Droplet behavior analysis on inclined, highly sticky, or slippery superhydrophobic nanostructured surfaces by observation and SPH simulation

[Display omitted] •Dynamic contact angle was observed on a superhydrophobic nanostructured surface.•Slippery surface: droplets slip off the unpinned portion of the receding angle.•Sticky surface: droplets remain attached even at an inclination angle of 90°.•Sticky solid surface was modeled by the momentum-based friction resistance. Although the motion of water droplets on a superhydrophobic surface is important for industrial processes, the characteristics of the slippery/sticky contact line are not fully understood at the macroscopic continuum fluid scale. In this study, we tracked the dynamic contact angle of the solid–liquid–gas phase when droplets moved on a superhydrophobic nanostructured surface. High-speed observations revealed the pinning/unpinning behavior of the droplets’ receding contact part. The droplets were modeled by introducing momentum attenuation as a friction model in the smoothed particle hydrodynamics (SPH) framework, which assumes a macroscopically smooth surface with different slippery properties. From a macroscopic perspective, droplet pinning requires both horizontal movement and the suppression of the rotational motion acting on the solid–liquid interface. A sticky solid surface was successfully modeled using this simple model; thus, it can be applied to more practical problems such as prediction of the motion of melts on coke.