Look, I can’t pretend to be the most worldly person ― after all, I only just realised the “correct” way to use an electric toothbrush; and the fact that IKEA is actually an acronym and not a Swedish name recently threw me for six.
So I hope you can forgive me for never quite being sure how that annoying tidemark of salt appears on the sides of my pot after cooking pasta or rice in it, despite understanding the basics of evaporation.
But I can at least take comfort in the fact that my bafflement ― why does the salt cluster in a circle before being boiled, and why do the rings take different shapes even when I use the same amount of salt on the same heat? ― has been the foundation of scientific research.
Recently, the American Insitute of Physics shared an experiment run by researchers who wanted to know how to create the best-looking salt rings.
Dr Mathieu Souzy, who took the idea for the test from his son, said: “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.”
What factors go into how the salt ring forms?
When a single grain of salt is dropped into water, it sinks to the bottom of the pan and creates something called a wake drag (like a tiny version of the ripple that follows speeding boats).
But when a “large number of particles are released at the same time,” as when pouring in a handful or even a spoonful of salt, Souzy explained in a statement that “neighbouring particles experience this flow perturbation generated by all surrounding particles”.
He explains, “It causes sedimenting (falling) particles to be progressively shifted horizontally, which leads to an expanding circular distribution of the particles.”
In other words, the more salt you drop in at once and the higher the speed, the more the water in the pan pushes it to the sides.
If you drop the grains from higher up, they have more time to spread out before they reach the bottom. Eventually, they spread so far apart that they stop influencing each other, and they fall straight down individually.
This creates a more evenly distributed, circular pile; the greater the height, the neater the circle.
Size mattered too; bigger grains got pushed out further. That “means you can sort particles by size just by dropping them into a water tank,” Souzy said.
So... do those scientists make really good salt rings now?
Having found that drop height, salt size, and volume of salt all affect how the usually annoying whitish ring forms, I for one wonder whether the scientists themselves have tried to control the “fascinating and varied pattern of deposits” previously left to the salt and steam gods.
Yes, it seems.
In a statement, Souzy said that he can now “create very nice salt rings almost every time.”