A swinging showerhead leads to discovery of a new mode of vibration in nature

JULY 25, 2024
by Kimm Fesenmaier, California Institute of Technology



Credit: Unsplash/CC0 Public Domain


During the hot summer of 2020, confined to his Pasadena home during the COVID-19 pandemic, National Medal of Science-winning applied physicist Amnon Yariv took frequent and long showers to cool off. A surprising result, to go with his record-breaking water bill, was a proposal and theoretical model for a new class of vibrations that can convert a constant force, such as wind or water, to a mechanical oscillation.

It wasn't exactly a eureka moment. The thought did not come all at once in the bathtub, a la Archimedes. But it was in the shower that Yariv first noticed something unusual about the way the water-spraying showerhead behaved when left dangling by its hose. To Yariv—a scientist who has been studying waves and their properties for most of his 70-plus-year career—that showerhead was more than just a fixture on a flexible tether spraying water at the wall. It was part of an oscillating system.

Oscillations are the rhythmic, or periodic, variations in the world around us. The ebbing and flowing of the sea is an oscillation. The vibrations of a plucked guitar string are oscillations. Even light is an oscillation, according to quantum theory.

Yariv observed that as he increased the water flow in the shower, the system began to behave unexpectedly. In fact, he saw a bimodal, joint oscillation—two different oscillations synchronized with each other. While the shower head was swinging back and forth like a pendulum, it was also twisting in sync in one direction and then the other. It was clear that those two oscillatory modes were driving each other since a damping of one would immediately cause the other mode to cease oscillating.

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