Dip-coated films of volatile liquids

Abstract

We examine experimentally the hydrodynamics of dip-coated, finite-length films of evaporative fluids, from the film tip through the film body all the way to the connection with the main meniscus. The characteristic film thickness has a power-law dependence on the withdrawing speed similar to that for the thickness of “infinite” films formed by nonvolatile liquids. The film length and cross-sectional area have power-law dependence on the withdrawing speed as well, but the prefactors of the power laws are controlled by the evaporation rate of the fluid. These power laws are consistent with the global mass balance over the film between mass lost by evaporation and mass input by the solid motion. We have also found that the apparent contact angle and the curvature at the film tip both have power-law dependencies on the withdrawing speed that are consistent with those found for the length and the film thickness. Film shape measurements near the film tip reach thicknesses ∼100 Å from the solid; but we did not detect any influence of the inner scale hydrodynamics and van der Waals forces on this shape. We have developed a systematic method for measuring the contributions of gravity, capillary force, viscous force, and vapor recoil on the pressure and flow fields in the film. This exercise reveals detailed information about the flow in evaporative films. The combined effects of evaporation and Marangoni flow on the hydrodynamics are deduced from experimental data, independent of evaporation models.