falling water or falling sand interating with shadow

New to OF and interactive art. Any one please give me a hint how this is done?(falling water or falling sand interacting with body) Especially the interaction of the particle with the shadow? Need to use physical engines like Box2d for the particle? Or there is simple ways?

http://www.youtube.com/watch?v=8YXvm9ZIiHU&feature=related

Thanks

  1. grab camera frames
  2. threshold them and get the contours (follow the opencvExample to learn how to do it)
  3. setup some kined of particle system that implements forces and collision; it can be something complex like box2d, but also a very easy, diy physics engine: a very simple way to do it is placing repelling particles on the contour of the silhouette