If you’ve ever cooked pasta, you’ve probably noticed the faint, whitish ring of salt deposits left behind in your pot. It’s a mundane observation, but one that recently inspired a team of scientists to dive into the physics behind this everyday phenomenon.
Their findings reveal that the humble salt ring is the result of a complex interplay of forces — and it could even teach us something about particle dynamics.
The study began, how else, over a casual dinner. Mathieu Souzy, a researcher at the University of Twente in the Netherlands, and his colleagues were enjoying a meal of pasta when they began to wonder: What makes salt form those rings? Why do they look the way they do? And could they control the process to create the “perfect” salt ring?
“By the end of our meal, we’d sketched an experimental protocol and written a succession of experiments we wanted to try on my youngest son’s small whiteboard,” said Souzy. “It was a great overall experience, because we soon realized our simple observation of daily life conceals a rich variety of physical mechanisms!”
The Physics of Pasta Rings
The team started with a simple experiment. They dropped salt particles into a tank of water and watched what happened. When a single particle falls, it creates a tiny wake — a disturbance in the water — as it sinks. But when many particles fall together, things get more interesting.
“If a large number of particles are released at the same time, neighboring particles experience this flow perturbation generated by all surrounding particles,” Souzy explained. “It causes sedimenting (falling) particles to be progressively shifted horizontally, which leads to an expanding circular distribution of the particles.”
This horizontal shift is key to the formation of the ring. As the particles sink, they spread out, creating a circular pattern on the bottom of the tank. The water flowing around the particles pushes them outward, leaving a clear, empty space in the center. The result? A perfect salt ring.
But the story doesn’t end there. The researchers found that the height from which you drop the particles also matters. When salt is released from a greater height, the particles have more time to spread out as they fall. This creates a wider, more uniform deposit. On the other hand, dropping salt from a lower height produces a tighter, more defined ring.
“Larger particles are more radially shifted than small ones,” said Souzy. “This means you can sort particles by size just by dropping them into a water tank!”
From pasta to particle physics
Beyond the kitchen, the same principles could be used to separate particles in industrial processes or even in environmental science, where understanding how particles settle in water is crucial for studying sediment transport in rivers and oceans.
The study also highlights how everyday observations can lead to profound scientific insights. “Despite its apparent simplicity, this phenomenon encompasses a wide range of physical concepts such as sedimentation, non-creeping flow, long-range interactions between multiple bodies, and wake entrainment,” Souzy said.
So the next time you’re boiling water for pasta, take a moment to appreciate the intricate physics at play in your kitchen.
The findings appeared in the journal Physics of Fluids.
