Scientists Observe Gravitational Instability in Which ‘Bubbles’ of Lighter Sand Form and Rise through Heavier Sand

Gravitational (Rayleigh-Taylor, or R-T) instabilities 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. Now, a team of researchers from Columbia University and ETH Zurich has observed an unexpected R-T-like instability in which lighter grains rise through heavier grains in the form of ‘fingers’ and ‘granular bubbles.’

Development of a ‘bubble’ of lighter sand (blue) forming in heavier sand (white). Image credit: Alex Penn / ETH Zurich.

Development of a ‘bubble’ of lighter sand (blue) forming in heavier sand (white). Image credit: Alex Penn / ETH Zurich.

Published in the Proceedings of the National Academy of Sciences, the study is the first to demonstrate that ‘bubbles’ of lighter sand form and rise through heavier sand when the two types of sand are subject to vertical vibration and upward gas flow, similar to the bubbles that form and rise in lava lamps.

The scientists found 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 of light sand rise through heavier sand even though two types of sand like to mix.

“We think our discovery is transformational. We found a granular analog of one of the last major fluid mechanical instabilities,” said senior author Dr. Chris Boyce, a researcher in the Department of Mechanical and Process Engineering at ETH Zürich and the Department of Chemical Engineering at Columbia University.

“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.”

 

Dr. Boyce and colleagues used experimental and computational modeling to show that gas channeling through lighter particles triggers the formation of finger and bubble patterns.

The gas channeling occurs because 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 arises from a competition between upward drag force increased locally by gas channeling and downward contact forces, a physical mechanism entirely different from that found in liquids.

The team found that this gas-channeling mechanism also generates other gravitational instabilities, including the cascading branching of a descending granular droplet.

The authors 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.

“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,” Dr. Boyce said.

“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.”

_____

Christopher P. McLaren et al. Gravitational instabilities in binary granular materials. PNAS, published online April 22, 2019; doi: 10.1073/pnas.1820820116

About Skype

Check Also

, Quantum Chromodynamics, #Bizwhiznetwork.com Innovation ΛI

Quantum Chromodynamics

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons …

Leave a Reply

Your email address will not be published. Required fields are marked *

Bizwhiznetwork Consultation