We explore the effects of thin films on the hydrodynamics of macroscop
ic fluid bodies spreading over solid surfaces. To examine these effect
s, we measure the interface shape within microns of moving contact lin
es and compare those measurements to two asymptotic models in the limi
t of small capillary number, Ca. One model requires that the films aff
ect the hydrodynamics only in a microscopic region near the contact li
ne and allows the macroscopic meniscus to exhibit a nonzero effective
contact angle. The other model describes the film as containing mobile
fluid and specifically models the flow as fluid moves into or out of
the film as the contact line moves. We examine fluids advancing and re
ceding on wetting and nonwetting surfaces with spontaneously forming (
molecular scale) and pre-existing (micron scale) films. Our results em
phasize the importance of the mobility of the molecules in these very
thin films in determining the hydrodynamics governing the moving conta
ct line. The first model, which describes fluids advancing over dry su
rfaces, also accounts for the hydrodynamics of liquids advancing over
very thin, immobile films. Surprisingly, the same model fails when flu
id recedes on a nonwetting surface and no film is present. For mobile
pre-existing films, the second model, based on Landau and Levich's the
ory, accounts for the hydrodynamics in the limit of small Ca. (C) 1998
American Institute of Physics.