Have you ever opened a bag of mixed snacks to find that all the biggest items are all on the top? This is called the ‘brazil-nut effect’, and it’s taken a surprisingly long time to figure out how it happens.
The brazil-nut effect occurs in granular solids – collections of solid objects that can pour like liquids (for example, sand or mixed nuts). The effect makes the biggest ‘grains’ rise to the top of the mixture when shaken.
Earlier attempts to understand this effect used computer simulations, but most of these couldn’t handle realistic shapes. Recently, some European scientists tried a different method.
They put some brazil nuts in a container and covered them with smaller peanuts. The researchers then gently shook the container, x-raying it between each shake. When they watched the x-rays together, they saw the brazil nuts bump around randomly until they pointed vertically, surrounded by peanuts. This position let the peanuts move downwards, pushing the brazil nuts upwards in the process.
This article was published in Issue 49 of Double Helix magazine (https://www.csiro.au/en/Education/Double-Helix). Copyright for this article is held by CSIRO.
Lachlan Writes Science 