Why Larger Nuts Always Rise to the Top When You Shake

Open a can of mixed nuts and you'll notice something odd: the Brazil nuts are always sitting on top, like they're too good to mingle with the peanuts and cashews below. Shake the can, and those big guys rise even higher. It's not Brazil nut snobbery—it's physics.

Scientists call this the Brazil nut effect, and it's a perfect example of granular convection. When you shake a container of differently sized particles, the larger ones work their way to the surface while the smaller ones sink. It defies what you'd expect from gravity alone, and it happens in everything from breakfast cereal to pharmaceutical pills.

The Physics of Rising Nuts

Three main forces conspire to push those Brazil nuts upward. First, there's percolation: when you shake the can, tiny gaps open up beneath the larger nuts, and smaller particles tumble down to fill them. Shake after shake, the big nut gets nudged upward while its smaller neighbors slip past.

Second, granular convection creates circulation patterns. The shaking motion causes particles to move up through the center of the container, across the surface, and down the sides—like a slow-motion lava lamp. Large particles get caught in the upward current, but once they reach the top, they're too big to fit into the downward flow along the walls. They're stuck at the surface.

Finally, the whole system wants to reach a lower energy state. Physics loves efficiency, and the most stable arrangement is heavy stuff on bottom, with the center of mass as low as possible. Since Brazil nuts are less dense but much larger, packing smaller, denser nuts underneath actually lowers the overall center of mass.

Seeing Inside the Can

For decades, scientists understood that the Brazil nut effect happened, but not exactly how. In 2021, researchers used X-ray CT scans to watch individual nuts move inside a shaking container, creating the first 3-D videos of the effect in action. They discovered something surprising: as the box shook, Brazil nuts gradually tilted to a more vertical orientation. This opened up space for peanuts higher in the mixture to tumble down beneath them, pushing the Brazil nuts upward.

Even weirder? The effect doesn't always need shaking. A 2023 study found that charged colloidal particles can experience the Brazil nut effect spontaneously, driven only by Brownian motion and electrical repulsion. The universe, it turns out, really loves sorting things.

Why It Matters Beyond Snack Time

Food manufacturers hate the Brazil nut effect. Once you've created a perfectly mixed bag of trail mix or a uniform batch of granola, the last thing you want is for it to separate during shipping. Every shake of the delivery truck is an opportunity for the big stuff to migrate topward, leaving customers with an uneven product.

• Pharmaceutical companies face the same challenge—uneven distribution of active ingredients in pills could mean inconsistent doses
• Construction workers deal with it when concrete aggregates separate, weakening the final product
• Even geologists study it to understand how rocks and sediments sort themselves during earthquakes

The next time you open a can of mixed nuts and find the Brazil nuts lounging on top, remember: they didn't climb there through ambition. They were pushed by the relentless, impersonal forces of physics, one tiny shake at a time.