Investigation and optimization of ZnO nanostructures for enhanced slippery liquid-infused nanostructured surface performance

In this study, ZnO nanostructures were fabricated on ITO/glass substrates via an electrochemical deposition process with a growth duration of 40 min and growth temperatures ranging from 70 to 90 °C. Subsequently, the samples were immersed in silicone oil (50 cSt) for 12 h and then left in a vertical position to drain the excess silicone oil to create slippery liquid-infused nanostructured surfaces (SLIPS). To optimize the physical morphologies and wettability of the surface, the SLIPS were characterized by field-emission scanning electron microscopy (FESEM) and a contact angle goniometer, respectively. Elemental and structural analyses were performed using an energy dispersive spectrometer (EDS) and an x-ray diffractometer (XRD), respectively. Raman spectroscopy was used to analyse the chemical composition of the lubricant oil layer. The results indicated that at elevated temperatures, the ZnO nanorod templates exhibited a vertically well-aligned morphology with an increased average length, while the average diameters and rod density decreased. This effect was attributed to the acceleration of decomposition of the OH- ions source. The liquid-repellent property of the SLIPS was confirmed under both static and dynamic water droplet conditions. The hydrophilic surface of the ZnO nanostructure template was drastically changed to a hydrophobic property with a static water contact angle of approximately 100° after the surface was overcoated with a low-surface-tension oil lubricant. At moderate growth temperatures, the optimal nanostructure morphology and the surface wettability create a stable, continuous lubricant layer for drag reduction and enhances fluid mobility. However, the morphology with a wider gap between the nanorods and lower nanorod density at higher growth temperatures causes the lubricant layer to be sheared away by the water droplet, leading to a transition of liquid/liquid to liquid/solid interfaces and a reduction in the slippery performance. These findings underscore the critical role of optimization in achieving efficient SLIPS and highlight their potential applications requiring low-resistance and high surfaces mobility.