Feynman’s Reversed Sprinkler Puzzle Finally Has A Solution

Imagine a sprinkler system with S-shaped arms. The water comes out and the sprinkler moves – so far, it seems pretty straightforward. Now imagine the complete opposite version: Your sprinkler is submerged and sucking in water. The question that physicist Richard Feynman asked was the following: in which direction does it rotate? We now have an answer, showing the complexity of the motion of fluids.

If you have a simple clear idea of how it would behave, you are in good company. Feynman believed that people would be either in the reverse rotation camp or in the same rotation camp, with sound logic or how that would work. Experiments since 1985 (when the book Surely You’re Joking, Mr. Feynman! was published) are more of a mixed bag, showing reverse rotation, unsteady rotation that changes direction, and motion completely dependent on the geometry of the system. It is a big whole mess.


The latest research set out to provide a global understanding of the mechanics of the system. Thanks to a precise experimental setup and successive modeling, the team got to the solution of the puzzle. The sprinkler does indeed reverse direction, but this motion is unsteady and much slower. So, reversing the flow of water in a sprinkler system is not the same as seeing the system playing backward.

Step one to understand the challenge is to submerge the sprinkler in water and make it rotate. This needs to happen with as little friction as possible in either direction. In the standard forward motion, the motion of the sprinkler is driven by jet propulsion. In the reverse version, the sprinkler is still being driven by jet propulsion but with an average rotation rate about 50 times slower.

The reverse approach still is puzzling if you can’t track what goes on inside the sprinkler. After all, the flow going inside should cancel out and not generate any net torque. The team used dyes and light to follow the behavior of the flow. In the forward case, the sprinkler beautifully moves as water comes out of the s-shaped arms. 

The arms shaped in the reverse sprinkler, which in the video above is kept stationary to help visualize the internal behavior, flung the water slightly off the center, creating a small but measurable motion. The flow is asymmetrical, giving rise to the peculiar profiles seen in the various experiments.

“The regular or ‘forward’ sprinkler is similar to a rocket, since it propels itself by shooting out jets,” senior author Leif Ristroph, from New York University, said in a statement. “But the reverse sprinkler is mysterious since the water being sucked in doesn’t look at all like jets. We discovered that the secret is hidden inside the sprinkler, where there are indeed jets that explain the observed motions.”

There is no need for sprinklers that suck in water, but applications for devices whose flow might be similar have now some solid modeling to rely upon. And while this is specific to water, the mechanics of this are shared among fluids.

A paper describing the results is published in the APS journal Physical Review Letters.

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