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These are bubbles of sand



Bubbles of light sand form in heavier sand.
Image: Alex Zurich

Engineers observed sand bubbles moving in different sands, like oil droplets through water. in a unique observation.

It is difficult for scientists to understand the behavior of countless small moving particles, and we are still learning new things about materials like sand, as some of our earlier stories show. In a new experiment, scientists observed two types of sand interacting with each other, with one sand forming bubbles in the other. A better understanding of the motions of these granular materials could lead to important results.

"There are various motivational factors" to investigate how two sands move toward each other, study author Christopher Boyce, Assistant Professor of Chemical Engineering at Columbia University said Gizmodo. These include construction, pharmaceuticals and even alternative energies.

The experiment consisted of a pair of sand, a white "heavy" sand on a black "light" sand, in a transparent rectangle. The black sand had slightly larger but lighter grains than the white sand. A machine shook the rectangle up and down as air flows up through the sand. The researchers observed grainy "bubbles" and "fingers" made up of the lighter black sand that flowed upwards through the heavier white grains.

The result looked very similar to Rayleigh-Taylor instability, as described in the paper published in the Proceedings of the International, National Academy of Sciences. This is a behavior that occurs when lighter fluids are forced into a heavier fluid. These instabilities arise, for example, when you put water on oil.

Except for two sands, water and oil are not, Boyce explains. This makes research exciting: water and oil do not like mixing, but they like the sands not to mix. In this case, computer simulations demonstrated that the gas flows preferentially through the lighter particles and, in combination with the vibrations, begins to push upwards in a wave-like shape. The upward forces of the gas and the light particles, along with the downward forces from the heavier sand, cause these waves to turn into fingers and then crush into bubbles. And these were sand bubbles, not gas; The scientists carefully controlled the jet to prevent the formation of gas bubbles.

Seems simple, but it's the first time that bubbles form at the interface between a pair of granular solids, Boyce said.

However, this might be the case They sound like a very specific setup that does not occur in the real world. Gases that blow under a pair of different granular materials can be used in industrial environments to mix chemicals that are to react with each other. Boyce said.

The team will continue to explore what kind of fluid-like behavior occurs in granular solids. When scientists find new behaviors, they often follow new uses.


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