82 research outputs found
Contact tribology also affects the slow flow behavior of granular emulsions
Recent work on suspension flows has shown that contact mechanics plays a role
in suspension flow dynamics. The contact mechanics between particulate matter
in dispersions should depend sensitively on the composition of the dispersed
phase: evidently emulsion droplets interact differently with each other than
angular sand particles. We therefore ask: what is the role of contact mechanics
in dispersed media flow? We focus on slow flows, where contacts are
long-lasting and hence contact mechanics effects should be most visible. To
answer our question, we synthesize soft hydrogel particles with different
friction coefficients. By making the particles soft, we can drive them at
finite confining pressure at all driving rates. For particles with a low
friction coefficient, we obtain a rheology similar to that of an emulsion, yet
with an effective friction much larger than expected from their microscopic
contact mechanics. Increasing the friction coefficient of the particles, we
find a flow instability in the suspension. Particle level flow and fluctuations
are also greatly affected by the microscopic friction coefficient of the
suspended particles. The specific rheology of our "granular emulsions" provides
further evidence that a better understanding of microscopic particle
interactions is of broad relevance for dispersed media flows
Universal fitness dynamics through an adaptive resource utilization model
The fitness of a species determines its abundance and survival in an
ecosystem. At the same time, species take up resources for growth, so their
abundance affects the availability of resources in an ecosystem. We show here
that such species-resource coupling can be used to assign a quantitative metric
for fitness to each species. This fitness metric also allows for the modeling
of drift in species composition, and hence ecosystem evolution through
speciation and adaptation. Our results provide a foundation for an entirely
computational exploration of evolutionary ecosystem dynamics on any length or
time scale. For example, we can evolve ecosystem dynamics even by initiating
dynamics out of a single primordial ancestor and show that there exists a well
defined ecosystem-averaged fitness dynamics that is resilient against resource
shocks
Reynolds Pressure and Relaxation in a Sheared Granular System
We describe experiments that probe the evolution of shear jammed states,
occurring for packing fractions , for frictional
granular disks, where above there are no stress-free static states. We
use a novel shear apparatus that avoids the formation of inhomogeneities known
as shear bands. This fixed system exhibits coupling between the shear
strain, , and the pressure, , which we characterize by the `Reynolds
pressure', and a `Reynolds coefficient', . depends only on , and diverges as , where , and . Under
cyclic shear, this system evolves logarithmically slowly towards limit cycle
dynamics, which we characterize in terms of pressure relaxation at cycle :
. depends only on the shear cycle
amplitude, suggesting an activated process where plays a
temperature-like role.Comment: 4 pages, 4 figure
Refractive Index Matched Scanning and Detection of Soft Particle
We describe here how to apply the three dimensional imaging technique of
refrecative index matched scanning to hydrogel spheres. Hydrogels are water
based materials with a low refractive index, which allows for index matching
with water-based solvent mixtures. We discuss here various experimental
techniques required to handle specifically hydrogel spheres as opposed to other
transparent materials. The deformability of hydrogel spheres makes their
identification in three dimensional images non-trivial. We will also discuss
numerical techniques that can be used in general to detect contacting,
non-spherical particles in a three dimensional image. The experimental and
numerical techniques presented here give experimental access to the stress
tensor of a packing of deformed particles.Comment: 9 pages, 9 figures, submitted to review of scientific instruments,
Issue 1
Rheology of Weakly Vibrated Granular Media
We probe the rheology of weakly vibrated granular flows as function of flow
rate, vibration strength and pressure by performing experiments in a vertically
vibrated split-bottom shear cell. For slow flows, we establish the existence of
a novel vibration dominated granular flow regime, where the driving stresses
smoothly vanish as the driving rate is diminished. We distinguish three
qualitatively different vibration dominated rheologies, most strikingly a
regime where the shear stresses no longer are proportional to the pressure.Comment: 14 pages, 19 figures, submitted to PR
Particle Diffusion in Slow Granular Bulk Flows
We probe the diffusive motion of particles in slowly sheared three
dimensional granular suspensions. For sufficiently large strains, the particle
dynamics exhibits diffusive Gaussian statistics, with the diffusivity
proportional to the local strain rate - consistent with a local, quasi static
picture. Surprisingly, the diffusivity is also inversely proportional to the
depth of the particles within the flow - at the free surface, diffusivity is
thus ill defined. We find that the crossover to Gaussian displacement
statistics is governed by the same depth dependence, evidencing a non-trivial
strain scale in three dimensional granular flows.Comment: 6 page
Darcy-Reynolds forces during intrusion into granular-fluid beds
We experimentally study intrusion into fluid-saturated granular beds by a free-falling sphere,
varying particle size and fluid viscosity. We test our results against Darcy-Reynolds theory, where
the deceleration of the sphere is controlled by Reynolds dilatancy and the Darcy flow resistance. We
find the observed intruder dynamics are consistent with Darcy-Reynolds theory for varied particle
size. We also find that our experimental results for varied viscosity are consistent with Darcy Reynolds theory, but only for a limited range of the viscosity. For large viscosities, observed forces
begin to decrease with increasing viscosity, in contrast with the theoretical prediction.Office of Naval ResearchOffice of Naval Research Global Visiting Scientist Program VSP 19-7-001N0001419WX0151
Obtaining Self-similar Scalings in Focusing Flows
The surface structure of converging thin fluid films displays self-similar
behavior, as was shown in the work by Diez et al [Q. Appl. Math 210, 155,
1990]. Extracting the related similarity scaling exponents from either
numerical or experimental data is non-trivial. Here we provide two such
methods. We apply them to experimental and numerical data on converging fluid
films driven by both surface tension and gravitational forcing. In the limit of
pure gravitational driving, we recover Diez' semi-analytic result, but our
methods also allow us to explore the entire regime of mixed capillary and
gravitational driving, up to entirely surface tension driven flows. We find
scaling forms of smoothly varying exponents up to surprisingly small Bond
numbers. Our experimental results are in reasonable agreement with our
numerical simulations, which confirm theoretically obtained relations between
the scaling exponents.Comment: 11 pages, 11 figures, accepted for Phys Rev
Spanning the Scales of Granular Materials: Microscopic Force Imaging
If you walk on sand, it supports your weight. How do the disordered forces
between particles in sand organize, to keep you from sinking? This simple
question is surprisingly difficult to answer experimentally: measuring forces
in three dimensions, between deeply buried grains, is challenging. We describe
here experiments in which we have succeeded in measuring forces inside a
granular packing subject to controlled deformations. We connect the measured
micro-scale forces to the macro-scale packing force response with an averaging,
mean field calculation. This calculation explains how the combination of
packing structure and contact deformations produce the unexpected mechanical
response of the packing, and reveals a surprising microscopic particle
deformation enhancement mechanism.Comment: Data and code available at http://www.phy.duke.edu/~nb108
Characterizing Granular Networks Using Topological Metrics
We carry out a direct comparison of experimental and numerical realizations
of the exact same granular system as it undergoes shear jamming. We adjust the
numerical methods used to optimally represent the experimental settings and
outcomes up to microscopic contact force dynamics. Measures presented here
range form microscopic, through mesoscopic to system-wide characteristics of
the system. Topological properties of the mesoscopic force networks provide a
key link between micro and macro scales. We report two main findings: the
number of particles in the packing that have at least two contacts is a good
predictor for the mechanical state of the system, regardless of strain history
and packing density. All measures explored in both experiments and numerics,
including stress tensor derived measures and contact numbers depend in a
universal manner on the fraction of non-rattler particles, . The force
network topology also tends to show this universality, yet the shape of the
master curve depends much more on the details of the numerical simulations. In
particular we show that adding force noise to the numerical data set can
significantly alter the topological features in the data. We conclude that both
and topological metrics are useful measures to consider when
quantifying the state of a granular system.Comment: 8 pages, 8 figure
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