How do I simulate water droplets realistically falling, gathering, and flowing down a window? For example, see http://www.youtube.com/watch?v=4jaGyv0KRPw. In particular, I want to simulate how smaller droplets merge together to form larger droplets that have enough weight to oppose the surface tension and flow downward, leaving a trail of water. I'm aware of fluid simulation, but how would it be applied in this situation?