Dynamic Wetting: Displacement of the Contact Line Induced by Air Pressure Oscillations in Its Close Vicinity

Abstract

In most studies devoted to dynamic wetting, the viscosity of the ambient air is neglected compared with that of the liquid, which equivalently means that the surrounding air is considered to be static. However, the presence of air flows underneath the sheet leads to an increase of the pressure in the wedge formed by the liquid sheet and the solid substrate. The pressure 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 contact line. This model incorporates the liquid wetting properties as well as parameters characteristic 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 same time, the air gap is reduced as compared to its value in the static case. [S0021-8936(00)01904-8]