Open a can of mixed nuts and chances are there’s a bunch of Brazil nuts on top of the heap – whether that’s good or bad depends on how you feel about Brazil nuts. It’s such a common phenomenon that it’s known as the “Brazil nut effect” (although muesli mix also gives rise to the same dynamics of granular convection). Now, for the first time on video, a team of scientists from the University of Manchester in England has captured the complicated dynamics that cause the Brazil nut effect, according to a new paper published in the journal Scientific Reports.
From a physical point of view, those mixed nuts are an example of a granular material, such as a sand pile. As I wrote at Gizmodo in 2016, the primary mechanisms behind the Brazil nut effect are percolation and convection. Percolation causes smaller grains to move through larger grains to the bottom of the stack, while convection pushes the larger grains upward. Complicating things are gravity, which pulls each grain down, as well as the fact that each individual grain knocks against everyone else in the container, producing friction and mechanical energy (lost as heat).
Scientists know that the size and shape of the nuts determine how much friction is produced, and their density also plays a role. Large particles that are less dense than other particles around it rise to the top and stay there, as do particles that are denser than the particles around it. If the difference in density between all the particles is too small, the particles will simply remain mixed. Air pressure plays a role, as that density dependence is not present when the particles (or nuts) are in a vacuum, just like the shape of the container.
Basically, the phenomenon is a bit more complicated than it initially appears, which motivates physicists to keep studying the Brazil nut effect. But it’s challenging to capture what’s going on with Brazil nuts as they mix in the can. That’s why the University of Manchester team turned to an advanced imaging technique called time-lapse X-ray computer tomography to track the movement of all those jostling notes as their container was moved repeatedly.
The Manchester team placed a mixture of peanuts and Brazil nuts in a shear box, with the Brazil nuts initially at the bottom. They placed the scissors box in a CT machine and ran 181 scans while the scissors box shook the mixed notes, with one shear cycle between each scan, to create the time-lapse video.
As expected, the peanuts in the mix fell over time, while several larger Brazil nuts gradually moved up. The Manchester team found that it took about 70 scissors cycles for the first Brazil nut to reach the top 10 percent of the mixed nut bed. Two others reached the same point after 150 shear cycles, with the rest of the Brazil nuts stuck to the bottom.
As it turns out, the orientation of a particular Brazil nut in the mix is key to that upward movement. The nuts usually start in a horizontal position and they don’t start to rise upward until they turn toward the vertical axis. Once a Brazil nut reaches the surface, it returns to the horizontal position.
“Our study highlights the important role of particle shape and orientation in segregation,” said study co-author Parmesh Gajjar. “Furthermore, this ability to track motion in 3D will pave the way for new experimental studies of separating mixtures and will open the door to even more realistic simulations and powerful predictive models. minimizing separation in size, leading to more uniform mixtures. This is critical for many industries, for example to ensure an even distribution of active ingredients in medicinal tablets, as well as in food processing, mining and construction. “
DOI: Scientific Reports, 2021.10.1038 / s41598-021-87280-1 (about DOIs).
Listing image by Melchior / Wikimedia Commons, CC BY-SA 3.0