232 research outputs found
Hypoconstrained Jammed Packings of Nonspherical Hard Particles: Ellipses and Ellipsoids
Continuing on recent computational and experimental work on jammed packings
of hard ellipsoids [Donev et al., Science, vol. 303, 990-993] we consider
jamming in packings of smooth strictly convex nonspherical hard particles. We
explain why the isocounting conjecture, which states that for large disordered
jammed packings the average contact number per particle is twice the number of
degrees of freedom per particle (\bar{Z}=2d_{f}), does not apply to
nonspherical particles. We develop first- and second-order conditions for
jamming, and demonstrate that packings of nonspherical particles can be jammed
even though they are hypoconstrained (\bar{Z}<2d_{f}). We apply an algorithm
using these conditions to computer-generated hypoconstrained ellipsoid and
ellipse packings and demonstrate that our algorithm does produce jammed
packings, even close to the sphere point. We also consider packings that are
nearly jammed and draw connections to packings of deformable (but stiff)
particles. Finally, we consider the jamming conditions for nearly spherical
particles and explain quantitatively the behavior we observe in the vicinity of
the sphere point.Comment: 33 pages, third revisio
Understanding the Frequency Distribution of Mechanically Stable Disk Packings
Relative frequencies of mechanically stable (MS) packings of frictionless
bidisperse disks are studied numerically in small systems. The packings are
created by successively compressing or decompressing a system of soft purely
repulsive disks, followed by energy minimization, until only infinitesimal
particle overlaps remain. For systems of up to 14 particles most of the MS
packings were generated. We find that the packings are not equally probable as
has been assumed in recent thermodynamic descriptions of granular systems.
Instead, the frequency distribution, averaged over each packing-fraction
interval , grows exponentially with increasing . Moreover,
within each packing-fraction interval MS packings occur with frequencies
that differ by many orders of magnitude. Also, key features of the frequency
distribution do not change when we significantly alter the packing-generation
algorithm--for example frequent packings remain frequent and rare ones remain
rare. These results indicate that the frequency distribution of MS packings is
strongly influenced by geometrical properties of the multidimensional
configuration space. By adding thermal fluctuations to a set of the MS
packings, we were able to examine a number of local features of configuration
space near each packing including the time required for a given packing to
break to a distinct one, which enabled us to estimate the energy barriers that
separate one packing from another. We found a positive correlation between the
packing frequencies and the heights of the lowest energy barriers .
We also examined displacement fluctuations away from the MS packings to
correlate the size and shape of the local basins near each packing to the
packing frequencies.Comment: 21 pages, 20 figures, 1 tabl
Rods are less fragile than spheres: Structural relaxation in dense liquids composed of anisotropic particles
We perform extensive molecular dynamics simulations of dense liquids composed
of bidisperse dimer- and ellipse-shaped particles in 2D that interact via
repulsive contact forces. We measure the structural relaxation times obtained
from the long-time decay of the self-part of the intermediate scattering
function for the translational and rotational degrees of freedom (DOF) as a
function of packing fraction \phi, temperature T, and aspect ratio \alpha. We
are able to collapse the \phi and T-dependent structural relaxation times for
disks, and dimers and ellipses over a wide range of \alpha, onto a universal
scaling function {\cal F}_{\pm}(|\phi-\phi_0|,T,\alpha), which is similar to
that employed in previous studies of dense liquids composed of purely repulsive
spherical particles in 3D. {\cal F_{\pm}} for both the translational and
rotational DOF are characterized by the \alpha-dependent scaling exponents \mu
and \delta and packing fraction \phi_0(\alpha) that signals the crossover in
the scaling form {\cal F}_{\pm} from hard-particle dynamics to super-Arrhenius
behavior for each aspect ratio. We find that the fragility at \phi_0,
m(\phi_0), decreases monotonically with increasing aspect ratio for both
ellipses and dimers. Moreover, the results for the slow dynamics of dense
liquids composed of dimer- and ellipse-shaped particles are qualitatively the
same, despite the fact that zero-temperature static packings of dimers are
isostatic, while static packings of ellipses are hypostatic.Comment: 10 pages, 17 figures, and 1 tabl
Phase Behavior of Colloidal Superballs: Shape Interpolation from Spheres to Cubes
The phase behavior of hard superballs is examined using molecular dynamics
within a deformable periodic simulation box. A superball's interior is defined
by the inequality , which provides a
versatile family of convex particles () with cube-like and
octahedron-like shapes as well as concave particles () with
octahedron-like shapes. Here, we consider the convex case with a deformation
parameter q between the sphere point (q = 1) and the cube (q = 1). We find that
the asphericity plays a significant role in the extent of cubatic ordering of
both the liquid and crystal phases. Calculation of the first few virial
coefficients shows that superballs that are visually similar to cubes can have
low-density equations of state closer to spheres than to cubes. Dense liquids
of superballs display cubatic orientational order that extends over several
particle lengths only for large q. Along the ordered, high-density equation of
state, superballs with 1 < q < 3 exhibit clear evidence of a phase transition
from a crystal state to a state with reduced long-ranged orientational order
upon the reduction of density. For , long-ranged orientational order
persists until the melting transition. The width of coexistence region between
the liquid and ordered, high-density phase decreases with q up to q = 4.0. The
structures of the high-density phases are examined using certain order
parameters, distribution functions, and orientational correlation functions. We
also find that a fixed simulation cell induces artificial phase transitions
that are out of equilibrium. Current fabrication techniques allow for the
synthesis of colloidal superballs, and thus the phase behavior of such systems
can be investigated experimentally.Comment: 33 pages, 14 figure
A first-order phase transition at the random close packing of hard spheres
Randomly packing spheres of equal size into a container consistently results
in a static configuration with a density of ~64%. The ubiquity of random close
packing (RCP) rather than the optimal crystalline array at 74% begs the
question of the physical law behind this empirically deduced state. Indeed,
there is no signature of any macroscopic quantity with a discontinuity
associated with the observed packing limit. Here we show that RCP can be
interpreted as a manifestation of a thermodynamic singularity, which defines it
as the "freezing point" in a first-order phase transition between ordered and
disordered packing phases. Despite the athermal nature of granular matter, we
show the thermodynamic character of the transition in that it is accompanied by
sharp discontinuities in volume and entropy. This occurs at a critical
compactivity, which is the intensive variable that plays the role of
temperature in granular matter. Our results predict the experimental conditions
necessary for the formation of a jammed crystal by calculating an analogue of
the "entropy of fusion". This approach is useful since it maps
out-of-equilibrium problems in complex systems onto simpler established
frameworks in statistical mechanics.Comment: 33 pages, 10 figure
Jammed frictionless discs: connecting local and global response
By calculating the linear response of packings of soft frictionless discs to
quasistatic external perturbations, we investigate the critical scaling
behavior of their elastic properties and non-affine deformations as a function
of the distance to jamming. Averaged over an ensemble of similar packings,
these systems are well described by elasticity, while in single packings we
determine a diverging length scale up to which the response of the
system is dominated by the local packing disorder. This length scale, which we
observe directly, diverges as , where is the difference
between contact number and its isostatic value, and appears to scale
identically to the length scale which had been introduced earlier in the
interpretation of the spectrum of vibrational modes. It governs the crossover
from isostatic behavior at the small scale to continuum behavior at the large
scale; indeed we identify this length scale with the coarse graining length
needed to obtain a smooth stress field. We characterize the non-affine
displacements of the particles using the \emph{displacement angle
distribution}, a local measure for the amount of relative sliding, and analyze
the connection between local relative displacements and the elastic moduli.Comment: 19 pages, 15 figures, submitted to Phys. Rev.
Edwards thermodynamics of the jamming transition for frictionless packings: ergodicity test and role of angoricity and compactivity
This paper illustrates how the tools of equilibrium statistical mechanics can
help to explain a far-from-equilibrium problem: the jamming transition in
frictionless granular materials. Edwards ideas consist of proposing a
statistical ensemble of volume and stress fluctuations through the
thermodynamic notion of entropy, compactivity, X, and angoricity, A (two
temperature-like variables). We find that Edwards thermodynamics is able to
describe the jamming transition (J-point). Using the ensemble formalism we
elucidate the following: (i)We test the combined volume-stress ensemble by
comparing the statistical properties of jammed configurations obtained by
dynamics with those averaged over the ensemble of minima in the potential
energy landscape as a test of ergodicity. Agreement between both methods
supports the idea of "thermalization" at a given angoricity and compactivity.
(ii) A microcanonical ensemble analysis supports the idea of maximum entropy
principle for grains. (iii) The intensive variables describe the approach to
jamming through a series of scaling relations as A {\to} 0+ and X {\to} 0-. Due
to the force-volume coupling, the jamming transition can be probed
thermodynamically by a "jamming temperature" TJ comprised of contributions from
A and X. (iv) The thermodynamic framework reveals the order of the jamming
phase transition by showing the absence of critical fluctuations at jamming in
observables like pressure and volume. (v) Finally, we elaborate on a comparison
with relevant studies showing a breakdown of equiprobability of microstates.Comment: 22pages, 24 figure
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