This book provides an important insight into the phenomena of air entrainment that occurs during almost all coating operations especially those that involve thin liquid film coating. The data presented on air entrainment speeds show that one cannot consider molecular effects only to describe dynamic wetting. Hydrodynamic effects of substrate roughness and air pressure play a major role and significantly affect air entrainment speed. In this book, previously conflicting effects (as predicted by the maximum speed of wetting concept and previous experimental data) of substrate roughness on air entrainment speed are reconciled. The data obtained in this research show that roughness can increase or decrease air entrainment velocity depending on the value of roughness and viscosity. This is an important result which can be exploited in industrial applications to coat viscous fluids (high solid content) at high speeds. The most important and novel aspect of this research is the experimental programme under vacuum, which demonstrates the striking effect of air pressure (viscosity) on air entrainment speed that increases exponentially as the pressure is reduced.