Wettability and Bactericidal Properties of Bioinspired ZnO Nanopillar Surfaces.

Nanomaterials of zinc oxide (ZnO) exhibit antibacterial activities under ambient illumination that result in cell membrane permeability and disorganization, representing an important opportunity for health-related applications. However, the development of antibiofouling surfaces incorporating ZnO nanomaterials has remained limited. In this work, we fabricate superhydrophobic surfaces based on ZnO nanopillars. Water droplets on these superhydrophobic surfaces exhibit small contact angle hysteresis (within 2-3°) and a minimal tilting angle of 1°. Further, falling droplets bounce off when impacting the superhydrophobic ZnO surfaces with a range of Weber numbers (8-46), demonstrating that the surface facilitates a robust Cassie-Baxter wetting state. In addition, the antibiofouling efficacy of the surfaces has been established against model pathogenic Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli). No viable colonies of E. coli were recoverable on the superhydrophobic surfaces of ZnO nanopillars incubated with cultured bacterial solutions for 18 h. Further, our tests demonstrate a substantial reduction in the quantity of S. aureus that attached to the superhydrophobic ZnO nanopillars. Thus, the superhydrophobic ZnO surfaces offer a viable design of antibiofouling materials that do not require additional UV illumination or antimicrobial agents.

Droplet impact on doubly re-entrant structures.

Doubly re-entrant pillars have been demonstrated to possess superior static and dynamic liquid repellency against highly wettable liquids compared to straight or re-entrant pillars. Nevertheless, there has been little insight into how the key structural parameters of doubly re-entrant pillars influence the hydrodynamics of impacting droplets. In this work, we carried out numerical simulations and experimental studies to portray the fundamental physical phenomena that can explain the alteration of the surface wettability from adjusting the design parameters of the doubly re-entrant pillars. On the one hand, three-dimensional multiphase flow simulations of droplet impact were conducted to probe the predominance of the overhang structure in dynamic liquid repellency. On the other hand, the numerical results of droplet impact behaviours are agreed by the experimental results for different pitch sizes and contact angles. Furthermore, the dimensions of the doubly re-entrant pillars, including the height, diameter, overhang length and overhang thickness, were altered to establish their effect on droplet repellency. These findings present the opportunity for manipulations of droplet behaviours by means of improving the critical dimensional parameters of doubly re-entrant structures.