Water droplets shoot out of a nozzle one after another, like cyclists on a narrow track. The faster ones catch up with the slower ones and merge into larger drops. This simple mechanism is called ballistic aggregation, and until now it had only been observed on cosmic scales: this is how dust particles stick together to form planets.
The laws turned out to be remarkably robust. Air slows droplets unevenly, so fast droplets start lagging behind slow ones — the exact opposite of what happens in a vacuum. But the big picture does not change: the mass distribution (how many small, medium, and large droplets form) follows the same mathematics. Everything is determined by a single parameter — the characteristic length over which droplets manage to meet.
The simple equations that usually describe particle coalescence (the Smoluchowski equations) fail here. They do not account for memory of the direction of motion, but in a jet, each droplet remembers where it came from and how fast it was traveling. It is this memory that creates a special order in the chaos of collisions.
The experiment used a water jet as thin as a human hair — about 70 micrometers. At such scales, droplets behave almost like celestial bodies.