Sardi Pax shares her technique for creating an effect similar to rain drops on window in Blender through the use of metaballs, which are like little balls within your scene. They allow you control over different parts and properties such as size or transparency so they can emulate various things from foggy windows all the way downpour!
This short tutorial introduces Metaballs to create a rain wet window effect
To create the refractive raindrop look, Sardi uses particles to coat his metaballs against an otherwise flat window. He also demonstrates how you can use this technique with other types of 3D objects like orbs or textured surfaces by adding various materials for different effects in SketchUp (the software).
Using Particles To Place The Metaballs Randomly And Against A Window’s Surface; Demonstrating How One Can Create An Effects-Heavy Shader For Refracting Drops.