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Encapsulation of solid particles and liquid agents in a liquid shell is of exceptional interest in biotechnological, chemical and pharmaceutical fields such as personalized medicine, fluidized catalytic cracking, wire fabrication, catalytic reactions. Looking at the interaction between liquid drop and solid particles, we can study the special, ideal case of drops impacting solid spheres, in order to understand the basic phenomena and the effect of the physical variables, such as surface tension, viscosity, but also surface wettability, on the spreading behavior. In fact, considering the importance of dynamics of drop-particle collision, which directly affects the quality of film deposition during encapsulation, various aspects of drop impact on dry solid spherical surfaces are still lacking in the existing literature. The cases are studied with a developed code, using 3D level set method implemented in the COMSOL platform. The impact Weber number, the size ratio of the droplet to the solid surface, and the surface contact angle are varied throughout the numerical simulations and for a suitable range. After providing a strong validation, it is found that impact on spherical surfaces generally presents a higher area of liquid to be in contact with the substrate with respect to the case of flat surfaces, when all other impact conditions are the same. The maximum spreading diameter increases with the impact velocity, with an increase of the sphere diameter, with a lower surface wettability and a lower surface tension. Typical outcomes of the impact include 1) complete rebound, 2) splash, and 3) a final deposition stage after a series of spreading and recoiling phases. The role of the centrifugal force is considered and quantified. Finally, a model is proposed, which can reasonably predict the maximum deformation of low Reynolds number impact of droplets onto hydrophobic or superhydrophobic spherical solid surfaces.
