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

Movement in flow of bulk materials such as sand and catalyst particles used in chemical applications

reactors, underlies a wide range ofnatural phenomena - from landslides to volcanic eruptions. The same mechanism is used in industrial processes ranging from pharmaceutical production to carbon capture. The movement and mixing of solids often exhibit striking similarities to liquids, as occurs in moving sand dunes, avalanches, and quicksand. At the same time, the physics underlying such granular flows is not as clear as liquid flows.

A recent discovery by Chris Boyce, associate professor of Engineering at Columbia University, explains the nature of a new family of gravitational instabilities in granular particles of varying densities that are driven by gas. Similar processes do not occur in liquids. In collaboration with the group of energy and engineering professor Christoph Müller at ETH Zurich, Beuys's team discovered an unexpected Rayleigh-Taylor-like instability where lighter grains rise through heavier grains in the form of "fingers" and "grain bubbles." Rayleigh-Taylor instability results from the interaction of two liquids of different densities that do not mix—for example, oil and water, because the lighter liquid repels the heavier one. This has never been seen before between two dry granular materials.

Boyce's study showed for the first time that the "bubble"from light sand can rise through heavy sand, provided that both types of sand are subject to vertical vibration and 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, bubbles from light sand pass through heavy sand, even if the two types of sand are thoroughly tried to mix.

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

The researchers also found that gasthe flow acting on the sand also creates other gravitational instabilities, including cascading branching of the downward granular droplet. They also showed that the Rayleigh-Taylor instability as an instability occurs over a wide range of gas flow rates 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.”