Granular convection

Brazil nuts ride on top of other assorted nuts

Granular convection is a phenomenon where granular material subjected to shaking or vibration will exhibit circulation patterns similar to types of fluid convection.[1] It is sometimes described as the Brazil nut effect when the largest particles end up on the surface of a granular material containing a mixture of variously sized objects; this derives from the example of a typical container of mixed nuts, where the largest will be Brazil nuts. The phenomenon is also known as the muesli effect since it is seen in packets of breakfast cereal containing particles of different sizes but similar density, such as muesli mix.

Under experimental conditions, granular convection of variously sized particles has been observed forming convection cells similar to fluid motion.[2][3] The convection of granular flows is becoming a well-understood phenomenon.[4]


It may be counterintuitive to find that the largest and (presumably) heaviest particles rise to the top, but several explanations are possible:

The phenomenon is related to Parrondo's paradox in as much as the Brazil nuts move to the top of the mixed nuts against the gravitational gradient when subjected to random shaking.[6]

Granular convection has been probed by the use of MRI [7] where convection rolls similar to those in fluids (Bénard cells) can be visualized.



The effect is of serious interest for some manufacturing operations; once a homogeneous mixture of granular materials has been produced, it is usually undesirable for the different particle types to segregate. Several factors determine the severity of the Brazil nut effect, including the sizes and densities of the particles, the pressure of any gas between the particles, and the shape of the container. A rectangular box (such as a box of breakfast cereal) or cylinder (such as a can of nuts) works well to favour the effect, while a cone-shaped container results in what is known as the reverse Brazil nut effect.


In astronomy, it is also seen in some low density, or rubble pile asteroids, for example the asteroid 25143 Itokawa.[8]


In geology, the effect is common in formerly glaciated areas such as New England and areas in regions of permafrost where the landscape is shaped into hummocks by frost heave — new stones appear in the fields every year from deeper underground. Horace Greeley noted "Picking stones, is a never-ending labor on one of those New England farms. Pick as closely as you may, the next plowing turns up a fresh eruption of boulders and pebbles, from the size of a hickory nut to that of a tea-kettle." [9] A hint to the cause appears in his further description that "this work is mainly to be done in March or April, when the earth is saturated with ice-cold water". Underground water freezes, lifting all particles above it. As the water starts to melt, smaller particles can settle into the opening spaces while larger particles are still raised. By the time ice no longer supports the larger rocks, they are at least partially supported by the smaller particles that slipped below them. Repeated freeze-thaw cycles in a single year speeds up the process.

This phenomenon is one of the causes of inverse grading which can be observed in many situations including soil liquefaction during earthquakes or mudslides. Granular convection is also exemplified by debris flow, which is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. These flows can carry material ranging in size from clay to boulders, including woody debris such as logs and tree stumps. Flows can be triggered by intense rainfall, glacial melt, or a combination of the two.

See also


  1. Granular Convection and Size Separation. The University of Chicago
  2. Rietz, Frank; Stannarius, Ralf (2008). "On the brink of jamming: Granular convection in densely filled containers". Physical Review Letters. 100 (7). Bibcode:2008PhRvL.100g8002R. doi:10.1103/PhysRevLett.100.078002.
  3. Baffling Patterns Form in Scientific Sandbox, Wired, Brandon Keim, October 28, 2009
  4. Grains of Sand Reveal Possible Fifth State of Matter, Wired, Brandon Keim, June 24, 2009
  5. "Sidney Nagel and Heinrich Jaeger Q&A". Retrieved 2010-09-27.
  6. Abbott, Derek (2009). "Developments in Parrondo's Paradox". Applications of Nonlinear Dynamics. Springer. pp. 307–321. ISBN 978-3-540-85631-3.
  7. Ehrichs, E. E.; Jaeger, H. M.; Karczmar, G. S.; Knight, J. B.; Kuperman, V. Yu.; Nagel, S. R. (1995). "Granular Convection Observed by Magnetic Resonance Imaging". Science. 267 (5204): 1632–4. Bibcode:1995Sci...267.1632E. doi:10.1126/science.267.5204.1632. PMID 17808181.
  8. Nemiroff, R.; Bonnell, J., eds. (22 April 2007). "Smooth Sections of Asteroid Itokawa". Astronomy Picture of the Day. NASA.
  9. excerpt from Recollections of a Busy Life, by Horace Greeley 1869
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