Challenge the laws of physics? Engineers demonstrate sand bubbles

The flow of granular materials, such as sand and catalytic particles used in chemical reactors, allows a wide range of natural phenomena, from mudflows to volcanoes, as well as a wide range of industrial processes, from production pharmaceutical industry to carbon capture. Although the movement and mixing of granular material often have striking similarities to liquids, such as in moving sand dunes, avalanches and quicksand, the physics underlying granular flow is not as well understood as the liquid flows.

A recent discovery by Chris Boyce, an assistant professor of chemical engineering at Columbia Engineering, explains a new family of gravitational instabilities in granular particles of different densities, driven by a gas channeling mechanism not observed in fluids. In collaboration with Professor Christoph Müller's group of energy and engineering sciences at the ETH Zurich, the Boyce team observed an unexpected instability resembling that of Rayleigh -Taylor (RT), in which lighter grains rise under the effect of "fingers" and "granular bubbles". The RT instabilities, which are produced by the interactions of two fluids of different densities that do not mix – oil and water, for example – because the lighter fluid spreads the heavier fluid, have not been observed between two dry granular materials.

The study, published today in the Proceedings of the National Academy of Sciences, is the first to demonstrate that "bubbles" of lighter sand form and rise in thicker sand when both types of sand are subjected to vertical vibration and upward gas flow, similar to bubbles that form and form in lava lamps. The team found that just as air and oil bubbles rise in the water because they are lighter than water and do not want to Mix it up, light sand bubbles rise into a thicker sand even though two types of sand mix together.

"We think our discovery is a transformation," says Boyce, "We have found a granular analogue of one of the last great instabilities of fluid mechanics, while analogues of other major instabilities have been discovered in granular flows in recent decades, the instability of the ambient temperature has escaped direct comparison.Our results could not only explain the formations and geological processes underlying the mineral deposits, but they could also be used in the technologies powder processing in the energy, construction and pharmaceutical industries. "

Boyce's group used experimental and computer modeling to show that channeling gases through lighter particles triggers the formation of finger patterns and bubbles. The gas pipeline occurs because clusters of lighter and larger particles have a higher permeability to gas flow than heavier and smaller grains. The RT-type instability in granular materials stems from a competition between the upward drag force locally augmented by the gas channeling forces and the downward contact forces, a physical mechanism totally different from that of the liquids.

They found that this gas channeling mechanism also generates other gravitational instabilities, including the cascading branching of a descending granular droplet. They also demonstrated that R-T type instability can occur in a wide variety of gas flow and vibration conditions, forming different structures under different excitation conditions.

"These instabilities, which can be applied to a variety of systems, shed new light on granular dynamics and suggest new possibilities for structuring within granular mixtures to form new products in the pharmaceutical industry," adds Boyce. . "We are particularly excited about the potential impact of our discoveries on the geological sciences, and these instabilities can help us understand how structures have formed over the long history of the Earth and predict how they will behave. others could be formed in the future. "

Boyce is now studying other structural and liquid-like phenomena in sand particles and quantifying their behavior. He is also in conversation with geologists and volcanologists to find out more about how this process and similar processes occur in the natural world.