Studies of two-dimensional liquid-wedge impact and their relevance to liquid-drop impact problems

In the initial stage of liquid-drop impact, the contact region expands faster than the wave speed in the liquid. This causes compressible behaviour in the liquid, and high transient pressures. High-velocity jetting results when the wave motion in the liquid overtakes the expanding contact edge and moves up the free surface of the drop. The detailed pressure fields in this early time history of impact have been calculated by Lesser (Proc. R. Soc. Lond. 377, 289 (1981)) for both two and three-dimensional liquid masses and for targets of finite admittance. An important result is that the edge pressures exceed the central ‘water-hammer’ pressure 3ρ0CUi and at the time of shock-detachment approach ca. 3ρ0CUi. At this stage the edge pressures, for both spherical drops and two-dimensional liquid wedges, depend only on the impact velocity and the instantaneous angle between the liquid and solid surfaces. This suggests that the essential features of the early stage of liquid impact can be usefully studied by producing impacts with two-dimensional liquid wedges, and predicted data for pressures, shock angles and velocities are presented. Experiments are described for producing impacts with preformed shapes by using water-gelatine mixtures and observing the impact events with high-speed photography. The results confirm the main features of the model and give information on edge pressures, jetting, cavitation in the liquid and the effect of the admittance of the solid. The relevance of the results to the damage and erosion of materials subjected to liquid impact is discussed. In particular, it is possible to explain the apparently low damage-threshold of some materials, the form of damage and its development with repeated impact. The study highlights the importance of the detailed surface geometry in the region of contact.