35 durability of the superhydrophobic coating against

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3.5 Durability of the Superhydrophobic Coating Against Water-Jet Impact and Pencil Hardness Test
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Page 11 of 28 Accepted Manuscript Under the impact of a high-speed, free-falling water drop or water jet, a superhydrophobic surface can lose its wetting properties momentarily or permanently. The high-speed water jet can either remove trapped air pockets from the rough cavities or damage the rough structure, resulting in a temporary or permanent transition of the wetting characteristics, respectively [35]. Generally, upon impacting the ideal superhydrophobic surface, a water jet is reflected without wetting the surface. Thus, the durability of the superhydrophobic state of the coating can be quantified under the impact of a water drop or water jet [36]. A water jet can impale and stay pinned or completely rebound from the superhydrophobic surface, depending on the robustness of the coating structure. A water-jet-impact study was performed on the superhydrophobic T-3 coatings, as shown in Fig. 7 . A needle diameter of 0.8 mm was used and a water jet was produced by applying normal pressure to the syringe. The water jet was directly impacted onto the superhydrophobic coating surface from the height of ~ 3cm. As the water jet hit the surface, it immediately reflected from the surface. The cylindrical shape of the water jets was preserved even after the rebounding. The trapped air pockets were intact in the rough structure and hence difficult to push out under the action of the water jets. Even though, the wetting properties of the superhydrophobic coatings were intact under water jet, the coating showed poor scratch resistance. A simple and gentle finger tips touching do not harm the coating surface and its wettability. We checked the scratch resistant property of the coating using pencil hardness test. The pencil was mounted with angle 45° in contact with the coating surface and moved horizontally over the surface. The pencils with hardness of 9H (hardest), HB (average hard) and 9B (softest) were tested on
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Page 12 of 28 Accepted Manuscript superhydrophobic T-3 coating; however coating surface was damaged and significant scratch observed for all pencil tests. This effect is obvious because the coating is made up of soft FAS layer which is physically adsorbed onto the glass surface during dip coating process. 3.6 Underwater Plastron Stability on the Superhydrophobic Coating The superhydrophobicity of the solid surfaces can be maintained underwater using a plastron layer. During immersion in water, a shiny layer (plastron layer) develops at the interface between the water and superhydrophobic surface owing to the total light reflection caused by the trapped air pockets in the rough interstices. Numerous aquatic arthropods having superhydrophobic skin use the plastron layer for underwater respiration and drag reduction [37]. A stable plastron layer allows superhydrophobic surfaces to remain dry underwater, which enables applications in the aquatics industry. The underwater stability of
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  • Fall '19
  • The Land, Surface tension, Contact angle, superhydrophobic coatings

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