Spectroscopy Probes Behavior of Granular Flows
LOS ANGELES -- Foods, pharmaceutical powders and pills, minerals and ores, road and building materials. All are examples of granular materials that handling systems must smoothly transport without jamming. But no one fully understands the features of jamming in granular flows. To gain insight into grain flow dynamics, Douglas J. Durian, an associate professor of physics at the University of California, uses 0.33-µm-diameter glass beads, which he calls "sand."
He drops the sand into a three-sided box with one end wall and two 30 x 30-cm plastic walls that are 9.5 mm apart. The sand forms a heap that is highest under the drop point and that gradually tapers to the open side of the box. At low flow rates, the angled slope formed by the sand remains stationary until it can no longer support grains higher on the heap, which then form an avalanche. As the flow rate increases, it reaches a point when the sand on the slope enters continuous flow.
Theories on granular flow dynamics exist for both continuous flows and avalanches, but they do not remain valid in the transition region between intermittent and continuous flow. To probe grain behavior in this transition region, Durian and graduate student Pierre-Anthony Lemieux use a 514-nm beam produced by a Coherent Innova argon-ion laser.
They target the laser at a fixed point on the slope to scatter a random speckle pattern. A gradient index lens coupled to a single-mode fiber samples a single speckle spot and transmits the light to a detector that monitors intensity fluctuations of grain motion down to 0.1 Å and for durations as short as 10 -8 seconds.
Durian and Lemieux found a few surprises. For example, as the slope flow changes from intermittent to continuous flow, the time at rest does not smoothly go to zero. This and other measurements led them to believe that "the rapid dynamics and grain-grain collisions during an avalanche are somehow different from the same actions during continuous flow."
"The transition between smooth and intermittent flow is hugely important," Durian said. "Whenever granular materials are transported, the handling system needs to avoid jamming. Our work could lead to the kind of understanding needed to correct the shockingly high jamming rate of commercial systems."
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