This post is part of a collaborative series with FYP on pilot-wave hydrodynamics. Previous entries: 1) Introduction; 2) Chladni patterns
In 1831, in an appendix to a paper on Chladni plate patterns, physicist Michael Faraday wrote:
“When the upper surface of a plate vibrating so as to produce sound is covered with a layer of water, the water usually presents a beautifully crispated appearance in the neighborhood of the centres of vibration.” #
Faraday was not the first to notice this, as he himself acknowledged, but it was his many clever observations and tests of the phenomenon that led to its naming as the Faraday instability. Like Chladni patterns, Faraday waves can take many forms, depending on the geometry of the vibrator and the frequency and amplitude of its vibration.
Beneath the “crispations” at the air interface, the liquid inside the pool is also moving, driven by the vibrations into streaming patterns. Sprinkling particles into this flow reveals discrete recirculation zones that depend on the vibrations’ characteristics, as seen above. This behavior can even be used to assemble particles into distinct formations.
When the vibrations are large enough at resonant frequencies, the rippling waves at the surface become violent enough to start ejecting droplets. You can experience this for yourself using a Chinese spouting bowl or a Tibetan singing bowl with some water. It’s also, bizarrely enough, a factor in alligator mating behaviors!
Next time, we’ll explore what happens to a droplet atop a Faraday wave.
(Image credits: N. Stanford, source; L. Gledhill, source; The Slow Mo Guys, source)