For a long time, the behavior of moving granular particles such as sand has been a mystery, even to scientists.
When it's in motion, sand is strikingly similar to liquid, as seen in shifting sand dunes and avalanches, among other events. However, understanding the physics of sand's flow remains elusive.
Now, engineers gain valuable insight in granular particles with a discovery that Columbia Engineering associate professor Chris Boyce calls "transformational," according to a report from the university.
Sand Bubbles Form In New Experiment
In a study published in the journal Proceedings of the National Academy of Sciences, researchers introduce a new family of gravitational instabilities in granular particles of different densities. Surprisingly, the mechanism is much more similar to gas than liquid.
Findings show a Raleigh-Taylor instability, which is when lighter grains rise through heavier grains in the form of "fingers" and "bubbles." This type of instability occurs when two fluids of different densities that don't mix interact, such as water and oil. Until the new study, it's behavior that has never been observed in two dry and solid granular particles.
For the first time ever, researchers show bubbles of lighter sand form and rise through heavier sand when both types of sand are exposed to vertical vibration and upward gas flow.
It's just like air bubbles and oil bubbles rising in water, because these particles don't mix with water. In the case of sand, though, the two types of sand do mix.
"We have found a granular analog of one of the last major fluid mechanical instabilities," explained Boyce, one of the study authors. "While analogs of the other major instabilities have been discovered in granular flows in recent decades, the R-T instability has eluded direct comparison. 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 Experiment And Its Impact On Science
To reach their findings, the researchers used experimental and computational modeling to demonstrate the gas channeling through lighter sand and creating the bubbles. The Raleigh-Taylor instability occurs due to the clash between the upward push of the lighter particles combined with the gas channeling and the downward force of the heavier particles, which is a behavior that liquid particles don't display.
In a report from Gizmodo, Boyce said that while the experiment setup may be highly unlikely to occur in the real world, it could be used in industrial settings on chemicals that are meant to react to each other.
The team is eager to see the potential impacts of their findings on the geological sciences since these types of instabilities can shed light on how the planet's many structures formed throughout history.