With Particle Playground, you should look into the particle emitter's position, the use of a Wall (using a closed mask shape), gravity and Kinetic Friction (under persistent property mapper).
Take a look at my particle playground tutorial here at the COW - Fun in Particle Playground Land for insights into, particle emitter position, Wall and kinetic friction.
Cheers
Roland Kahlenberg
broadcastGEMs
customizable animated backdrops with Adobe After Effects project files