Ultrasound in a liquid is like shaking a bottle of soda vigorously: shake it, and bubbles appear. In science, this is called cavitation, and the bubbles don't just float around: they are born, grow, and collapse with tremendous force.
On the Fugaku supercomputer in Japan, researchers simulated an entire ocean of a hundred billion atoms — imagine every drop of water in a small pond being a single atom. They directed ultrasound at this 'ocean' and watched what happens near the sound source. Previously, such simulations involved at most a few hundred million atoms — enough to see a single bubble, but not a whole crowd.
It turned out that bubbles gather into large clusters near the sound emitter, like foam at a faucet. These clusters periodically break into smaller groups and then merge again. The whole cycle repeats in sync with the oscillations of the sound source.
The researchers also noticed that the bubble oscillations are slower than the sound itself: this is called subharmonics. This behavior had been seen in experiments before, but couldn't be explained at the molecular level. Now it's clear that it's related to how bubbles assemble and break apart.
Interestingly, the bubbles themselves barely hinder the sound as it passes through the liquid. That is, cavitation near the source doesn't significantly affect the overall acoustics. This is important for practical applications: for example, in ultrasonic cleaning or in medical devices where bubbles are used to break up kidney stones or deliver drugs.
A hundred billion atoms is like a hundred billion tiny spheres moving according to the laws of physics. And now we can see how foam (which cleans, heals, and destroys) emerges from these spheres.
Cavitation bubbles can heat up to several thousand degrees when they collapse, like the surface of the Sun, but only for a tiny instant.
In Stanisław Lem's novel 'Solaris', the ocean, a thinking substance, behaves unpredictably, much like the billions of atoms in the simulation that suddenly start gathering into bubbles.