Despite my last entry clearly dictating that I have no idea what's going on when it comes to fluid dynamics, here I go again.
This time, it's droplets of water and propylene glycol and how they interact when on a glass surface.
First, watch https://www.youtube.com/watch?v=K8Wx2PHIYGI. Now. Okay, watched it? No? Then the rest of this won't make much sense and will be pretty boring. I promise it's cool, and the music's pretty sweet too.
Now that you've all either watched it or closed out of this post due to my incessant nagging, let's try to explain what's going on.
The different types of droplets create a film on the glass that attracts other droplets due to surface tension differences. Once they actually meet, things happen and one droplet ends up "chasing" another. Add hydrophobic markers to the mix and a few curious scientists with a cool camera and a lot of awesome effects can be achieved.
Read a much better explanation of what's going on (not including the one in the video itself) by people who actually understand it here.