  Molecular dynamics studies of nono-sized rigid grains, droplets and bubbles
in nano-sized pores indicate that the drag force may have a hydrodynamic form
if the moving object is dense and small compared to the pore diameter.
Otherwise, the behavior is non-hydrodynamic. The terminal speed is insensitive
to whether the falling droplet is made of liquid or a solid. The velocity
profiles within droplets and bubbles that move in the pore are usually
non-parabolic and distinct from those corresponding to individual fluids. The
density profiles indicate motional shape distortion of the moving objects.
