Roles of viscosity, wettability, and geometry in the post-impact behavior of a droplet impacting on a cylindrical cavity

The impact of droplets on solid, microstructured surfaces is broadly relevant to many academic and industrial applications. However, previous studies have focused on the convex structures, with the post-impact behavior of droplets on concave structures receiving little attention. In this study, we combine laboratory experiments with numerical simulations to investigate the impact of droplets on a solid surface with a cavity. The influence of viscosity, wettability, and geometry in the spread and splashing of the droplets is studied for a wide range of Reynolds numbers Re, contact angle θ, cavity width W, and cavity depth H. Based on experimental observations, we map the crown formation regimes that occur on Re-W and Re-H diagrams and comprehensively evaluate the effect of the controlling parameters on the height of the crown. Our numerical simulations are in good agreement with the experimental observations and provide detailed comparative analyses of the underlying physics behind the observations, including the flow field, distribution of the streamlines, and the forces exerted by the droplet on the solid surface. The numerical simulations reveal that the higher forces that arise in narrower and deeper cavities with larger Re and θ redirect more of the liquid's momentum, leading to the formation of a higher crown. The present study improves our understanding of the underlying mechanisms governing droplet impact and thereby provides valuable guidance for the design of concave surfaces.