You should try to disable the rigidbody's [useGravity][1] property when the collision happens and use [AddForce][2] to fake your upwards gravity, pretty much like the example in [AddForce][2] documentation.


  [1]: http://docs.unity3d.com/ScriptReference/Rigidbody-useGravity.html
  [2]: http://docs.unity3d.com/ScriptReference/Rigidbody.AddForce.html