Classical Approach to Understanding the Impact Dynamics of Hollow Droplets
Compound droplets are utilized in applications ranging from the preparation of emulsion to biological cell printing and additive manufacturing. Here, we report on the impact dynamics of a compound hollow droplet on a solid substrate. Contrary to the impact of simple droplets and compound droplets with liquids of similar densities, the compound droplet with an encapsulated air bubble demonstrates the formation of a counterjet in addition to the lamella. Here, we experimentally investigate the influence of the size of the air bubble, liquid viscosity, and height of impact on the evolution of counterjet and the spreading characteristics of the lamella. For a given hollow droplet, the volume of the counterjet is observed to depend on the volume of air and liquid in the droplet and is independent of the viscosity of the liquid and impact velocity of the droplet. We observe that the spread characteristics, counterintuitively, do not vary significantly compared to that of a simple droplet having an identical liquid volume as the hollow droplet. We propose a model to predict the maximum spread during the impact of a hollow droplet on a substrate based on the energy interaction between the spreading liquid and the liquid in the counterjet during the impact process. Furthermore, the maximum spread diameter during the impact of a HD obtained using the model developed is in excellent agreement with that observed in experiments.