Impact dynamics of air-in-liquid compound droplets

Compound droplets are utilized in applications ranging from preparation of emulsion to biological cell printing and additive manufacturing. Here, we report on the impact dynamics of an air-in-liquid hollow compound droplet on a solid substrate. Contrary to the impact of pure droplets and compound droplets with liquids of similar densities, a compound droplet with an encapsulated air bubble demonstrates the formation of a counterjet in addition to the lamella. 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 pure 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 based on the energy interaction between the spreading liquid and the liquid in the counterjet during the impact process.