This study investigates the light-induced hydrophilicity of metal-oxide (MOx) thin films, focusing on TiO₂ and ZnO, which exhibit photocatalytic effects such as water splitting, carbon dioxide reduction, and organic compound decomposition. The authors propose a model for photo-induced hydrophilicity in MOx films and test it using TiO₂/Si and ZnO/Si heterojunctions. They employ a wet exposure technique, where UV light is applied to a water droplet sitting on the MOx surface, allowing continuous recording of contact angles during illumination. The model and experimental techniques enable the determination of minority carrier diffusion lengths and suggest design rules for materials with photocatalytic hydrophilicity. The study highlights the importance of optical constants and film thickness in photocatalytic surface phenomena and provides insights into the activation of surface groups by UV light. The findings have implications for improving the performance of MOx heterostructures in various applications, including microfluidic channels.This study investigates the light-induced hydrophilicity of metal-oxide (MOx) thin films, focusing on TiO₂ and ZnO, which exhibit photocatalytic effects such as water splitting, carbon dioxide reduction, and organic compound decomposition. The authors propose a model for photo-induced hydrophilicity in MOx films and test it using TiO₂/Si and ZnO/Si heterojunctions. They employ a wet exposure technique, where UV light is applied to a water droplet sitting on the MOx surface, allowing continuous recording of contact angles during illumination. The model and experimental techniques enable the determination of minority carrier diffusion lengths and suggest design rules for materials with photocatalytic hydrophilicity. The study highlights the importance of optical constants and film thickness in photocatalytic surface phenomena and provides insights into the activation of surface groups by UV light. The findings have implications for improving the performance of MOx heterostructures in various applications, including microfluidic channels.