Air entrainment in web handling: To be avoided or mastered?

The presence of ambient air is of prime importance in various industrial processes involving web handling. This paper is an attempt to answer the following questions: how does air influence the quality of the final product? How is it possible to cope with such a situation?After a brief description of a few basic problems of fluid mechanics, namely: (i) the development of boundary layers on moving webs and (ii) the flow structure and pressure generation in wedges (i.e. corner flows ), several illustrative examples are presented.(1) When a flexible web passes over a spindle, a thin air layer is formed between the two surfaces. This is typically a foil bearing configuration, which is important to master in order to reduce wear reduction or to avoid any misfunction at the head-tape interface. A brief survey of the historical works on this topic will be given.(2) In wound roll models, the stress field generated in the roll depends on the winding conditions (i.e. geometry and processing parameters) and on the flexible media bulk properties (elasticity or viscoelasticity) and surface properties (topography). It is well known that there is a strong link between the roughness of a surface (resulting from microparticles added to the resin) and its behavior in terms of air entrainment and evacuation. A first attempt to study the complex mechanisms governing this link is proposed.(3j In high velocity coating flows which are present in numerous processes (magnetic tape manufacturing, paper industry, ... ) some air can be entrained between the solid substrate and the liquid layer being coated on it. After a qualitative description of the complex phenomena occurring in the vicinity of the three-phase junction, the amount of air likely to be entrained is evaluated on the basis of a theoretical model.As a conclusion, a few recommendations for practical applications will be tentatively drawn