The Application of Dislocation Etching Techniques to the Study of Liquid Impacts [and Discussion]

Erosion damage is very often the cumulative result of a series of liquid droplet impacts which individually do not produce any deformation visible under the optical microscope. Such collisions do, however, produce dislocations in the crystalline structure surrounding the area of impact, and in suitable materials these dislocations can be revealed by chemical etch pitting. The technique is particularly easy to apply to freshly cleaved lithium fluoride crystals, and it has been used to study several types of impact. The impact of solid balls produces symmetrical rosettes of dislocations lying on {110} planes, and the dimensions of the rosettes can be related to the area of contact and stress distribution calculated from the theory of the collision of elastic/plastic bodies. Similar, but less symmetrical, rosettes are produced by liquid impacts and, by comparison of the extent and distribution of the dislocation loops in the two cases, it has been possible to make an estimate of the pressure and effective area of contact for liquid drops of various sizes, quantities which are otherwise difficult to measure. The behaviour of liquids other than water has also been investigated.