If you’ve ever added a substantial pinch of salt to the water in your pasta pan for flavor, or to speed up the boiling process, you may have noticed a whitish ring of deposits forming inside the pan. This occurrence led a team of scientists to explore the intriguing question of how to generate the most aesthetically pleasing salt ring in a pasta pot.
If you’ve ever added a substantial pinch of salt to the water in your pasta pan for flavor, or to speed up the boiling process, you may have noticed a whitish ring of deposits forming inside the pan.
Scientists from the University of Twente in the Netherlands and the French National Institute for Agriculture, Food, and Environment (INRAE) were inspired to investigate this phenomenon during a pasta dinner while playing board games. They pondered what factors influence the formation of a beautiful salt ring in a pasta pan: Does the size of the salt grains matter? What about their quantity or the speed at which they are added? Is there a perfect water level for optimal results?
The group shared their discoveries in the journal Physics of Fluids, published by AIP Publishing. Their experimental approach is straightforward, easily replicable, and cost-effective.
“By the end of our meal, we had designed an experimental protocol and documented a series of tests we wanted to conduct on a small whiteboard owned by my youngest son,” recalled Mathieu Souzy. “It turned out to be a delightful experience, as we quickly recognized that our simple everyday observation hides a complex array of physical processes!”
So, what happens inside the pan? When a single salt particle is introduced into a container of water, gravity causes it to sink to the bottom, creating a slight disturbance in the surrounding water flow.
“If many particles are introduced simultaneously, each one affects its neighbors due to the flow disturbances they create,” explained Souzy. “This results in the sedimenting particles being gradually pushed sideways, forming an expanding circular pattern.”
When particles settle at the bottom quickly, they form a circular deposit, while the water caught in the wake of the particle cloud helps to push them outward, leading to a clear central region devoid of particles.
However, if particles are dropped from a higher position, they have more time to fall, allowing the particle cloud to spread further. Eventually, the particles become spaced far enough apart that their interactions diminish, and they settle down uniformly, creating a consistent circular deposit.
“These are the essential physical elements at play, and although it seems simple, this phenomenon incorporates a variety of physical concepts, like sedimentation, non-creeping flow, interactions among multiple bodies, and wake entrainment,” Souzy noted. “What’s particularly fascinating is that larger particles diffusing in water tend to spread out more than smaller ones. This means you can sort particles by size simply by dropping them into a water tank!”
Now, Souzy can easily “create very nice salt rings almost every time” he cooks.
