Demonstrating bubbles of sand

Defying the laws of physics?

Development of a
Development of a "bubble" of lighter sand (blue) forming in heavier sand (white).

The flow of granular materials, such as sand and catalytic particles, is critical to a wide range of natural phenomena and industrial processes. However, the physics underlying granular flows are poorly understood.

In a new study, Chris Boyce, assistant professor of chemical engineering at Columbia Engineering, explained a new family of gravitational instabilities in granular particles of different densities that are driven by a gas-channeling mechanism not seen in fluids.

He along with Energy and Engineering Science Professor Christoph Müller’s group at ETH Zurich, Boyce’s team observed an unexpected Rayleigh-Taylor (R-T)-like instability in which lighter grains rise through heavier grains in the form of “fingers” and “granular bubbles.”

R-T 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 pushes aside the heavier one, have not been seen between two dry granular materials.

This is for the time that scientists have demonstrated that “bubbles” of lighter sand form and rise through heavier sand. This phenomenon can be explained as When two types of sand are subject to vertical vibration and upward gas flow, similar to the bubbles that form and rise in lava lamps.

This is as same as air and oil bubbles. When air and oil bubbles rise in water because they are lighter than water and do not want to mix with it, bubbles of light sand rise through heavier sand even though two types of sand like to mix.

Boyce said, “We have found a granular analog of one of the last major fluid mechanical instabilities. 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 gas channeling happens on the grounds that the clusters of lighter, larger particles have a higher permeability to gas flow than do the heavier, smaller grains. The R-T-like instability in granular materials emerges from a competition between upward drag force increased locally by gas channeling and downward contact forces, a physical mechanism entirely different in relation to that found in liquids.

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

Boyce said, “These instabilities, which can be applied to a variety of systems, shed new light on granular dynamics and suggest new opportunities for patterning within granular mixtures to form new products in the pharmaceutical industry, for example. We are especially excited about the potential impact of our findings on the geological sciences—these instabilities can help us understand how structures have formed over the long history of the Earth and predict how others may form in the future.”

Boyce is now investigating other liquid-like and structured phenomena in sand particles and quantifying their behavior. He is also in conversations with geologists and volcanologists to explore more about how this process and similar ones occur in the natural world.

The study is published today in the Proceedings of the National Academy of Sciences.