Nanostructures Increase Water Droplet Adhesion on Hierarchically Rough Superhydrophobic Surfaces

Abstract

Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.

Achieving a slippery, liquid-infused porous surface with anti-icing properties by direct deposition of flame synthesized aerosol nanoparticles on a thermally fragile substrate

“…This is explained by water's ability to penetrate into the porous structure (the droplet resides partially in the Wenzel state). 14 The initial surface of the oil-infused structure displays a complete opposite behavior with a water sliding angle value of 3 , which satisfies the additional requirement for superhydrophobicity of WSA lower than 10 . This value shows an increase in the level of the plain oiled surface after the first cycle of the ice adhesion testing.…”

Chemical and Physical Pathways for Fabricating Flexible Superamphiphobic Surfaces with High Transparency

“…Many researchers have used the presence of hybrid states to explain particular water adhesion behaviors on surfaces with dualscale roughness, e.g., Cassie-Baxter in the microscale and Wenzel in the nanoscale, or vice versa, or a semi-Cassie-Baxter state where the liquid meniscus partially penetrates the rough features. Similarly, the water-filled Wenzel state with micro/nano structures is illustrated in Figure 2e, and the waterfilled Cassie state with nanostructures is demonstrated in Figure 2f [30][31][32][33]. …”

“…Many researchers have used the presence of hybrid states to explain particular water adhesion behaviors on surfaces with dualscale roughness, e.g., Cassie-Baxter in the microscale and Wenzel in the nanoscale, or vice versa, or a semi-Cassie-Baxter state where the liquid meniscus partially penetrates the rough features. Similarly, the water-filled Wenzel state with micro/nano structures is illustrated in Figure 2e, and the waterfilled Cassie state with nanostructures is demonstrated in Figure 2f [30][31][32][33]. Engineering transparent surfaces that actively repel water [34], oil [35], low-surface-tension liquids [36], and/or organic liquids have many applications in the fields of fluids [37,38], chemical shielding [39], stain-proof coatings [40,41], membrane technologies [42], and antifouling coatings [43].…”

Bio-Nanotechnology for Active Food Packaging

“…The R812S silica nanoparticles and Polydimethylsiloxane (PDMS) silicone oil revealed that this complex is highly water resistant because of its unique nanostructured morphology. The efforts are being made to develop TiO 2 nanoparticles coating to provide hydrophobic property to the paper with the help of liquid flame spray technique (Teisala et al, 2012). TiO 2 and Ag nanoparticles were synthesized and coated onto a paper prepared from rice straw.…”