Tank draining through an orifice: wetting effects*

We performed an experimental study of wetting effects on tank draining. Water flows from a tank through an orifice set at the tank bottom under its own weight. The hole's size is on the order of magnitude of the capillary length. As expected, flows follow Torricelli-like behavior, however, important wetting effects are highlighted. Wetting effects are manifested in two main aspects. First of all, the speed of drainage exhibits a non-monotonous variation with the static wetting angle of the fluid on the bottom plate of the tank. The fluid flow rate goes through a minimum as the outside surface of the tank bottom plate changes from hydrophilic to hydrophobic. The second striking aspect of the effect of wetting on fluid flows through an orifice is the existence of a jet shape disturbance right at the hole outlet. To our knowledge such a disturbance of the jets has never been described before. We believe that the origin of both phenomena lies in the meniscus formed by the fluid at hole's outlet. I will present experimental results showing the effects of surface wettability, as well as a model we developed, which could explain wetting effects on the drainage speed.