a coupla physicists sitting around talking How to get a perfect salt ring deposit in your pasta pot Particle diameter, height from which they fall through water, and particle volume are key factors. Jennifer Ouellette Jan 21, 2025 12:41 pm | 18 Releasing a handful of salt into a pasta pan can result in a circular ring deposit Credit: Mathieu Souzy Releasing a handful of salt into a pasta pan can result in a circular ring deposit Credit: Mathieu Souzy Story textSizeSmallStandardLargeWidth *StandardWideLinksStandardOrange* Subscribers only Learn morePhysicist Mathieu Souzy of the University Twente was enjoying an evening of pasta and board games with several colleagues when the conversation turned to how adding salt to a pasta pot to make it boil faster can leave a white ring on the bottom of the pot. Ever the curious scientists, they wondered about the various factors that would contribute to creating the perfect circular pattern for a salt ring.By the end of our meal, wed sketched an experimental protocol and written a succession of experiments we wanted to try on my youngest sons small whiteboard, said Souzy. It all comes down to three factors: the diameter of the particles (grains of salt, in this case), the settling height, and the number of particles released simultaneously, according to a new paper published in the journal Physics of Fluids.We've previously reported on physicists' longstanding interest in similar phenomena like the "coffee ring effect," when a single liquid evaporates and the solids that had been dissolved in the liquid (like coffee grounds) form a ring. It happens because the evaporation occurs faster at the edge than at the center. Any remaining liquid flows outward to the edge to fill in the gaps, dragging those solids with it. Mixing in solvents (water or alcohol) reduces the effect as long as the drops are very small. Large drops produce more uniform stains.There are also the so-called "whiskey webs" formed by American whiskeys (but not their Scottish counterparts): an unusual web-like pattern as droplets dry up. Those webs are different for different brands, making them a kind of "fingerprint." Similarly, when a drop of watercolor paint dries, the pigment particles of color break outward, toward the rim of the drop.A pinch of saltSouzy et al. specifically wanted to explore what mechanisms produce the salt rings and whether other deposit shapes are possible, as well as identify the key parameters involved. For their experiments, they used a large transparent water tank filled with waterlarge enough that the physicists could discount any effects a nearby wall might have on the results. Deposit morphologies for a settling particle. When increasing either the injection volume or the settling height, the deposit radius increases. Credit: M. Souzy et al., 2025 They used spherical borosilicate glass beads of varying diameters to represent the grains of salt and loaded different fixed volumes of beads into cylindrical tubes. Then they slid open the tube's bottom to release the beads, capturing how they fell and settled with a Nikon D300 camera placed at the top of the tank. The tank was illuminated from below by a uniform LED light screen and diffuser to get an even background.The physicists found that gravity will pull a single particle to the bottom of the tank, creating a small wake drag that affects the flow of water around it. That perturbation becomes much more complicated when many large particles are released at once, each with its own wake that affects its neighbors. So, the falling particles start to shift horizontally, distributing the falling particles in an expanding circular pattern.Particles released from a smaller height fall faster and form a pattern with a clean central region. Those released from a greater height take longer to fall to the bottom, and the cloud of particles expands radially until the particles are far enough apart not to be influenced by the wakes of neighboring particles such that they no longer form a cloud. In that case, you end up with a homogeneous salt ring deposit.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, said 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. It was a great overall experience, because we soon realized our simple observation of daily life conceals a rich variety of physical mechanisms.Those phenomena are just as relevant outside the kitchen, according to the authors, most notably in such geophysical and industrial contexts as "the discharge of dredged materials and industrial waste into rivers lakes and oceans," they wrote. "In scenarios involving contaminated waste, comprehending the behavior of both the solid waste and the interacting fluid is crucial."Physics of Fluids, 2025. DOI: 10.1063/5.0239386 (About DOIs).Jennifer OuelletteSenior WriterJennifer OuelletteSenior Writer Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 18 Comments