Researchers at New York University's Courant Institute and collaborators have confirmed the momentum flux theory that explains how water flow drives rotation in reverse sprinklers, showing the same principles apply to “silly sprinklers” used for lawn watering.

The reverse sprinkler problem, originally discussed by Ernst Mach and later popularized by Richard Feynman, had remained unresolved; experiments had produced mixed results about whether such devices rotate opposite to normal sprinklers.

In 2024, Leif Ristroph and team built a low‑friction rotary sprinkler, tested it with controlled inflow and outflow rates, and used dyes and high‑speed laser imaging to map flow. They found the reverse sprinkler turned at one‑fiftieth the speed of a standard model but followed the same fluid dynamics, supporting the momentum flux theory and contradicting Mach’s and Feynman’s earlier hypotheses. The study also showed that arm geometry influences jet flow and can be used to design devices that generate torque, with implications for turbines and other energy‑conversion technologies. Co‑author Brennan Sprinkle noted that the findings help understand how fluid flows can be harnessed for energy applications. Previous work by the same lab on bubbles, stone‑forest formation, Tesla valves, and paper‑airplane aerodynamics illustrates a pattern of tackling whimsical yet scientifically significant problems.

The results provide a clearer basis for engineering designs that exploit fluid dynamics, though further research is needed to generalize the theory to other sprinkler configurations.