Sand, bubbling through sand


Fluid-like motion observed in granular material for the first time.


A bubble of lighter sand, seen here as blue, forming inside a matrix of heavier grains.

ALEX PENN/ETH ZURICH

Sand will sometimes form bubbles. A study led by chemical engineer Chris Boyce from the Columbia University School of Engineering and Applied Science, US, has uncovered a new family of gravitational instabilities in granular particles – two different types of sand, in this case – that are driven by a gas-channelling mechanism not seen in fluids.

Boyce and his team observed what looks like a phenomenon known as a Rayleigh-Taylor (R-T) instability in the interaction of two types of sand with different grade granules. RT instabilities occur commonly in fluid physics, when two fluids of different densities come into contact. The rising and falling bubbles inside a lava lamp constitute a perfect example.

Several other liquid-like properties have previously been observed in the behaviour of sand, but, Boyce says, this is the first time R-T instabilities have been observed.

“We think our discovery is transformational,” he says. “ Our findings could not only explain geological formations and processes that underlie mineral deposits, but could also be used in powder-processing technologies in the energy, construction, and pharmaceuticals industries.”

The research is published in the journal Proceedings of the National Academy of Sciences.

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  1. https://www.pnas.org/cgi/doi/10.1073/pnas.1820820116
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