Biomedical subjects

DJ Durian

Publications and source records attributed to DJ Durian.

18 recordsLinked to original sources

PubMed2000

From avalanches to fluid flow: A continuous picture of grain dynamics down a heap

Surface flows are excited by steadily adding spherical glass beads to the top of a heap. To simultaneously characterize the fast single-grain dynamics and the much slower collective intermittency of the flow, we extend photon-correlation spectroscopy via fourth-order temporal correlations in the scattered light intensity. We find that microscopic grain dynamics during an avalanche are similar to those in the continuous flow just above the transition. We also find that there is a minimum jamming time, even arbitrarily close to the transition.

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PubMed2000

Instabilities in a liquid-fluidized bed of gas bubbles

Gas bubbles in an aqueous foam can be unjammed, or fluidized, by introducing a forced flow of the continuous liquid phase at a sufficiently high rate. We observe that the resulting bubble dynamics are spatially inhomogeneous, exhibiting a sequence of instabilities vs increasing flow rate. First irregular swirls appear, then a single convective roll, and finally a series of stratified convection rolls each with a different average bubble size.

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PubMed2000

Hysteresis and packing in gas-fluidized beds

The packing fraction and the pressure drop across gas-fluidized beds of granular media exhibit hysteresis as the gas-flow rate is cycled up and down across the fluidization transition. Presumably this is due to contact forces and transfer of stress to the surrounding walls, and hence should vary nontrivially with the aspect ratio of the sample. Here we present systematic measurements of the variation of hysteresis with particle size and aspect ratio of the sample. Remarkably, the hysteresis scales in a trivial way with these parameters, showing no evidence of long-range effects of the wall. Our measurements also show that the packing fraction becomes 0. 590+/-0.004, independent of particle size and container shape, when the fluidizing flow of gas flow is slowly removed.

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PubMed1999

Shear-Induced "Melting" of an Aqueous Foam.

We present diffusing-wave spectroscopy measurements of bubble dynamics in a continuously sheared aqueous foam. At slow strain rates, isolated clusters of bubbles intermittently rearrange from one solidly packed configuration to another, even though the macroscopic flow appears continuous. At fast rates, bubbles instead move smoothly and continuously throughout the entire foam. In other words, shear flow that appears macroscopically laminar is similarly laminar down to the bubble scale; effectively the foam "melts." The crossover to this regime can be understood in terms of elastic energy accumulation and viscous dissipation mechanisms. In particular, the strain rate needed for shear-induced melting to occur is set by the ratio of a yield strain to the rearrangement event duration. To explore the implications for macroscopic flow, we compare these bubble-scale dynamics directly with viscosity measurements. Copyright 1999 Academic Press.

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PubMed1997

Diffusing-Wave Spectroscopy of Dynamics in a Three-Dimensional Granular Flow

Diffusing-wave spectroscopy was used to measure the microscopic dynamics of grains in the interior of a three-dimensional flow of sand. The correlation functions show that minutely separated grains fly from collision to collision with large random velocities. On a time scale 10(3) to 10(4) times longer than the average time between collisions, the grains displayed slow, collective rearrangements, which, at the long-time limit, produced diffusive dynamics.

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PubMed1996