The Help Manual says:
Could you confim this parameter is not taking the contribution from RW Surface?@ Friction
This parameter is only available when "On surface" is set to "Place". You can adjust the particles' friction to make them stay closer to the object for a longer time.
You can check two comparative videos:
@ Friction 0.0 http://www.vimeo.com/15733232
@ Friction 0.85 http://www.vimeo.com/15734047