Twisting arms to understand the Feynman sprinkler

Flowing fluids often drive the dynamics of solid structures in nature and in human inventions. However, the underlying flow-structure couplings can be subtle or unclear, as epitomized by Feynman's paradox regarding chiral sprinklers that may rotate in response to flow throughput from internal sinks (in so-called "reverse sprinklers") or sources. We carry out direct torque, motion, and flow measurements on such sprinklers and interrogate hypotheses for their operation by tailoring the geometry of the tubular arms through which flow is routed. Our findings contradict the prevailing explanations of the "reverse" torques, which variously rely on total angular momentum swirl-up in the fluid, local force balances near the outer orifices, vortex-formation, or interaction between jets in the sprinkler body. Instead, our results suggest that the reverse sprinkler is a machine powered by the conversion of isotropic flow in the far field into the subtly chiral flow in its interior. The physics learned here is fundamental to our reasoning about open systems and suggests routes towards the geometric control of flow-structure dynamics.