Forced Coating of Polypropylene Fibers with Non-wetting Fluids: the Scaling of the Film Thickness

The film thickness of free-meniscus coating of a polymeric fiber with a non-wetting fluid was investigated. A polypropylene monofilament fiber was coated with various glycerol/water mixtures. With a small capillary number (Ca), a detectable liquid film did not form on the fiber due to the non-wettability of the fiber-liquid system. Above a certain threshold velocity, liquid was forced to wet the fiber by hydrodynamic forces, thus forming a film. However, the film thickness in this region is lower than Landau– Levich–Derjaguin (LLD) theoretical value for the wetting system. At a moderate velocity range, the film thickness increases to a value predicted either by the LLD law or White–Tallmadge (WT) model, depending on the velocity, mimicking that of the wetting system. At a higher Ca, the film thickness increases much more rapidly, deviating from any predictions, due to the inertial effect.