Open up a container of mixed nuts and you may notice something: chances are, they are no longer mixed. The larger nuts — Brazil nuts and walnuts, typically, have somehow risen to the top, defying gravity. The smaller peanuts, much to the chagrin of latecomers to the nut mix, have fallen to the bottom. This phenomenon is colloquially called the Brazil nut effect, after the mixed nut nuttiness, but more formally, is called “granular convection.”
The effect is exhibited whenever items of different sizes, packaged together and originally mixed up, are shaken or vibrated. As the amount of jostling increases, the amount of unmixing increases, with the larger items shuffling to the top. The same effect happens with a lot of breakfast cereals, most notably muesli — oats, fruits, and nuts. There is no agreed-upon reason as to why it happens, but there are a lot of theories.
One theory holds that the larger items may have more empty space around them (as their size disallows them to fill in the gaps). Over time, the jostling fixes this, as small items move down into the spaces and the larger items shift upward. Another uses a similar theory — but focuses on the center of mass of the can taken as a whole. Again, this is caused by the empty space created by the larger nuts — if these nuts are at the bottom, the can’s center of mass is too high, and gravity solves the problem by moving these spaces (and, really, the bigger nuts) upward.
Another theory takes the opposite tack. When shaken, the nuts — all of them, regardless of size — circulate throughout the package in a convection-like flow, shifting up through the middle of the container while filtering down around the sides. The larger nuts get stuck at the top, unable to make their way back down to the bottom. The convection flow of nuts keeps on going, as smaller and smaller nuts filter down as the process repeats.
Either way, it’s a serious issue for the manufacturing industry and the science community has published many studies on the topic. After all, what’s the point of mixing nuts if nature is just going to unmix them?
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Related: A two-pack of assorted nuts, in case you want to try it out at home. Twice.