  The plasma force on grains of specified charge and height in a collisional DC
plasma sheath are calculated using the multidimensional particle in cell code
COPTIC. The background ion velocity distribution functions for the unperturbed
sheath vary substantially with collisionality. The grain force is found to
agree quite well with a combination of background electric field force plus ion
drag force. However, the drag force must take account of the non-Maxwellian
(and spatially varying) ion distribution function, and the collisional drag
enhancement. It is shown how to translate the dimensionless results into
practical equilibrium including other forces such as gravity.
