Has this ever happened to you? You wake up and want to make a bowl of cereal or pour some granola over cereal only to find that your raisins and bran have separated. You shake the box hoping to distribute things, but it only makes it worse as bigger chunks rise to the top! What you’ve just witnessed, much to your consternation, is granular convection. Granular convection doesn’t just happen with cereal but with any collection of similarly sized particles. In fact, it is often referred to as “the Brazil Nut Effect” after the way Brazil nuts tend to rise to the surface of bags of mixed nuts despite their large size.
Boxes of cereal and bags of nuts are really just piles of particles interacting through a bunch of physical characteristics and processes like size, shape, friction, and density. Granular convection arises from the immensely complex interactions between these and dozens of other factors, which explains why it is of constant interest to scientists.
For example, a group of scientists in 2019 used a complicated computer imaging system consisting of CT scan time-lapses of shaking nut boxes in order to examine how Brazil nuts and peanuts shifted positions. In doing this, the study discovered a new factor in the effect: shape and orientation. The Brazil nuts wouldn’t rise until they were vertically oriented and then once they rise to the top, they settle horizontally!
Such studies have an immense impact on the logistics of delicate infrastructure industries like mining, shipping, and construction, so while it may seem nuts, we should all be thinking more about granular convection.
A special thanks goes to Parmesh Gajjar, The University of Manchester, for reviewing today's episode.
Further Reading
- Size segregation of irregular granular materials captured by time-resolved 3D imaging, Scientific Reports
- Cracking the Case, Ars Technica
- Defying gravity with the Brazil nut effect, Phys.org