Tag Archives: #percolation

The Brazil Nut Effect Is The Mysterious Reason Big Nuts End Up On Top (Science)

Everyone knows what “mixed nuts” really means: a decidedly un-mixed tin with a layer of small peanuts and cashews on the bottom and massive walnuts and Brazil nuts on top. This spontaneous snack sorting is due to the peculiarities of fluid dynamics, and it has a name: the Brazil nut effect.

Despite the nut-centric title, any container full of particles of different sizes can fall prey to this effect when jostled enough (which is why shaking the box won’t help!). Think granola, where the first bowl you pour gets all the delicious clusters and the last bowl gets nothing but oat dust, or coffee grounds, which look perfectly even when they go into the coffee maker but have big partially ground beans at the top when they come out. This everyday phenomenon is deceptively complex, with multiple mechanisms at work. One is percolation. When all of the particles in a container are tossed up together and come back down, small particles move into the spaces beneath larger ones, thereby pushing them upward. Another mechanism is convection: the way particles in the center of a shaken pile will push upward until they get to the top, then fall down in the spaces created at the sides of a container, repeating in a swirling path.

The Brazil nut effect happens when a large particle reaches the top, then just hangs out there. Scientists aren’t sure why this occurs. One reason could be that the particles are too big to fit in the available spaces on the sides of the container. Sometimes, it’s because that particle is a different density than the particles around it—scientists have found that when a particle is either much more or much less dense than its fellow particles, it moves toward the surface much faster. Here’s where things get mysterious: that doesn’t happen when the container is placed in a vacuum chamber, suggesting that the pressure of the air between the particles has something to do with it. Scientists are still studying this phenomenon to learn more about how particles move. And you thought it was just a tin of mixed nuts! Dig deeper into this nutty phenomenon, and other scientific priniciples on display in your food, below.