Diffuse (cos(theta) or Random) Scattering of Particle Collisions I'm trying to get particles to bounce off a surface in a scattered fashion. Currently the collisions have the particles bouncing normally, but is there a way to have them bounce off randomly? Do I have to program this into blender through python or is there a setting I can use for this? If python is needed, is there somewhere I can go to get info on this? I've attached a picture to help clarify things a bit. The green arrows are normal collisions and the orange ones are the random scattering I'm trying to get.
Rough Surface
You could add some roughness to your collision object or to your particles. Fist Subdivide it. Next give it a Displace modifier with some texture. Cloud texture seems to work just fine. Then enable physics for Collision. The order of your Modifier stack is important. Collisions has to be last and Subdivisions has to be first.


Force Field Turbulence
Another option would be to add a Turbulence Force Field next to the wall.
Strength of 100 and Max Distance of 100 cm seem to work well for me.