Feynman's reverse sprinkler

Settled in 2024 by Kaizhe Wang, Mingxuan Zuo, Brennan Sprinkle, Leif Ristroph (NYU / Colorado School of Mines).

Ernst Mach posed the puzzle in the 1880s, and it became famous after Richard Feynman recounted (in 'Surely You're Joking, Mr. Feynman!') breaking a bottle while testing it as a Princeton graduate student around 1940. For decades the problem resisted clean resolution because tabletop experiments were dominated by bearing friction and turbulent inflow noise, leading to genuinely conflicting reported results. In January 2024 Leif Ristroph's NYU group built a low-friction 'floating' sprinkler using a capillary bearing and combined it with high-resolution CFD, showing the reverse sprinkler spins gently in the reverse direction because the two inward jets collide slightly off-center inside the hub. The paper was explicitly framed as finally settling a question still listed as 'open' in fluid-mechanics textbooks.

A century-old thought experiment that seems answerable by symmetry alone turns out to hinge on a subtle asymmetry in how colliding jets meet inside the sprinkler head.

Explanations that circulated

• That a naive time-reversal argument implies equal-and-opposite torque to the forward case (in fact the reverse case is a qualitatively different, weaker, 'inside-out rocket' effect).
• That the effect is essentially zero/negligible in the idealized inviscid limit, which does not match the small but real and consistent rotation measured experimentally once bearing friction is eliminated.

References

• Centrifugal Flows Drive Reverse Rotation of Feynman's Sprinkler, K. Wang, M. Zuo, B. Sprinkle, L. Ristroph, Physical Review Letters, 2024