THE EFFECTS OF THIN-FILMS ON THE HYDRODYNAMICS NEAR MOVING CONTACT LINES

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.