Engineers observed bubbles of sand moving within other sand, like oil droplets through water, in a first-of-its-kind observation.
It’s difficult for scientists to understand the behaviour of countless small moving particles, and we’re still learning new things about materials like sand, as some of our past stories show. In a new experiment, scientists observed two kinds of sands interacting, where one sand would form bubbles in the other sand. Better understanding the movements of these granular materials could lead to important results.
“There are a variety of different motivating factors” for studying how two sands move around one another, study author Christopher Boyce, assistant professor in chemical engineering at Columbia University, told Gizmodo. That includes construction, pharmaceuticals, and even alternative energies.
The experiment consisted of a pair of sands, a white “heavy” sand atop a black “light” sand, in a see-through rectangle. The black sand had slightly larger, but lighter, grains than the white sand. A machine shook the rectangle up and down, while air flows upward through the sand. The researchers observed as granular “bubbles” and “fingers” composed of the lighter black sand flowed upward through the heavier white grains.
The result looked much like a Rayleigh–Taylor instability, according to the paperpublished in the Proceedings of the National Academy of Sciences. That’s a behaviour that occurs when lighter fluids push into a heavier fluid. These kinds of instabilities form when you put water on top of oil, for example.
Except two sands aren’t water and oil, Boyce explained, which is what makes the research exciting—water and oil don’t like to mix, but these sands otherwise don’t mind mixing. In this case, computational simulations demonstrated the gas flowing preferentially through the lighter particles, and combined with the vibrations, it starts pushing up in a wave-like shape. The upward forces of the gas and light particles, combined with the downward forces from the heavier sand, cause these waves to turn into fingers, and then pinch off into bubbles. And these were bubbles of sand, not gas; the scientists carefully controlled the jet to prevent the formation of gas bubbles.
Seems simple, but it’s the first time bubbles have been shown to form at the interface between a pair of granular solids, Boyce said.
Though this might sound like a highly specific setup that wouldn’t occur in the real world, gases blowing beneath a pair of different granular materials can be used in industrial settings to mix chemicals meant to react with one another, Boyce said.
The team will continue researching what sorts of liquid-like behaviours also appear in granular solids. After all, when scientists find novel behaviours, novel uses for them often follow.