Hybrid contact and interfacial adhesion on well-defined periodic hierarchical pillars

Abstract

Herein, we describe a simple fabrication procedure for creating artificial hierarchical micro/nanopillars on silicon substrates that allows an effective, precise control of the interfacial adhesion and surface hydrophobicity. These well-defined hierarchical micro/nanostructures have four possible wetting states: Cassie-Cassie (C-C), Cassie-Wenzel (C-W), Wenzel-Cassie (W-C) and Wenzel-Wenzel (W-W). By controlling the critical height of the micro/nanopillars, it is possible to fabricate hierarchical micro/nanostructures in these four states. Thus, the hierarchical superhydrophobic surfaces proposed and fabricated in this study are promising for mimicking either lotus leaves with low adhesion or rose petals with high adhesion.

Molecular Dynamics Study on the Wetting Transition of a Hierarchical Groove

“…When a water droplet is deposited on such a grooved surface, the micro‐ and nanogrooves can be dewetted or wetted by the droplet, giving rise to the so‐called Cassie–Baxter (CB) or Wenzel (WZ) states. There can be in principle four possible composite states for a dual‐scale groove: CB–CB, CB–WZ, WZ–CB, and WZ–WZ states . These composite states can be tuned from one state into another by changing the physical and chemical properties of the hierarchical structures …”

“…There can be in principle four possible composite states for a dual‐scale groove: CB–CB, CB–WZ, WZ–CB, and WZ–WZ states . These composite states can be tuned from one state into another by changing the physical and chemical properties of the hierarchical structures …”

“…Superhydrophobic surfaces with self‐cleaning properties (lotus effect) are frequently achieved by controlling the surface morphology so that both the major and minor grooves are maintained in the CB state . However, the liquid–gas interfaces are subject to instability caused by an external stimulus, leading to CB to WZ (wetting) and WZ to CB (dewetting) transitions .…”

“…Compared with the single‐scale groove, d * = 0.0, the free energy barriers for the wetting and dewetting transitions significantly increase (by 9.8 k B T ) and decrease (by 1.4 k B T ). This arises from the fact that the liquid–solid (gas–solid) interface area decreases (increases), giving rise to an increased hydrophobicity of surface . The density profiles are shown for the stable CB states in the bottom of Figure (inset).…”

Hierarchical surface coatings of polystyrene nanofibers and silica microparticles with rose petal wetting properties

“…Rose petal surfaces contain distinct features with dimensions that can be classified on at least two different scales (1 -13). The hierarchical roughness of these surfaces enables water droplets to penetrate into the void spaces between the larger, microscale structures while limiting further wetting and penetration of water into the surfaces with the inclusion of nanoscale hydrophobic features (3)(4)(5)(6)(7)(8)(9). The wetting properties of rose petal surfaces is a delicate balance between the Cassie-Baxter state (14) where a water droplet is supported on surfaces with entrapped air pocketsand the Wenzel state (15)where a water droplet is fully wetting the surfaces.…”