The whole point of this post is to show cool video, so let’s just cut to the chase:
That is a drop of silicon oil apparently hovering above a bowl filled with the stuff. It’s definitely the coolest liquid trick I’ve seen since the bouncing shampoo demonstration a few years back. Denis Terwagne of the University of Liège in Belgium and John Bush of MIT have a paper in the journal Nonlinearity describing the drop mathematically. Here’s how it works.
First off that bowl isn’t just any bowl, it’s a Tibetan singing bowl. Singing bowls are made of a bronze alloy, and it’s probably better to think of them as bells than bowls. Like a wine glass, rubbing their edge makes them vibrate at a specific frequency, creating their characteristic sound.
Fill them up with water, and something very different happens:
At a certain point, the water in the bowl appears to be boiling, but it’s not. Just because we can, let’s go to the slow-mo:
The capital gamma (Γ) refers to the acceleration at the bowl’s rim, in other words how violently the bowl is vibrating. As you can see, the vibration is transfering energy to the water, and the more the more the bowl vibrates the more pronounced the waves on the surface become. At a critical frequency the waves become unstable at the bowl’s edge and begin breaking into little droplets. It’s known as Faraday instability, and it’s a complex, nonlinear effect (and hence a good match for the journal where it was published).
That’s not all. Take the same bowl, vibrate it, and put a tiny drop on the surface, and it will just bounce there. That’s because the rim vibration makes the liquid in the bowl to pulse up and down. Each pulse generates a little air cushion on which the drop bounces. As long as the bowl is vibrating, the drop can basically stay there forever, Terwagne told me.
You could actually do the same thing with a wine glass, but because the resonant frequencies of wine glasses are so much higher, the drops are a lot smaller.
Pretty cool stuff eh? To end it let’s take a look at the drops forming right at the bowl’s edge.
All videos courtesy of D. Terwagne