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تسجيل مستخدم جديدJamming and the onset of granulation in a model particle system
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Granulation is a ubiquitous process crucial for many products ranging from food and care products to pharmaceuticals. Granulation is the process in which a powder is mixed with a small amount of liquid (binder) to form solid agglomerates surrounded by air. By contrast, at low solid volume fractions {phi}, the mixing of solid and liquid produces suspensions. At intermediate {phi}, either granules or dense suspensions are produced, depending on the applied stress. We address the question of how and when high shear mixing can lead to the formation of jammed, non-flowing granules as {phi} is varied. In particular, we measure the shear rheology of a model system - a suspension of glass beads with an average diameter of $sim$ 7 {mu}m - at solid volume fractions {phi} $gtrsim$ 0.40. We show that recent insights into the role of inter-particle friction in suspension rheology allow us to use flow data to predict some of the boundaries between different types of granulation as {phi} increases from $sim$ 0.4 towards and beyond the maximum packing point of random close packing.
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We investigate the jamming transition in a quasi-2D granular material composed of regular pentagons or disks subjected to quasistatic uniaxial compression. We report six major findings based on experiments with monodisperse photoelastic particles wit
h static friction coefficient $muapprox 1$. (1) For both pentagons and disks, the onset of rigidity occurs when the average coordination number of non-rattlers, $Z_{nr}$ , reaches 3, and the dependence of $Z_{nr}$ on the packing fraction $phi$ changes again when $Z_{nr}$ reaches 4. (2) Though the packing fractions $phi_{c1}$ and $phi_{c2}$ at these transitions differ from run to run, for both shapes the data from all runs with different initial configurations collapses when plotted as a function of the non-rattler fraction. (3) The averaged values of $phi_{c1}$ and $phi_{c2}$ for pentagons are around 1% smaller than those for disks. (4) Both jammed pentagons and disks show Gamma distribution of the Voronoi cell area with same parameters. (5) The jammed pentagons have similar translational order for particle centers but slightly less orientational order for contacting pairs comparing to jammed disks. (6) For jammed pentagons, the angle between edges at a face-to-vertex contact point shows a uniform distribution and the size of a cluster connected by face-to-face contacts shows a power-law distribution.
The jamming scenario of disordered media, formulated about 10 years ago, has in recent years been advanced by analyzing model systems of granular media. This has led to various new concepts that are increasingly being explored in in a variety of syst
ems. This chapter contains an introductory review of these recent developments and provides an outlook on their applicability to different physical systems and on future directions. The first part of the paper is devoted to an overview of the findings for model systems of frictionless spheres, focussing on the excess of low-frequency modes as the jamming point is approached. Particular attention is paid to a discussion of the cross-over frequency and length scales that govern this approach. We then discuss the effects of particle asphericity and static friction, the applicability to bubble models for wet foams in which the friction is dynamic, the dynamical arrest in colloids, and the implications for molecular glasses.
We investigate the mechanical response of jammed packings of repulsive, frictionless spherical particles undergoing isotropic compression. Prior simulations of the soft-particle model, where the repulsive interactions scale as a power-law in the inte
rparticle overlap with exponent $alpha$, have found that the ensemble-averaged shear modulus $langle G rangle$ increases with pressure $P$ as $sim P^{(alpha-3/2)/(alpha-1)}$ at large pressures. However, a deep theoretical understanding of this scaling behavior is lacking. We show that the shear modulus of jammed packings of frictionless, spherical particles has two key contributions: 1) continuous variations as a function of pressure along geometrical families, for which the interparticle contact network does not change, and 2) discontinuous jumps during compression that arise from changes in the contact network. We show that the shear modulus of the first geometrical family for jammed packings can be collapsed onto a master curve: $G^{(1)}/G_0 = (P/P_0)^{(alpha-2)/(alpha-1)} - P/P_0$, where $P_0 sim N^{-2(alpha-1)}$ is a characteristic pressure that separates the two power-law scaling regions and $G_0 sim N^{-2(alpha-3/2)}$. Deviations from this form can occur when there is significant non-affine particle motion near changes in the contact network. We further show that $langle G (P)rangle$ is not simply a sum of two power-laws, but $langle G rangle sim (P/P_c)^a$, where $a approx (alpha -2)/(alpha-1)$ in the $P rightarrow 0$ limit and $langle G rangle sim (P/P_c)^b$, where $b gtrsim (alpha -3/2)/(alpha-1)$ above a characteristic pressure $P_c$. In addition, the magnitudes of both contributions to $langle Grangle$ from geometrical families and changes in the contact network remain comparable in the large-system limit for $P >P_c$.
We study the vibrational modes of three-dimensional jammed packings of soft ellipsoids of revolution as a function of particle aspect ratio $epsilon$ and packing fraction. At the jamming transition for ellipsoids, as distinct from the idealized case
using spheres where $epsilon = 1$, there are many unconstrained and non-trivial rotational degrees of freedom. These constitute a set of zero-frequency modes that are gradually mobilized into a new rotational band as $|epsilon - 1|$ increases. Quite surprisingly, as this new band is separated from zero frequency by a gap, and lies below the onset frequency for translational vibrations, $omega^*$, the presence of these new degrees of freedom leaves unaltered the basic scenario that the translational spectrum is determined only by the average contact number. Indeed, $omega^*$ depends solely on coordination as it does for compressed packings of spheres. We also discuss the regime of large $|epsilon - 1|$, where the two bands merge.
In this work we study onset of nonlinear rheological behavior of a colloidal dispersion of a synthetic hectorite clay, Laponite, at the critical gel state while undergoing sol-gel transition. When subjected to step strain in the nonlinear regime, the
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