and space, independent of the value of $h$. In particular the system exhibits aging (or lack of time translational invariance on intermediate time scales) although no indications of coarsening are present.Comment: 4 pages RevTeX, 2 eps-figures include

Geometric Laws of Vortex Quantum Tunneling

In the semiclassical domain the exponent of vortex quantum tunneling is dominated by a volume which is associated with the path the vortex line traces out during its escape from the metastable well. We explicitly show the influence of geometrical quantities on this volume by describing point vortex motion in the presence of an ellipse. It is argued that for the semiclassical description to hold the introduction of an additional geometric constraint, the distance of closest approach, is required. This constraint implies that the semiclassical description of vortex nucleation by tunneling at a boundary is in general not possible. Geometry dependence of the tunneling volume provides a means to verify experimental observation of vortex quantum tunneling in the superfluid Helium II.Comment: 4 pages, 2 figures, revised version to appear in Phys. Rev.

The flow of a very concentrated slurry in a parallel-plate device: influence of gravity

We investigate, both experimentally and theoretically, the fow and structure of a slurry when sheared between 2 horizontal plates. The slurry, otherwise called a "wet granular material", is made of non-Brownian particles immersed in a viscous fluid. The particles are heavier than the fluid, consequently, gravity influences the structure and flow profiles of the sheared material. Experiments are carried out in a plane Couette device, with a model slurry composed of approximately monodisperse spherical PMMA particles in oil, at high average solid concentration (about 58%). Optical observation reveals a typical 2-phase configuration, with a fluidized layer in contact with the upper plate and on top of an amorphous solid phase. We provide data on velocity profiles, wall-slip and shear stress versus the average shear rate. To interpret the data, we propose a model for the ideal case of infinite horizontal flat plates. The model, of mean field type, is based on local constitutive equations for the tangential and normal components of the stress tensor and on expressions relating the material viscometric coefficients (the shear viscosity eta and the normal viscosity psi) with the local concentration (phi) and the local shear rate. 1-,2- and 3-phase configurations are predicted, with non linear flow and concentration profiles. We conclude that the model equations correctly describe the experimental data, provided that appropriate forms are chosen for the divergence of eta and psi near the packing concentration (phi_max), namely a (phi_max-phi)^-1 singularity.Comment: 26 pages, 12 figures ; submitted to Physics of Fluid