Physicists have discovered that sand granules behave like liquids of different densities when mixed — they bubble and float.

Movement in the flow of bulk materials such as sand and catalytic particles used in chemical

reactors underlies a wide rangenatural phenomena - from landslides to volcanic eruptions. The same mechanism is used in industrial processes - from pharmaceutical production to carbon sequestration. The movement and mixing of bulk solids often show striking similarities with liquids, as happens in moving sand dunes, avalanches and quicksand. At the same time, the physics behind these free-flowing flows are not as clear as liquid flows.

The recent discovery of Chris Boyce, associate professor of chemicalat Columbia University, explains the nature of a new family of gravitational instabilities in granular particles of various densities that are driven by gas. Similar processes do not occur in liquids. In collaboration with a group of energy and engineering professors Christoph Muller of the Swiss High School of Zurich, the Boyes team discovered unexpected instability, similar to Rayleigh-Taylor, when lighter grains rise through heavier grains in the form of “fingers” and “granular bubbles”. Rayleigh-Taylor instability arises from the interaction of two liquids of different densities that do not mix — for example, oil and water, because a lighter fluid repels a heavier one. This has never been seen before between two dry granular materials.

Beuys research for the first time showed that the "bubble"from light sand can rise through heavy sand, provided that both types of sand are subject to vertical vibration and to the effects of an upward gas flow. Scientists have discovered that just as air and oil bubbles rise in water, because they are lighter than water and do not want to mix with it, light sand bubbles pass through heavy sand, even if you carefully mix the two types of sand.

“We believe that our discovery transformsscience, ”notes Boyce. “We found a“ granular ”analog of fluid mechanical instabilities. Our results can not only explain the geological formations and processes underlying the formation of mineral deposits, but can also be used in powder processing technology in the energy, construction and pharmaceutical industries. ”

Researchers also found that gasthe flow acting on the sand also creates other gravitational instabilities, including the cascade branching of a descending granular drop. They also showed that the Rayleigh-Taylor instability as instability occurs in a wide range of gas flow and vibration conditions, forming different structures under different excitation conditions.

"These instabilities that can be appliedto different systems, shed light on the dynamics of the granular environment and offer new possibilities for the formation of a pattern within the granular mixtures for the formation of new products in the pharmaceutical industry, ”adds Beuys. “We are particularly excited about the potential impact of our findings on the geological sciences — these instabilities will help us understand how the structure of the planet has formed over the long history of the Earth and predict how others will form in the future.”