Numerical investigation of hollow-droplet impact dynamics on superhydrophobic corrugated surfaces at low Weber numbers

Hollow-droplet impact on superhydrophobic corrugated surfaces plays a critical role in spray cooling, coating, anti-icing, and additive manufacturing, yet the interplay between internal cavity deformation and surface topography remains poorly understood. To investigate impact dynamics of hollow droplets on periodically corrugated superhydrophobic surfaces at low Weber numbers, we employ an improved embedded-boundary method combined with the volume-of-fluid method. By systematically varying the Weber number (We) and corrugation density (ηe), we identify four distinct wetting regimes: Wenzel state mode, Cassie–Wenzel transition mode, sticky Cassie state (SC) mode, and retracting Cassie state(RC) mode. A phase diagram in the We–ηe plane is constructed to classify these regimes and illustrate their transition pathways. Sparse corrugations promote liquid impregnation and Wenzel-type pinning, whereas denser textures suppress deep penetration and favor Cassie-type wetting. Within the Cassie regime, insufficient retraction inertia results in temporary adhesion (SC mode), whereas higher We allows the droplet to overcome localized suction and rebound in the RC mode. The competition between adhesive and retractive capillary pressures provides a unified interpretation of the observed wetting transitions. These results reveal how corrugation geometry and impact inertia jointly govern hollow-droplet adhesion, retraction, and rebound, providing design guidelines for textured superhydrophobic surfaces with tailored liquid retention or repellency.