134 research outputs found
Simulations of driven overdamped frictionless hard spheres
We introduce an event-driven simulation scheme for overdamped dynamics of
frictionless hard spheres subjected to external forces, neglecting hydrodynamic
interactions. Our event-driven approach is based on an exact equation of motion
which relates the driving force to the resulting velocities through the
geometric information characterizing the underlying network of contacts between
the hard spheres. Our method allows for a robust extraction of the
instantaneous coordination of the particles as well as contact force statistics
and dynamics, under any chosen driving force, in addition to shear flow and
compression. It can also be used for generating high-precision jammed packings
under shear, compression, or both. We present a number of additional
applications of our method.Comment: 12 pages, 9 figure
Glassiness, Rigidity and Jamming of Frictionless Soft Core Disks
The jamming of bi-disperse soft core disks is considered, using a variety of
different protocols to produce the jammed state. In agreement with other works,
we find that cooling and compression can lead to a broad range of jamming
packing fractions , depending on cooling rate and initial
configuration; the larger the degree of big particle clustering in the initial
configuration, the larger will be the value of . In contrast, we find
that shearing disrupts particle clustering, leading to a much narrower range of
as the shear strain rate varies. In the limit of vanishingly small
shear strain rate, we find a unique non-trivial value for the jamming density
that is independent of the initial system configuration. We conclude that shear
driven jamming is a unique and well defined critical point in the space of
shear driven steady states. We clarify the relation between glassy behavior,
rigidity and jamming in such systems and relate our results to recent
experiments.Comment: 10 pages, 11 figures, significantly expanded version as accepted for
publication in PR
Interparticle friction leads to non-monotonic flow curves and hysteresis in viscous suspensions
Hysteresis is a major feature of the solid-liquid transition in granular
materials. This property, by allowing metastable states, can potentially yield
catastrophic phenomena such as earthquakes or aerial landslides. The origin of
hysteresis in granular flows is still debated. However, most mechanisms put
forward so far rely on the presence of inertia at the particle level. In this
paper, we study the avalanche dynamics of non-Brownian suspensions in slowly
rotating drums and reveal large hysteresis of the avalanche angle even in the
absence of inertia. By using micro-silica particles whose interparticle
friction coefficient can be turned off, we show that microscopic friction,
conversely to inertia, is key to triggering hysteresis in granular suspensions.
To understand this link between friction and hysteresis, we use the rotating
drum as a rheometer to extract the suspension rheology close to the flow onset
for both frictional and frictionless suspensions. This analysis shows that the
flow rule for frictionless particles is monotonous and follows a power law of
exponent , in close agreement with the previous
theoretical prediction, . By contrast, the flow rule for
frictional particles suggests a velocity-weakening behavior, thereby explaining
the flow instability and the emergence of hysteresis. These findings show that
hysteresis can also occur in particulate media without inertia, questioning the
intimate nature of this phenomenon. By highlighting the role of microscopic
friction, our results may be of interest in the geophysical context to
understand the failure mechanism at the origin of undersea landslides.Comment: 10 pages, 8 figure
Unifying Suspension and Granular flows near Jamming
Rheological properties of dense flows of hard particles are singular as one
approaches the jamming threshold where flow ceases, both for granular flows
dominated by inertia, and for over-damped suspensions. Concomitantly, the
lengthscale characterizing velocity correlations appears to diverge at jamming.
Here we review a theoretical framework that gives a scaling description of
stationary flows of frictionless particles. Our analysis applies both to
suspensions and inertial flows of hard particles. We report numerical results
in support of the theory, and show the phase diagram that results when friction
is added, delineating the regime of validity of the frictionless theory.Comment: Short review to appear in Powders and Grains 201
Duality in Shearing Rheology Near the Athermal Jamming Transition
We consider the rheology of soft-core frictionless disks in two dimensions in
the neighborhood of the athermal jamming transition. From numerical simulations
of bidisperse, overdamped, particles, we argue that the divergence of the
viscosity below jamming is characteristic of the hard-core limit, independent
of the particular soft-core interaction. We develop a mapping from soft-core to
hard-core particles that recovers all the critical behavior found in earlier
scaling analyses. Using this mapping we derive a duality relation that gives
the exponent of the non-linear Herschel-Bulkley rheology above jamming in terms
of the exponent of the diverging viscosity below jamming.Comment: 5 pages, 4 figures. Manuscript revisions: new title, additional text
concerning connections to experiment, revised Fig. 4, other minor changes and
clarifications in text. Conclusions remain essentially unchanged. Accepted
for publication in Phys. Rev. Let
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