Unraveling the Effect of Surface Roughness on Hydrophobicity and Stability of Water-Repellent Surfaces on Chemically Etched Copper Substrate

Surface roughness plays an important role in controlling the hydrophobicity and stability of water-repellent surfaces. In this study, we analyzed the effect of surface roughness on the water repellency of superhydrophobic and liquid-infused surfaces (LIS), fabricated on a chemically etched copper substrate with a stearic acid monolayer coating, in terms of (super)hydrophobicity, stability, and dynamics of droplet-shedding properties. To control the surface roughness, the copper substrate was etched in an NH₄OH aqueous solution with different concentrations and times. Wetting measurement studies reveal that hydrophobicity and its stability significantly increase with surface roughness, with the highest apparent contact angle of 163^◦ observed after 48 h of immersion in 0.03 M NH₄OH. The combination of topographical analysis of field emission-scanning electron microscopy and optical 3D profilometry images, along with chemical analysis by Fourier transform infrared and X-ray photoelectron spectroscopy techniques, confirms the formation of optimum hierarchical (micro/nanometer-sized) structures with a stearic acid monolayer coating on the superhydrophobic surfaces. To explain the observed results, the Wenzel and Cassie models were employed to estimate the apparent contact angle, where the roughness parameters, such as rms roughness and the fractional liquid-solid area, were calculated using fractal analysis of the random roughness. However, the equilibrium apparent contact angle on LIS, governed by the wettability of the underlying substrate, is independent of surface roughness for a hydrophobic LIS substrate. Moreover, our study finds that the nanometer-sized surface roughness plays a crucial role in retaining the lubricant in the structures by increasing the lubricant-solid interfacial area, thereby improving the lubricant stability of LIS. Therefore, this article unravels the intricate interplay between surface roughness and liquid repellency properties to design extreme water-repellent surfaces for various applications.