The impact of raindrops on sand

When a raindrop hits a sand bed, it leaves behind a small crater with a mixture of liquid and grains located at the center. This event is frequently observed in nature, but when absent, sprinklers may artificially produce these impacting drops to facilitate irrigation. Also in industry, the interaction between liquid and grains is important in for instance, a pharmaceutical production process called wet granulation. In order to understand and influence this interplay between liquid and sand, we investigate the physics of this subtle and complex phenomenon, by studying experimentally the early stage of rain impacting on sand, i.e., the impact of a single drop on a still dry bed of grains.
As soon as a drop impacts on a granular bed, both the drop and the target start to deform, and for wettable grains mixing occurs. In this thesis we succeeded in disentangling these three processes by using high-speed laser profilometry in combination with traditional high-speed imaging, such that we were able to capture both the deformation of the sand bed and the droplet. With this work, we managed to understand the effect of the mixing of liquid and sand on the maximum droplet spreading diameter and to take significant steps in understanding the influence of the deformability of an intruder on the crater formation.