Open a box of mixed nuts and you may find a bunch of Brazil nuts covering the pile – whether it’s a good thing or a bad thing depends on how you feel about the Brazil nuts. It is such a common phenomenon that it is known as the “Brazil nut effect” (although the mixture of muesli gives rise to the same dynamics of granular convection). Now, for the first time in the video, a team of scientists from the University of Manchester in England captured the complicated dynamics that cause the effect of nuts in Brazil, according to a new article published in the journal Scientific Reports.
From a physical point of view, those mixed nuts are an example of granular material, like a pile of sand. As I wrote at Gizmodo in 2016, the main mechanisms behind the nut effect in Brazil are percolation and convection. Percolation causes smaller grains to move through larger grains at the bottom of the pile, while convection pushes larger grains toward the top. Complicating the problems is gravity, pulling down each grain, and the fact that each individual grain pushes against all the others in the container, producing friction and mechanical energy (lost as heat).
Scientists know that the size and shape of the nuts determines how much friction is produced, and their density also plays a role. Large particles, which are less dense than other particles around them, rise to the top and remain there, as are particles that are denser than those around them. If the difference in density between all particles is too small, the particles will remain mixed. Air pressure plays a role, as density dependence is not present if the particles (or nuts) are in a vacuum, as is the shape of the container.
In short, the phenomenon is a little more complicated than it initially seems, which motivates physicists to continue studying the effect of nuts in Brazil. But capturing what happens to Brazil nuts as they mix in the box is a challenge. That’s why the University of Manchester team used an advanced imaging technique called time-lapse computed tomography to track the movement of all those nuts, as their container was repeatedly shaken.
The Manchester team placed a mixture of peanuts and Brazil nuts in a hair clipper, with the Brazil nuts initially at the bottom. They placed the shear box inside a CT machine and performed 181 scans while the shear box shook the mixed nuts, with a shear cycle between each scan, to create the time-lapse video.
As expected, over time, the peanuts in the mixture percolated downward, while a few larger Brazil nuts gradually grew upward. The Manchester team found that it took about 70 shear cycles for the first Brazilian nut to reach the top 10 percent of the mixed nut bed. Two others arrived at the same point after 150 shear cycles, while the rest of the Brazil nuts remained trapped at the bottom.
It seems that the orientation of a certain Brazil nut in the mixture is the key to this upward movement. The nuts usually start in a horizontal position and will not start to rise up until they rotate towards the vertical axis. Once a Brazilian nut reaches the surface, it will return to the horizontal position.
“Our study highlights the important role of particle shape and orientation in segregation,” said co-author Parmesh Gajjar. “Moreover, this ability to track motion in 3D will pave the way for new experimental studies of segregation mixtures and open the door to even more realistic simulations and powerful predictive models. This will allow us to better design industrial equipment to minimize size segregation thus leading to more uniform mixtures. This is essential for many industries, for example, ensuring a uniform distribution of active ingredients in medicinal tablets, but also in food processing, exploitation and construction. “
DOI: Scientific Reports, 2021. 10.1038 / s41598-021-87280-1 (About DOI).
Image listed by Melchior / Wikimedia Commons, CC BY-SA 3.0