The problem of a liquid sheet issuing from an ejector and being coated on a moving solid substrate is classically considered like a free surface flow problem. The viscosity of ambient air is neglected compared with that of the liquid, which equivalently means that the surrounding air is considered to be steady. However, the free surface is affected by the presence of air flows underneath the sheet and by the increase of pressure, which deforms the interface and tends to separate the liquid sheet from the solid substrate. This is a coupled problem, the two unknown functions being the meniscus shape and the pressure generated in the air wedge.
A simple model is proposed here to investigate the possible influence of slight pressure fluctuations in the air on the displacement of the triple line. This model incorporates the liquid wetting properties as well as parameters characteristic of the process and of air pressure oscillations.