Dynamic wetting: Displacement of the contact line induced by air pressure oscillations in its close vicinity

In most studies devoted to dynamic wetting, the viscosity of the ambient ai r is neglected compared with that of the liquid, which equivalently means t hat the surrounding air is considered to be static. However, the presence o f air flows underneath the sheet leads to an increase of the pressure in th e wedge formed by the liquid sheet and the solid substrate. The pressure de forms the interface and tends to separate the liquid sheet from the solid s ubstrate. This is a coupled problem, the two unknown functions being the me niscus shape and the pressure generated in the air wedge. A simple model is proposed here to investigate the possible influence of slight pressure flu ctuations in the air on the displacement of the contact line. This model in corporates the liquid wetting properties as well as parameters characterist ic of the process and of the air pressure oscillations. The main result is that small amplitude pressure oscillations in the air close to the contact line may generate large lateral displacements of the contact line. At the s ame time, the air gap is reduced as compared to its value in the static cas e. [S0021-8936(00)01904-8].