Wetting thresholds for long-lasting superwettability: From intrinsic wetting boundary to critical roughness value

Fabricating superlyophilic and superlyophobic materials is of great significance in both academic research and practical application, but the boundary thresholds of intrinsic composition and surface roughness to achieve long-lasting superwettability are not completely understood yet. Herein, silicon nanowire (SiNW) surfaces are chosen as a model to study the intrinsic wetting boundary (IWB) and critical roughness value (CRV) of super-wettability by adjusting intrinsic composition and surface roughness systematically. Taking account of the influence of surface polarity, we define a critical f value as the ratio of polar components of the surface to dispersive components. Different liquids correspond to different IWBs reflected critical f values, for water, the IWB reflected in f value is around 0.5. For surface structure determined CRV, roughness is defined as a ratio of actual area to projected area, and the CRV of SiNWs to reach superwettability is tested to be around 18 (corresponding to 1 mu m SiNWs), which is the transition point of wetting behavior from Wenzel state to Cassie state. With these thresholds, long-lasting superhydrophilicity with superspreading behavior and superhydrophobicity with ultralow adhesion for contaminants are achieved, which provide significant guidance for designing functional interfaces related to practical applications such as solar cells, microfluidics and electrodes.