Researchers have experimentally resolved a long-standing physics conundrum known as the Feynman sprinkler problem, which involves the counterintuitive behavior of a submerged sprinkler drawing water inward.
The problem, originally posed by physicist Richard Feynman in the 1940s, asks whether reversing the flow of a sprinkler—sucking water in rather than expelling it—would cause rotation in the opposite direction, the same direction, or no rotation at all. Despite its simplicity in description, the phenomenon defied consensus for over a century due to the complex interplay of fluid dynamics and mechanical forces.
Led by Leif Ristroph of New York University, the team conducted precise experiments using modified sprinkler designs to test the hypothesis. Their results, published in the Proceedings of the National Academy of Sciences, confirm that the reverse-flow sprinkler rotates in the opposite direction of a standard sprinkler, albeit at roughly 1/40th the speed. The key factor, they found, lies in the geometry of the sprinkler arms: the bends near the pivot create an offset in the incoming water jets, generating a twisting force akin to the inertial effect felt when a vehicle turns.
The study addresses prior criticism by testing variations of the design, including “silly sprinkler” configurations with additional bends. These trials consistently showed rotation in the same direction as the original S-shaped model, validating the researchers’ earlier 2024 findings. Detlef Lohse, a physicist at the University of Twente, praised the work, stating, “This Feynman problem, as put by Feynman—this is solved.”
While the practical applications of reverse sprinklers remain unclear, the research sheds light on the interactions between solid objects and fluid flows, with potential implications for energy-harvesting technologies such as ocean wave or wind power systems. However, the team acknowledges that a complete theoretical explanation—specifically, a computational simulation accurately modeling fluid pressure dynamics—remains elusive due to the challenge of capturing high-pressure, rapidly moving flows.
Brennan Sprinkle, a collaborator and applied mathematics professor at the Colorado School of Mines, continues working on numerical models to further dissect the problem, noting the simulations present significant computational hurdles. The findings mark a significant step toward reconciling a puzzle that has intrigued physicists since Ernst Mach first formalized it in 1883.
