How come when I open a can of mixed nuts, the Brazil nuts are always at the top?
How come when I open a can of mixed nuts, the Brazil nuts (which I don’t even like) are always at the top? asks a reader.
It’s like a Brazil nut conspiracy. The big, heavy nuts sit like bullying boulders at the top of the can, shoving all the tasty almonds, pecans, cashews, and (not-so-thrilling) peanuts to the bottom. Doesn’t gravity make heavier things sink? Or is it somehow rendered powerless in the confines of a Planters can?
Scientists first officially identified the mystery in the 1930s, around the same time the first Planters peanut stores opened across the U.S. When a container of particles is shaken up and down, a big particle buried inside will tend to rise to the surface.
More than 70 years later, the Brazil Nut Effect still isn’t completely understood. Why all the attention to a problem with party nuts? Scientists say that the Brazil Nut Effect goes way beyond oppressed almonds. The separation of a material’s particles by size affects everything from geological processes to food and drug manufacturing. On the positive side, the effect allows manufacturers to use vibration to separate particles (like grains of rice) by size. On the negative side, it can result in unevenly-mixed medicine.
The Brazil Nut Effect also interests scientists investigating the effects of vibration on sand and snow, and how materials sort during avalanches and landslides. Meanwhile, farmers encounter the effect every spring, when boulders mysteriously appear on open fields, heaved up through the freezing/thawing ground.
The oldest explanation for the effect is also the simplest. With each vertical shake or jostle (say, in a truck on a bumpy road), particles rise off the bottom and then settle. As spaces are created in the material, smaller particles in a container sift down into the gaps under bigger particles. Gradually, the biggest particles are pushed to the top.
But there’s more: Material that is thrown against the container walls experiences friction, and a stream of small particles flows downward along the walls. Meanwhile, in the center, particles heave up, like spaghetti in a pan of rolling boiling water. The big particles can’t return to the bottom through the narrow margins along the side walls. So they remain stuck at the top.
Scientists have also discovered that how fast a nut rises through, say, a box of cereal flakes also involves air in the spaces between particles. Air apparently causes a drag on the smallest particles, keeping them from rising as quickly. In experiments using an air-free vacuum, all the particles, big and small, moved toward the top at the same rate.
To complicate matters, some experiments have shown that big but lightweight particles may actually sink to the bottom, in a kind of reverse Brazil Nut Effect. Meanwhile, other experiments have shown that heavy and lightweight particles rose faster than medium-weight particles of the same size. The upshot? The Brazil Nut Effect is far more complex than anyone imagined 70 — or even 10 — years ago.
Do your own Brazil Nut Effect experiment; visit the website http://199.6.131.12/en/scictr/lab/brazilnut/index.htm.
