245 research outputs found
Convective instability and mass transport of diffusion layers in a Hele-Shaw geometry
We consider experimentally the instability and mass transport of a
porous-medium flow in a Hele-Shaw geometry. In an initially stable
configuration, a lighter fluid (water) is located over a heavier fluid
(propylene glycol). The fluids mix via diffusion with some regions of the
resulting mixture being heavier than either pure fluid. Density-driven
convection occurs with downward penetrating dense fingers that transport mass
much more effectively than diffusion alone. We investigate the initial
instability and the quasi steady state. The convective time and velocity
scales, finger width, wave number selection, and normalized mass transport are
determined for 6,000<Ra<90,000. The results have important implications for
determining the time scales and rates of dissolution trapping of carbon dioxide
in brine aquifers proposed as possible geologic repositories for sequestering
carbon dioxide.Comment: 4 page, 3 figure
Rapid granular flows on a rough incline: phase diagram, gas transition, and effects of air drag
We report experiments on the overall phase diagram of granular flows on an
incline with emphasis on high inclination angles where the mean layer velocity
approaches the terminal velocity of a single particle free falling in air. The
granular flow was characterized by measurements of the surface velocity, the
average layer height, and the mean density of the layer as functions of the
hopper opening, the plane inclination angle and the downstream distance x of
the flow. At high inclination angles the flow does not reach an x-invariant
steady state over the length of the inclined plane. For low volume flow rates,
a transition was detected between dense and very dilute (gas) flow regimes. We
show using a vacuum flow channel that air did not qualitatively change the
phase diagram and did not quantitatively modify mean flow velocities of the
granular layer except for small changes in the very dilute gas-like phase.Comment: 10 pages, 16 figures, accepted to Phys. Rev.
Fragility and hysteretic creep in frictional granular jamming
The granular jamming transition is experimentally investigated in a
two-dimensional system of frictional, bi-dispersed disks subject to
quasi-static, uniaxial compression at zero granular temperature. Currently
accepted results show the jamming transition occurs at a critical packing
fraction . In contrast, we observe the first compression cycle exhibits
{\it fragility} - metastable configuration with simultaneous jammed and
un-jammed clusters - over a small interval in packing fraction (). The fragile state separates the two conditions that define
with an exponential rise in pressure starting at and an exponential
fall in disk displacements ending at . The results are explained
through a percolation mechanism of stressed contacts where cluster growth
exhibits strong spatial correlation with disk displacements. Measurements with
several disk materials of varying elastic moduli and friction coefficients
, show friction directly controls the start of the fragile state, but
indirectly controls the exponential slope. Additionally, we experimentally
confirm recent predictions relating the dependence of on . Under
repetitive loading (compression), the system exhibits hysteresis in pressure,
and the onset increases slowly with repetition number. This friction
induced hysteretic creep is interpreted as the granular pack's evolution from a
metastable to an eventual structurally stable configuration. It is shown to
depend upon the quasi-static step size which provides the only
perturbative mechanism in the experimental protocol, and the friction
coefficient which acts to stabilize the pack.Comment: 12 pages, 10 figure
Two scenarios for avalanche dynamics in inclined granular layers
We report experimental measurements of avalanche behavior of thin granular
layers on an inclined plane for low volume flow rate. The dynamical properties
of avalanches were quantitatively and qualitatively different for smooth glass
beads compared to irregular granular materials such as sand. Two scenarios for
granular avalanches on an incline are identified and a theoretical explanation
for these different scenarios is developed based on a depth-averaged approach
that takes into account the differing rheologies of the granular materials.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Frustration and Melting of Colloidal Molecular Crystals
Using numerical simulations we show that a variety of novel colloidal
crystalline states and multi-step melting phenomena occur on square and
triangular two-dimensional periodic substrates. At half-integer fillings
different kinds of frustration effects can be realized. A two-step melting
transition can occur in which individual colloidal molecules initially rotate,
destroying the overall orientational order, followed by the onset of interwell
colloidal hopping, in good agreement with recent experiments.Comment: 6 pages, 3 postscript figures. Procedings of International Conference
on Strongly Coupled Coulomb Systems, Santa Fe, 200
Stretching fields and mixing near the transition to nonperiodic two-dimensional flow
Although time-periodic fluid flows sometimes produce mixing via Lagrangian chaos, the additional contribution to mixing caused by nonperiodicity has not been quantified experimentally. Here, we do so for a quasi-two-dimensional flow generated by electromagnetic forcing. Several distinct measures of mixing are found to vary continuously with the Reynolds number, with no evident change in magnitude or slope at the onset of nonperiodicity. Furthermore, the scaled probability distributions of the mean Lyapunov exponent have the same form in the periodic and nonperiodic flow states
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