We might not need to “unwater” our lawns, but results could help control fluid flows.
A typical lawn sprinkler features various nozzles arranged at angles on a rotating wheel; when water is pumped in, they release jets that cause the wheel to rotate. But what would happen if the water were sucked into the sprinkler instead? In which direction would the wheel turn then, or would it even turn at all? That’s the essence of the “reverse sprinkler” problem that physicists like Richard Feynman, among others, have grappled with since the 1940s. Now, applied mathematicians at New York University think they’ve cracked the conundrum, per a recent paper published in the journal Physical Review Letters—and the answer challenges conventional wisdom on the matter.
“Our study solves the problem by combining precision lab experiments with mathematical modeling that explains how a reverse sprinkler operates,” said co-author Leif Ristroph of NYU’s Courant Institute. “We found that the reverse sprinkler spins in the ‘reverse’ or opposite direction when taking in water as it does when ejecting it, and the cause is subtle and surprising.”
Ristroph’s lab frequently addresses these kinds of colorful real-world puzzles. For instance, back in 2018, Ristroph and colleagues fine-tuned the recipe for the perfect bubble based on experiments with soapy thin films. (You want a circular wand with a 1.5-inch perimeter, and you should gently blow at a consistent 6.9 cm/s.) In 2021, the Ristroph lab looked into the formation processes underlying so-called “stone forests” common in certain regions of China and Madagascar. These pointed rock formations, like the famed Stone Forest in China’s Yunnan Province, are the result of solids dissolving into liquids in the presence of gravity, which produces natural convective flows.
The big question then becomes: “is that behaviour inherent to all systems like this, or just this one?” Like, if you go to the store, buy a basic sprinkler, and then test it and it behaves exactly opposite to how you might expect it to. Or it does something completely unexpected, like phases into another dimension and starts pumping strawberry jam. Your next step shouldn’t be to say “Oh, weird, I guess that’s that.” You’d start knocking down variables. Is it the same with every sprinkler or just this one? Does the amount of suction applied affect it? If I replace the water with something else does the outcome change?"
If you’re doing research like this, you’re kind of expected to do the same sort of elaboration even if the result of a basic experiment conforms precisely to your hypothesis, because the question isn’t if any given sprinkler setup behaves in this way, it’s about whether this is a universal phenomenon across all similar setups. Because there’s an xkcd for everything, it’s this.