For more than 140 years, physicists have debated one of the most famous thought experiments in fluid mechanics: What happens when a submerged lawn sprinkler draws water inward instead of spraying it outward? Known as the Feynman sprinkler problem, the puzzle fascinated generations of scientists, including Nobel Prize-winning physicist Richard Feynman, who famously attempted to solve it through an experiment that ended with the apparatus exploding before yielding an answer, tells The New York Times (full article available to subscribers).
Now, researchers led by Leif Ristroph at New York University believe they have finally settled the debate. Their latest findings, published in the Proceedings of the National Academy of Sciences, build on earlier experiments and provide the strongest experimental evidence yet that a reverse sprinkler rotates in the opposite direction of a conventional sprinkler. However, the reverse motion is much weaker, spinning at only about one-fortieth the speed of a normal sprinkler.
The team’s explanation centers on the behavior of water flowing through the sprinkler’s curved arms. As water is drawn inward, the bends near the central pivot redirect the incoming streams so they no longer collide head-on. This offset creates a twisting force, or torque, that causes the sprinkler to rotate in the opposite direction. According to the researchers, the amount of bending close to the pivot, rather than the orientation of the nozzle openings, is the key factor controlling the motion.
To test competing theories, the researchers designed several unconventional sprinkler configurations with additional bends and altered shapes. Despite these modifications, every design rotated in the same direction, demonstrating that earlier explanations based on nozzle orientation or suction forces were incorrect. These experiments strengthened the team’s confidence that its proposed mechanism accurately explains the phenomenon.
Although reverse sprinklers have little direct practical value, the research improves scientists’ understanding of the complex interaction between flowing fluids and moving structures. Such knowledge could support the design of energy harvesting systems, underwater devices, and other engineering technologies involving fluid-structure interactions. The researchers note that one challenge remains: creating a detailed computer simulation that fully captures the intricate pressure distribution inside the moving sprinkler. Achieving that goal would provide a complete theoretical solution to one of physics’ longest-running and most intriguing puzzles.