216 research outputs found
Relaxing in foam
We investigate the mechanical response of an aqueous foam, and its relation
to the microscopic rearrangement dynamics of the bubble-packing structure. At
rest, even though the foam is coarsening, the rheology is demonstrated to be
linear. Under flow, shear-induced rearrangements compete with
coarsening-induced rearrangements. The macroscopic consequences are captured by
a novel rheological method in which a step-strain is superposed on an otherwise
steady flow
Effect of hydrogel particle additives on water-accessible pore structure of sandy soils: A custom pressure plate apparatus and capillary bundle model
To probe the effects of hydrogel particle additives on the water-accessible
pore structure of sandy soils, we introduce a custom pressure plate method in
which the volume of water expelled from a wet granular packing is measured as a
function of applied pressure. Using a capillary bundle model, we show that the
differential change in retained water per pressure increment is directly
related to the cumulative cross-sectional area distribution of the
water-accessible pores with radii less than . This is validated by
measurements of water expelled from a model sandy soil composed of 2 mm
diameter glass beads. In particular, the expelled water is found to depend
dramatically on sample height and that analysis using the capillary bundle
model gives the same pore size distribution for all samples. The distribution
is found to be approximately log-normal, and the total cross-sectional area
fraction of the accessible pore space is found to be . We then report
on how the pore distribution and total water-accessible area fraction are
affected by superabsorbent hydrogel particle additives, uniformly mixed into a
fixed-height sample at varying concentrations. Under both fixed volume and free
swelling conditions, the total area fraction of water-accessible pore space in
a packing decreases exponentially as the gel concentration increases. The size
distribution of the pores is significantly modified by the swollen hydrogel
particles, such that large pores are clogged while small pores are formed
Rain water transport and storage in a model sandy soil with hydrogel particle additives
We study rain water infiltration and drainage in a dry model sandy soil with
superabsorbent hydrogel particle additives by measuring the mass of retained
water for non-ponding rainfall using a self-built 3D laboratory set-up. In the
pure model sandy soil, the retained water curve measurements indicate that
instead of a stable horizontal wetting front that grows downward uniformly, a
narrow fingered flow forms under the top layer of water-saturated soil. This
rain water channelization phenomenon not only further reduces the available
rain water in the plant root zone, but also affects the efficiency of soil
additives, such as superabsorbent hydrogel particles. Our studies show that the
shape of the retained water curve for a soil packing with hydrogel particle
additives strongly depends on the location and the concentration of the
hydrogel particles in the model sandy soil. By carefully choosing the particle
size and distribution methods, we may use the swollen hydrogel particles to
modify the soil pore structure, to clog or extend the water channels in sandy
soils, or to build water reservoirs in the plant root zone
Topological persistence and dynamical heterogeneities near jamming
We introduce topological methods for quantifying spatially heterogeneous
dynamics, and use these tools to analyze particle-tracking data for a
quasi-two-dimensional granular system of air-fluidized beads on approach to
jamming. In particular we define two overlap order parameters, which quantify
the correlation between particle configurations at different times, based on a
Voronoi construction and the persistence in the resulting cells and nearest
neighbors. Temporal fluctuations in the decay of the persistent area and bond
order parameters define two alternative dynamic four-point susceptibilities,
XA(t) and XB(t), well-suited for characterizing spatially-heterogeneous
dynamics. These are analogous to the standard four-point dynamic susceptibility
X4(l,t), but where the space-dependence is fixed uniquely by topology rather
than by discretionary choice of cutoff function. While these three
susceptibilities yield characteristic time scales that are somewhat different,
they give domain sizes for the dynamical heterogeneities that are in good
agreement and that diverge on approach to jamming
The partition of energy for air-fluidized grains
The dynamics of one and two identical spheres rolling in a nearly-levitating
upflow of air obey the Langevin Equation and the Fluctuation-Dissipation
Relation [Ojha et al. Nature 427, 521 (2004) and Phys. Rev. E 71, 01631
(2005)]. To probe the range of validity of this statistical mechanical
description, we perturb the original experiments in four ways. First, we break
the circular symmetry of the confining potential by using a stadium-shaped
trap, and find that the velocity distributions remain circularly symmetric.
Second, we fluidize multiple spheres of different density, and find that all
have the same effective temperature. Third, we fluidize two spheres of
different size, and find that the thermal analogy progressively fails according
to the size ratio. Fourth, we fluidize individual grains of aspherical shape,
and find that the applicability of statistical mechanics depends on whether or
not the grain chatters along its length, in the direction of airflow.Comment: experimen
The Hindered Settling Function at Low Re Has Two Branches
We analyze hindered settling speed versus volume fraction for
dispersions of monodisperse spherical particles sedimenting under gravity,
using data from 15 different studies drawn from the literature, as well as 12
measurements of our own. We discuss and analyze the results in terms of popular
empirical forms for the hindered settling function, and compare to the known
limiting behaviors. A significant finding is that the data fall onto two
distinct branches, both of which are well-described by a hindered settling
function of the Richardson-Zaki form but with different
exponents: for Brownian systems with P\'eclet number , and for non-Brownian systems with . The crossover P\'eclet number is ,
which is surprisingly large.Comment: Supplementary material available on reques
Characterizing Pixel and Point Patterns with a Hyperuniformity Disorder Length
We introduce the concept of a hyperuniformity disorder length that controls
the variance of volume fraction fluctuations for randomly placed windows of
fixed size. In particular, fluctuations are determined by the average number of
particles within a distance from the boundary of the window. We first
compute special expectations and bounds in dimensions, and then illustrate
the range of behavior of versus window size by analyzing three
different types of simulated two-dimensional pixel pattern - where particle
positions are stored as a binary digital image in which pixels have value
zero/one if empty/contain a particle. The first are random binomial patterns,
where pixels are randomly flipped from zero to one with probability equal to
area fraction. These have long-ranged density fluctuations, and simulations
confirm the exact result . Next we consider vacancy patterns, where a
fraction of particles on a lattice are randomly removed. These also display
long-range density fluctuations, but with for small . For a
hyperuniform system with no long-range density fluctuations, we consider
Einstein patterns where each particle is independently displaced from a lattice
site by a Gaussian-distributed amount. For these, at large , approaches
a constant equal to about half the root-mean-square displacement in each
dimension. Then we turn to grayscale pixel patterns that represent simulated
arrangements of polydisperse particles, where the volume of a particle is
encoded in the value of its central pixel. And we discuss the continuum limit
of point patterns, where pixel size vanishes. In general, we thus propose to
quantify particle configurations not just by the scaling of the density
fluctuation spectrum but rather by the real-space spectrum of versus
. We call this approach Hyperuniformity Disorder Length Spectroscopy
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