Sticky stuff
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  "Viscosity"  is a viscous drag force which applies a resistive force
on the masses proportional to their velocity.

  "Stickiness" is not a real force.  When a mass hits a wall, it loses
part of its velocity component in the  direction of  the  wall (in  an
amount proportional to  the  stickiness).   If  it  loses all of  this
component, it remains stuck to the wall.  It will remain  stuck to the
wall  until  a  force  (which  exceeds  an  amount proportional to the
Stickiness) pulls it off the wall.