This is a raindrops setups in Houdini using pop solver. The behaviour of the drops are controlled by a custom brain written in vex. They search for little drops to consume and if they come too close to a trail of another big drop, they merge to it to follow the path of least resistance.
They also get bigger as they consume little drops and their acceleration increases.
You can download the setup at: gum.co/XBYZx
***This file is compatible with Houdini 17.5.327. Only simulation setup is provided. There is no lighting or rendering setups.