Nonwettable Hierarchical Structure Effect on Droplet Impact and Spreading Dynamics

In this study, the nano/micro hierarchical
structure effect of
a nonwettable surface on droplet impact was investigated by high-speed
visualization. A dual-scale structure of a superhydrophobic surface
was designed for manipulating a wide range of capillary pressures
(103–106 Pa), and it was supposed to
trigger a hierarchical effect on the droplet dynamics. Distilled water
droplets of various sizes and initial velocity were subjected to the
prepared samples, and the impact behavior, the spreading diameter,
and contacted time, were quantitatively measured. The apparent maximum
spreading and contact time of the low Weber number (We#) condition was less dependent on the microscaled design factor
of the multiscale-fabricated surface. However, in the high We# condition, the wavy formation shape and the fragmented
criteria of the droplet impact were affected by the configuration
of the surface morphology. The hierarchical effect from the dual-scale
structure on droplet spreading dynamics has been discussed through
a balance between capillary pressure induced by the structure and
the dynamic pressure of droplet impact.