46 research outputs found
Softening and Yielding of Soft Glassy Materials
Solids deform and fluids flow, but soft glassy materials, such as emulsions,
foams, suspensions, and pastes, exhibit an intricate mix of solid and
liquid-like behavior. While much progress has been made to understand their
elastic (small strain) and flow (infinite strain) properties, such
understanding is lacking for the softening and yielding phenomena that connect
these asymptotic regimes. Here we present a comprehensive framework for
softening and yielding of soft glassy materials, based on extensive numerical
simulations of oscillatory rheological tests, and show that two distinct
scenarios unfold depending on the material's packing density. For dense
systems, there is a single, pressure-independent strain where the elastic
modulus drops and the particle motion becomes diffusive. In contrast, for
weakly jammed systems, a two-step process arises: at an intermediate softening
strain, the elastic and loss moduli both drop down and then reach a new plateau
value, whereas the particle motion becomes diffusive at the distinctly larger
yield strain. We show that softening is associated with an extensive number of
microscopic contact changes leading to a non-analytic rheological signature.
Moreover, the scaling of the softening strain with pressure suggest the
existence of a novel pressure scale above which softening and yielding
coincide, and we verify the existence of this crossover scale numerically. Our
findings thus evidence the existence of two distinct classes of soft glassy
materials -- jamming dominated and dense -- and show how these can be
distinguished by their rheological fingerprint.Comment: 9 pages, 11 figures, to appear in Soft Matte
Anisotropic diffusion limited aggregation in three dimensions : universality and nonuniversality
We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions
Effects of grain shape on packing and dilatancy of sheared granular materials
Granular material exposed to shear shows a variety of unique phenomena:
Reynolds dilatancy, positional order and orientational order effects may
compete in the shear zone. We study granular packings consisting of macroscopic
prolate, oblate and spherical grains and compare their behaviour. X-ray
tomography is used to determine the particle positions and orientations in a
cylindrical split bottom shear cell. Packing densities and the arrangements of
individual particles in the shear zone are evaluated. For anisometric
particles, we observe the competition of two opposite effects. One the one
hand, the sheared granulate is dilated, but on the other hand the particles
reorient and align with respect to the streamlines. Even though aligned
cylinders in principle may achieve higher packing densities, this alignment
compensates for the effect of dilatancy only partially. The complex
rearrangements lead to a depression of the surface above the well oriented
region while neigbouring parts still show the effect of dilation in the form of
heaps. For grains with isotropic shapes, the surface remains rather flat.
Perfect monodisperse spheres crystallize in the shear zone, whereby positional
order partially overcompensates dilatancy effects. However, already slight
deviations from the ideal monodisperse sphere shape inhibit crystallization.Comment: 12 pages, 13 figures, accepted in Soft Matte
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
Heaping, Secondary Flows and Broken Symmetry in Flows of Elongated Granular Particles
In this paper we report experiments where we shear granular rods in
split-bottom geometries, and find that a significant heap of height of least
40% of the filling height can form at the particle surface. We show that
heaping is caused by a significant secondary flow, absent for spherical
particles. Flow reversal transiently reverses the secondary flow, leading to a
quick collapse and slower regeneration of the heap. We present a symmetry
argument and experimental data that show that the generation of the secondary
flow is driven by a misalignment of the mean particle orientation with the
streamlines of the flow. This general mechanism is expected to be important in
all flows of sufficiently anisometric grains.Comment: Accepted for Soft Matte
Critical scaling in linear response of frictionless granular packings near jamming
We study the origin of the scaling behavior in frictionless granular media
above the jamming transition by analyzing their linear response. The response
to local forcing is non-self-averaging and fluctuates over a length scale that
diverges at the jamming transition. The response to global forcing becomes
increasingly non-affine near the jamming transition. This is due to the
proximity of floppy modes, the influence of which we characterize by the local
linear response. We show that the local response also governs the anomalous
scaling of elastic constants and contact number.Comment: 4 pages, 3 figures. v2: Added new results; removed part of
discussion; changed Fig.
Coherent structures in Dissipative Particle Dynamics simulations of the transition to turbulence in compressible shear flows
We present simulations of coherent structures in compressible flows near the
transition to turbulence using the Dissipative Particle Dynamics (DPD) method.
The structures we find are remarkably consistent with experimental observations
and DNS simulations of incompressible flows, despite a difference in Mach
number of several orders of magnitude. The bifurcation from the laminar flow is
bistable and shifts to higher Reynolds numbers when the fluid becomes more
compressible. This work underlines the robustness of coherent structures in the
transition to turbulence and illustrates the ability of particle-based methods
to reproduce complex non-linear instabilities.Comment: 4 pages, 5 figure
Packing, alignment and flow of shape-anisotropic grains in a 3D silo experiment
Granular material flowing through bottlenecks, like the openings of silos, tend to clog and thus inhibit further flow. We study this phenomenon in a three-dimensional hopper for spherical and shape-anisotropic particles by means of x-ray tomography. The x-ray tomograms provide information on the bulk of the granular filling, and allows us to determine the particle positions and orientations inside the silo. In addition, it allows us to calculate local packing densities in different parts of the container. We find that in the flowing zone of the silo particles show a preferred orientation and thereby a higher order. Similarly to simple shear flows, the average orientation of the particles is not parallel to the streamlines but encloses a certain angle with it. In most parts of the hopper, the angular distribution of the particles did not reach the one corresponding to stationary shear flow, thus the average orientation angle in the hopper deviates more from the streamlines than in stationary shear flows. In the flowing parts of the silo, shear induced dilation is observed, which is more pronounced for elongated grains than for nearly spherical particles. The clogged state is characterized by a dome, i.e. the geometry of the layer of grains blocking the outflow. The shape of the dome depends on the particle shape