Open a can of mixed nuts, and chances are you’ll see a bunch of paranotes on top of the heap – whether it’s good or bad depends on how you feel about paranotes. It is such a common phenomenon that it is known as the “mother-of-pearl effect” (although muesli mixture also gives rise to the same dynamics of granular convection). A team of scientists from the University of Manchester in England, who are now on video for the first time, have captured the complex dynamics that cause the nose effect, according to a new article published in the journal Scientific Reports.
From a physical point of view, the mixed nuts are an example of a granular material, such as a sand pile. As I wrote in 2016 at Gizmodo, the primary mechanisms behind the nose 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. The complexity is gravity, the taking down of each grain, as well as the fact that each individual grain pushes against all the others in the container and produces friction and mechanical energy (lost as heat).
Scientists know that the size and shape of the nuts determine how much friction is produced, and its density also plays a role. Large particles that are less dense than other particles around them rise to the top and stay there, just like particles that are denser than those around them. If the difference in density between all the particles is too small, the particles remain mixed. Air pressure plays a role because the density dependence is not available when the particles (or nuts) are in a vacuum, just like the shape of the container.
In short, the phenomenon is a bit more complicated than it initially seems, which motivates physicists to continue studying the paranote effect. But it’s a challenge to figure out what’s going on with paranote when it mixes in the can. Therefore, the team from the University of Manchester focused on an advanced imaging technique, called time-lapse X-ray computer tomography, to track the movement of all the nuts while stirring their container repeatedly.
The Manchester team placed a mixture of peanuts and paranotes in a cupboard, with the nuts initially at the bottom. They placed the slider in a CT machine and did 181 scans while the slider shakes up the mixed nuts, with one shift cycle between each scan to create the time-lapse video.
As expected, the peanuts in the mixture subsided over time, while several larger paranotes gradually rose upward. The Manchester team found that it took about 70 shaving cycles before the first nut to the top 10 per cent of the bed was mixed nuts. Two more reached the same point after 150 shift cycles, while the rest of the paranotes remained trapped at the bottom.
It turns out that the orientation of a given mother-of-pearl in the mix is the key to that upward movement. The nuts usually start in a horizontal position and will only start to rise upwards until they turn to the vertical axis. Once a mother-of-pearl 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. “Furthermore, this ability to detect movement in 3D will pave the way for new experimental studies on separation of mixtures and open the door to even more realistic simulations and powerful predictive models. This will enable us to better industrial equipment designed to minimize size segregation This leads to more uniform mixtures. This is critical for many industries, for example to ensure an even distribution of active ingredients in medicinal tablets, but also in food processing, mining and construction. “
DOI: Scientific Reports, 2021. 10.1038 / s41598-021-87280-1 (on DOIs).
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