Nonwettable Hierarchical Structure Effect on Droplet Impact and Spreading Dynamics

Abstract

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 (10-10 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.

Bioinspired nanoscale hierarchical pillars for extreme superhydrophobicity and wide angular transmittance

“…The two pressures applied at the direction normal to the surface by impact are P WH and P D and are expressed as P WH ≈ 0.2 ρcv , P D = 0.5 ρv 2 , where ρ is the density of water, c is the speed of sound in water (∼1490 m s −1 ), and v is the impact velocity. P C is created by the surface tension between the structures of the surface during impact as 47 P C = −2 σ cos θ / g s , where σ is the surface tension of the liquid, θ is the contact angle, and g s is the gap between the nanostructures. In this study, owing to the colloidal lithography of the hexagonally packed PS beads as a mask, g s can be expressed using the structure diameter ( D ) and the contact area fraction ( Φ C ) as …”

Fabrication of Micro-/Submicro-/Nanostructured Polypropylene/Graphene Superhydrophobic Surfaces with Extreme Dynamic Pressure Resistance Assisted by Single Hierarchically Porous Anodic Aluminum Oxide Template

“…Figure depicts schematics of mechanism for the impact dynamics of droplets on HMSNS-PP/GP surfaces. The wetting pressures are the effective water hammer pressure, P WH ≈ 0.2ρ w cv , and the dynamic pressure, P D = 0.5 ρ w v 2 , where c is the speed of sound in the water ( c ≈ 1490 m/s) …”

“…The wetting pressures are the effective water hammer pressure, P WH ≈ 0.2ρ w cv, and the dynamic pressure, P D = 0.5ρ w v 2 , where c is the speed of sound in the water (c ≈ 1490 m/s). 49 P WH is generated by the shock wave built up by the compression of droplets at the contact stage, and P D is induced by the kinetic energy of droplets. The antiwetting pressure caused by the air trapped in the surface roughness is the capillary pressure…”

Hypergyrating Droplets Generated on a Selective Laser-Textured Heterogeneous Wettability Surface

“…The droplet with an impacting velocity of 2.3 m/s was released from a height of 26 cm. Because the droplet size affects the bounce behavior, , the volume of the droplet should match with the size of the wettability pattern to maximize torque and gyrating speed. A series of experiments were done, and 10 μL was found to be the most suitable size.…”