Open a can of mixed nuts, and chances are you’ll find a bunch of Brazil nuts topping the heap—whether that’s a good or bad thing depends on how you feel about Brazil nuts. It’s such a common phenomenon that it’s known as the “Brazil nut effect” (though muesli mix also gives rise to the same dynamics of granular convection). Now, on video for the first time, 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 physics standpoint, those mixed nuts are an example of a granular material, like a sand pile. As I wrote at Gizmodo back 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 pile, while convection pushes the larger grains toward the top. Complicating matters is gravity, pulling down on every grain, as well as the fact that every individual grain is jostling against all the others in the container, producing friction and mechanical energy (lost as heat).