Ellen Phiddian
Cosmos science journalist
Ever noticed a white ring forming in the bottom of your pasta pan?
It’s salt, and physicists have figured out how to make the best one – providing insights into molecular behaviour.
Unsurprisingly, the researchers, who published this crucial insight in Physics of Fluids, first started wondering about salt rings at a pasta dinner.
“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,” says lead author Dr Mathieu Souzy, a researcher at the French National Research Institute for Agriculture, Food and Environment.
“It was a great overall experience, because we soon realised our simple observation of daily life conceals a rich variety of physical mechanisms!”
The team dropped a range of different types and amounts of salt into water to see what different rings formed.
They found that falling particles cause small wakes flowing behind them, which can disturb nearby particles.
“If a large number of particles are released at the same time, neighbouring particles experience this flow perturbation generated by all surrounding particles,” says Souzy.
“It causes sedimenting (falling) particles to be progressively shifted horizontally, which leads to an expanding circular distribution of the particles.”
When the particles fall quickly, the turbulence creates an expanding ring of salt, with depleted levels in the centre.
But if they fall from a greater distance and a longer time, they expand far enough that their turbulence stops affecting each other and the donut-shaped cloud collapses back into a simple circle of salt.
“These are the main physical ingredients, and 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,” says Souzy.
“Things get even more interesting once you realize larger particles are more radially shifted than small ones, which means you can sort particles by size just by dropping them into a water tank!”
Souzy adds that he can now “create very nice salt rings almost every time” he’s in the kitchen.
