Maximum spreading and energy analysis of ellipsoidal impact droplets

Droplet impacts on solid surfaces are ubiquitous in nature and industry. Before impact, the droplet shape may be affected by gravity, shear flow, and the electric and magnetic fields, inducing non-spherical droplets. However, most previous studies focused on the impact dynamics of spherical droplets. In this study, we conduct experiments, simulations, and theoretical analyses to investigate the impact behaviors of ellipsoidal water droplets whose symmetry axis is perpendicular to the surface. In particular, we explore the maximum spreading and energy evolution during impact. A numerical model adopting the Volume of Fluid method and Kistler's dynamic contact angle model achieves good agreement with the experimental results for both the temporal droplet profile and spreading factor. The effects of Weber number, contact angle, and aspect ratio on the impact dynamics are systematically investigated, and the outcomes show that both the maximum spreading time and factor enlarge with the increasing aspect ratio. Their relations approximately follow the 2/3-power and 1/6-power laws, respectively. Reducing the aspect ratio enhances the viscous dissipation during impact. Based on the theoretical analyses of above results, we modify the viscous dissipation in the conventional energy balance model to include the effects of aspect ratio on the maximum spreading factor. The modified theoretical model reduces the deviations from -23%-51% to -5%-25% and elucidates the scaling law between the maximum spreading factor and aspect ratio. This work deepens our understanding of the interaction between non-spherical impact droplets and surfaces and may contribute to associated applications.