Applied mathematicians at New York University’s Courant Institute have finally resolved a decades-old physics mystery originally popularized by Nobel laureate Richard Feynman, demonstrating exactly how a ‘reverse sprinkler’ behaves when submerged and sucking in water. Publishing their findings in the journal Physical Review Letters, the research team used custom-built, ultra-low-friction backyard-style sprinklers to observe the precise fluid mechanics that govern this elusive phenomenon. The breakthrough ends a long-standing academic debate by proving that reverse sprinklers do indeed rotate in the opposite direction of normal sprinklers, driven by internal momentum collisions.
The Legacy of Feynman’s Sprinkler Problem
The puzzle, known widely as the Feynman sprinkler problem, asks a seemingly simple question: if a common lawn sprinkler rotates when spraying water outward, which way will it spin if it is submerged in a fluid and sucks that fluid inward? First debated in the mid-20th century, the problem has perplexed physicists, including Feynman himself, who famously declined to publish a definitive answer after a laboratory apparatus he constructed exploded under pressure.
For decades, the scientific community split into competing camps. Some researchers argued the reverse sprinkler would spin in reverse due to the suction forces, others claimed it would not move at all because suction lacks a directed nozzle jet, and a third group suggested it would rotate in the normal direction due to complex fluid drag.
Previous experiments yielded contradictory results, primarily because underwater friction and turbulent flows masked the subtle forces at play. To overcome these experimental hurdles, the NYU team designed a highly sensitive, near-frictionless rotary device that could operate underwater for extended periods without interference.
Inside the Chamber: How Water Collides
The researchers constructed a transparent, custom-engineered sprinkler system that allowed them to visualize the flow of water both outside and inside the device. They added tiny microscopic particles to the water, illuminating them with green lasers to track the fluid’s exact path using high-speed cameras.
The experiments revealed that when water is sucked into the sprinkler’s arms, it forms inward-directed jets that meet inside the central chamber. Instead of a simple suction effect at the nozzle tips, the key driver of the motion is the collision of these water jets inside the hub.
As the incoming streams of water collide, they generate a localized force that pushes against the internal walls of the sprinkler chamber. This internal momentum transfer generates a torque that rotates the device in the reverse direction, opposite to its standard outward-spraying motion.
Crucially, the team discovered that this mechanism remains consistent regardless of the shape or curvature of the sprinkler arms. Whether the arms are straight, curved, or angled, the internal collision of fluid momentum dictates the rotational behavior, providing a unifying explanation for previous experimental discrepancies.
Validating the Physics with Precise Data
‘We found that the reverse sprinkler behaves as a steady device, meaning it rotates continuously once the suction begins,’ said Leif Ristroph, an associate professor at NYU’s Courant Institute and the study’s lead author. ‘The force is incredibly weak—about 50 times weaker than the forward rotation—which explains why previous scientists struggled to measure it accurately without ultra-low-friction setups.’
The mathematical models developed by the NYU team successfully predicted the exact rotational speeds and forces observed during the physical experiments. By measuring the angle and velocity of the colliding internal jets, the researchers established a direct mathematical relationship between fluid flow rate and rotational torque.
This quantitative approach allowed the team to rule out alternative theories, such as pure boundary layer drag or external suction forces, as the primary drivers of the rotation. The data conclusively showed that internal fluid dynamics, rather than external forces, govern the reverse sprinkler’s behavior.
Engineering Implications and Future Frontiers
While solving a theoretical physics puzzle satisfies a decades-old intellectual curiosity, the underlying principles of the research have immediate practical applications. Understanding how internal fluid collisions generate rotational force is vital for improving the efficiency of devices that process, pump, or harvest energy from fluids.
Engineers can apply these fluid dynamics models to optimize microfluidic devices, which manipulate tiny amounts of liquids in medical diagnostic kits and chemical sensors. Additionally, the findings could inform the design of novel propulsion systems for underwater drones and micro-submersibles that must navigate complex, confined environments.
Looking ahead, the researchers plan to investigate how these principles apply to more complex fluids, such as polymer solutions or viscoelastic liquids, which behave differently than water under pressure. Observing how these non-Newtonian fluids collide inside micro-chambers could unlock new methods for controlling fluid flow at the nanoscale, potentially revolutionizing targeted drug delivery systems and advanced manufacturing processes.
