Sustainable superhydrophobic branched hierarchical ZnO nanowires: Stability and wettability phase diagram

Highlights: • Superhydrophilic branched hierarchical ZnO nanostructures were fabricated. • Surface of the samples were modified by methyltrimethoxysilane. • Surface modification of samples exhibited superhydrophobic property. • Phase transition from Cassie-Baxter to Wenzel state was studied via critical pressure. Stability of Cassie-Baxter (CB) state is very critical in application of superhydrophobic surfaces. The most industrial applications of superhydrophobic surfaces are limited by the transition from the CB state to Wenzel (W) state. In this research, CB state stability of branched hierarchical ZnO nanowires (BH-ZnO NWs) was investigated as compared with ZnO nanowires (ZnO NWs) by using theoretical and experimental approaches. For this purpose, surface of the BH-ZnO NWs and ZnO NWs were modified by thin layers of methyltrimethoxysilane (MTMS). The MTMS thickness was optimized by varying NH₄F (0, 5, 10, 20 μL) as used catalyst. The highest water contact angle (WCA) was measured at about 153 ± 3° with sliding angle of 15 ± 3° for the M (10)/BH-ZnO NWs. Based on the theoretical results, the critical pressure (P_c) for the transition from the CB to W state was obtained at about of 1155 ± 230 and 36770 ± 7350 Pa for the M (10)/ZnO NWs and first level of the M (10)/BH-ZnO NWs samples, respectively. Finally, it was found that the M (10)/BH-ZnO NWs sample showed the higher CB stability as compared to the M (10)/ ZnO NWs sample due to presence of hierarchical nanostructures.