We've seen some pretty cool things in our time, but this is right up there. Watch as water droplets are levitated using nothing but sound, and then see them explode like glistening fireworks in the latest episode of Smarter Every Day. And did we mention the whole thing is filmed in glorious slow motion at 20,000 frames per second?
Of course, what you're seeing here is acoustic levitation, which involves using two sound waves to create a column of standing waves. These standing waves have peaks that oscillate between high and low pressure, but they also have a mid-point in each wave where the pressure doesn't change at all. This creates a little sweet spot where something light, such as a styrofoam ball or a water droplet, can float, constantly being bumped up against the force of gravity by the standing wave below it.
As Destin from Smarter Every Day explains, the standing wave acts sort of like a ping pong paddle, and the levitating object is the ball being rapidly bounced up above it.
This levitation itself is pretty incredible to watch. But Destin also had the opportunity to film levitating water droplets with two high-speed Phantom Cams, and manages to capture what happens when they explode. And the results are pretty spectacular, as you can see above.
So what's causing these water droplets to burst? When you increase amplitude of the sound waves, it effectively changes the length of the standing waves and puts pressure on the levitating water droplets. At some point, that pressure gets so great that surface tension can't hold the drop anymore, and we get an explosion that looks spectacular in slow motion. But don't take our word for it, watch the video to be mesmerised by the beauty of physics.