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Proc Natl Acad Sci U S A. 2019 May 7;116(19):9263-9268. doi: 10.1073/pnas.1820820116. Epub 2019 Apr 22.

Gravitational instabilities in binary granular materials.

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Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland.
Department of Chemical Engineering, Columbia University, New York, NY 10027.
Institute for Biomedical Engineering, University of Zürich and ETH Zürich, 8092 Zürich, Switzerland.
Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland;


The motion and mixing of granular media are observed in several contexts in nature, often displaying striking similarities to liquids. Granular dynamics occur in geological phenomena and also enable technologies ranging from pharmaceuticals production to carbon capture. Here, we report the discovery of a family of gravitational instabilities in granular particle mixtures subject to vertical vibration and upward gas flow, including a Rayleigh-Taylor (RT)-like instability in which lighter grains rise through heavier grains in the form of "fingers" and "granular bubbles." We demonstrate that this RT-like instability arises due to a competition between upward drag force increased locally by gas channeling and downward contact forces, and thus the physical mechanism is entirely different from that found in liquids. This gas channeling mechanism also generates other gravitational instabilities: the rise of a granular bubble which leaves a trail of particles behind it and the cascading branching of a descending granular droplet. These instabilities suggest opportunities for patterning within granular mixtures.


fluidization; granular material; instabilities

[Available on 2019-10-22]

Conflict of interest statement

The authors declare no conflict of interest.

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