Development of high-hydrophobic ZnO nanowires on metal surface for graded wetting and no-voltage droplet displacement by negligible tilt angle

A highly hydrophobic ZnO nanowire (NW) surface with a wettability gradient is fabricated on a NiSi/Si substrate using a novel voltage-assisted double-step chemical bath deposition technique. X-ray diffraction analyses confirmed the formation of c-axis oriented ZnO NWs. Detailed morphological and compositional characterization using field emission scanning electron microscopy, energy-dispersive spectroscopy, and atomic force microscopy signifies a progressive increase in NW areal density, Zn/O stoichiometric ratio, and effective surface roughness from the positive to grounded terminal of the biased NiSi strip. This morphological gradient resulted in a tunable wettability profile, studied with static contact angles of water ranging from 130.15° to 142.72°. The obtained hydrophobic nature is attributed to the stoichiometric ratio, morphological variations, and enhanced surface roughness. Furthermore, tilt-induced no-voltage droplet displacement is observed and analyzed. Additionally, this droplet movement behavior is compared with that on a Teflon surface. The current study utilizes a simple, cost-effective approach to fabricate high-hydrophobic ZnO nanowires on a metal surface with a wettability gradient. Such development is critical for advanced, energy-efficient microfluidics integrated biosensor systems.