9 research outputs found
Force Mobilization and Generalized Isostaticity in Jammed Packings of Frictional Grains
We show that in slowly generated 2d packings of frictional spheres, a
significant fraction of the friction forces lies at the Coulomb threshold - for
small pressure p and friction coefficient mu, about half of the contacts.
Interpreting these contacts as constrained leads to a generalized concept of
isostaticity, which relates the maximal fraction of fully mobilized contacts
and contact number. For p->0, our frictional packings approximately satisfy
this relation over the full range of mu. This is in agreement with a previous
conjecture that gently built packings should be marginal solids at jamming. In
addition, the contact numbers and packing densities scale with both p and mu.Comment: 4 pages, 4 figures, submitte
Local contact numbers in two dimensional packings of frictional disks
We analyze the local structure of two dimensional packings of frictional
disks numerically. We focus on the fractions x_i of particles that are in
contact with i neighbors, and systematically vary the confining pressure p and
friction coefficient \mu. We find that for all \mu, the fractions x_i exhibit
powerlaw scaling with p, which allows us to obtain an accurate estimate for x_i
at zero pressure. We uncover how these zero pressure fractions x_i vary with
\mu, and introduce a simple model that captures most of this variation. We also
probe the correlations between the contact numbers of neighboring particles.Comment: 4 pages, 5 figure
Critical and non-critical jamming of frictional grains
We probe the nature of the jamming transition of frictional granular media by
studying their vibrational properties as a function of the applied pressure p
and friction coefficient mu. The density of vibrational states exhibits a
crossover from a plateau at frequencies omega \gtrsim omega^*(p,mu) to a linear
growth for omega \lesssim omega^*(p,mu). We show that omega^* is proportional
to Delta z, the excess number of contacts per grains relative to the minimally
allowed, isostatic value. For zero and infinitely large friction, typical
packings at the jamming threshold have Delta z -> 0, and then exhibit critical
scaling. We study the nature of the soft modes in these two limits, and find
that the ratio of elastic moduli is governed by the distance from isostaticity.Comment: 4 pages, 4 figures; discussion update
Isotropic-nematic transition in hard-rod fluids: relation between continuous and restricted-orientation models
We explore models of hard-rod fluids with a finite number of allowed
orientations, and construct their bulk phase diagrams within Onsager's second
virial theory. For a one-component fluid, we show that the discretization of
the orientations leads to the existence of an artificial (almost) perfectly
aligned nematic phase, which coexists with the (physical) nematic phase if the
number of orientations is sufficiently large, or with the isotropic phase if
the number of orientations is small. Its appearance correlates with the
accuracy of sampling the nematic orientation distribution within its typical
opening angle. For a binary mixture this artificial phase also exists, and a
much larger number of orientations is required to shift it to such high
densities that it does not interfere with the physical part of the phase
diagram.Comment: 4 pages, 2 figures, submitted to PR