Study Proves Two Kinds of Sand can Act like Heavy and Light Liquids

The flow of granular materials, such as catalytic particles and sand used in chemical reactors, describes a broad variety of natural phenomena, from volcanos to mudslides, and enables a wide range of industrial processes, from pharmaceutical manufacture to carbon capture. Although the motion and mixing of granular matter frequently display striking similarities to liquids, as in moving sand dunes, quicksand, and avalanches, the physics behind granular flows is not as properly understood as liquid flows.

Development of a "bubble" of lighter sand (blue) forming in heavier sand (white). (Image credit: Columbia University)

Now, in a new discovery by Chris Boyce, assistant professor of chemical engineering at Columbia Engineering, explains a new group of gravitational uncertainties in granular particles of varying densities that are triggered by a gas-channeling mechanism not observed in fluids. In partnership with Energy and Engineering Science Professor Christoph Müller’s group at ETH Zurich, Boyce’s team witnessed an unforeseen Rayleigh-Taylor (R-T)-like instability in which lighter grains rise through heavier grains in the shape of “fingers” and “granular bubbles.” R-T instabilities, which are created by the interactions of two fluids of varying densities that do not mix—oil and water, for instance—because the lighter fluid forces aside the heavier one, have not been observed between two dry granular materials.

The research paper, published recently in the Proceedings of the National Academy of Sciences, is the first to show that “bubbles” of lighter sand form and rise through heavier sand when the two kinds of sand are exposed to vertical vibration and upward gas flow, akin to the bubbles that develop and rise in lava lamps. The researchers learned that, just like how air and oil bubbles rise in water as they are lighter than water and do not want to mix with it, bubbles of light sand rise through heavier sand even though two kinds of sand like to combine.

We think our discovery is transformational. 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.

Chris Boyce, Assistant Professor of Chemical Engineering, Columbia Engineering.

Boyce’s team used experimental and computational modeling to demonstrate that gas channeling through lighter particles activates the creation of finger and bubble patterns. The gas channeling happens because the clusters of lighter, larger particles have more permeability to gas flow than do the heavier, smaller grains. The R-T-like instability in granular materials is caused due to a competition between upward drag force increased locally by gas channeling and downward contact forces, a physical mechanism totally different from that witnessed in liquids.

They discovered that this gas-channeling mechanism also produces other gravitational uncertainties, such as the cascading branching of a descending granular droplet. They also showed that the R-T-like instability can occur under a broad range of gas flow and vibration environments, developing various structures under various excitation conditions.

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.

Chris Boyce, Assistant Professor of Chemical Engineering, Columbia Engineering.

Boyce is currently exploring other liquid-like and structured occurrences in sand particles and quantifying their behavior. He is also discussing with geologists and volcanologists ways to explore this process and similar ones taking place in the natural world.

Formation of lava-lamp like structures and "bubbles" of lighter sand and breakup of a "droplet" of heavier sand. (Credit: Columbia University)

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