Dynamics of hollow hole and lamella formed by droplet impact on rotating surfaces

The study presents an investigation into the dynamic behavior of water droplets impacting the center of two rotating stainless-steel surfaces, with an emphasis on the initiation of lamella hollow holes and the coupled roles of rotational speed and Weber number. These results show that a smoother surface (Ra = 0.025 μm) promotes broader lamella spreading and earlier hole initiation, while a rougher surface (Ra = 0.15 μm) increases viscous dissipation, delays rupture, and induces anisotropic elliptical deformation. The diameter ratio (β) confirms inertia-dominated early spreading, followed by centrifugal enhancement and roughness-induced suppression during post-spreading, with surface A consistently supporting higher expansion. The hollow hole initiation time (τhole) decreases with increasing rotational speed and Weber number, though its reduction rate diminishes at higher ω due to lamella thinning saturation and radial re-stabilization. The power-law scaling of the lamella-hole Weber number (Wel.hole) with We captures the transition from inertia-driven to shear and rotation-dominated regimes, while the hollow hole Weber number ratio (ξhole) and the centroidal distance (d) reveal critical thresholds and roughness effects. The widening speed of the lamella area (υA,l) exhibits a non-monotonic trend, with surface B showing consistently lower values, while hollow hole growth rates remain roughness-independent. These insights highlight how modest topographical variations substantially influence lamella stability, symmetry, and rupture thresholds, providing predictive insights for optimizing droplet-based technologies, including spray cooling, coating, and inkjet printing.