603 research outputs found
Permeability of mixed soft and hard granular material: hydrogels as drainage modifiers
We measure the flow of water through mixed packings of glass spheres and soft
swellable hydrogel grains, at constant sample volume. Permeability values are
obtained at constant sample volume and at porosities smaller than random close
packing, for different glass bead diameters and for variable gel grain
diameter , as controlled by the salinity of the water. The gel content is
also varied. We find that the permeability decays exponentially in
where is the gel to glass bead number ratio and is
approximately 3. Therefore, flow properties are determined by the volume
fraction of gel beads. A simple model based on the porosity of overlapping
spheres is used to account for these observations
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
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
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
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
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