The problem is I think that the particle collision happens based on the center of the particle, not the surface. You might be able to do something like have an invisible plane near the bottom of your cylinder so the particle collides with the plane and it keeps surface a little higher than the bottom of the cylinder.

Regards,
Eric Winemiller
Digital Carvers Guild
Carrara plug-ins

Eric Winemiller
Digital Carvers Guild
Carrara and LightWave plug-ins