Forming Granular Plugs
Fluid-grain flows, such as mudslides and blood, are difficult to model because of complex frictional forces that act on and between the grains. New experiments explore the behavior of a three-phase flow that combines liquid, air, and grains in a narrow tube. At relatively slow flow rates, the grains pile up and form a periodic pattern of tube-blocking plugs. The researchers explain this process by accounting for the frictional forces acting in the system.
Many different forces are at work inside fluid-grain flows. The fluid exerts a viscous force that pushes grains downstream, while the grains feel friction from rubbing against each other and on the walls of the container. If a second fluid is added, then capillary forces from the fluid-fluid boundary (or meniscus) may also act on the grains.
Guillaume Dumazer from the University of Oslo, Norway, and his colleagues performed experiments on a system containing water, air, and grains (small glass beads). They placed the water-grain mixture in a long, 2-mm-diameter tube. Water was drawn from one end of the tube via a syringe, forcing an air column to be pulled in through the other end. At high water-flow rates, the viscous forces dragged the grains along, evacuating the tube completely. But at low flow rates, the air-water meniscus pushed grains into piles that clogged the tubes because of the friction from the tube walls. This plug-forming process occurred multiple times at evenly spaced locations along the tube. The team constructed a model that explained the plug pattern and predicted the minimum flow rate needed to prevent plug formation. This work may lead to a better understanding of other fluid-grain flows, like, for example, the hydraulically driven flows of ingredients found in the pharmaceutical industry.
This research is published in Physical Review Letters.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.