Impact Dynamics of Hollow Droplets*

The impact of droplets on rigid substrates is accompanied by the outward spreading of the liquid on the substrate, forming a lamella. Here, we report on the impact dynamics of an air-in-liquid hollow droplet on a rigid substrate. We show that during the impact of a hollow droplet (HD), the formation of the lamella is accompanied by the formation of a central jet of liquid moving upwards against the direction of impact. The liquid jet is referred to as the counterjet (CJ) and comprises 30 – 60% of the volume of the liquid in the HD. The volume of the liquid entering the CJ during the impact of a HD with a given liquid volume is observed to be independent of the impact height and increases with the volume of air encapsulated in the HD. Interestingly, even though almost half of the liquid in the droplet goes into forming the counterjet, the maximum spread of a HD is almost similar to that of a simple droplet with liquid volume and height of impact same as that of the HD. We establish that there exists an energy transfer from the liquid in the CJ to the liquid in the lamella and that the viscous loss during impact of a HD is relatively less compared to that of a simple droplet.