Rolling and Impacting Caustic Drops on Super Liquid-Repellent Surfaces : In Situ Force and Energy Monitoring of Surface Degradation

Super liquid-repellent surfaces are increasingly deployed in harsh environments, including marine electronics, electrochemistry, and microfluidics. However, prolonged exposure to corrosive or reactive media causes progressive surface degradation that compromises performance. Conventional evaluation methods based on immersion and time-resolved studies, are coarse, discontinuous, and susceptible to bias due to the heterogeneous nature of degradation. Here, two continuous, drop-based force-probing techniques are introduced for in situ assessment of surface degradation by analyzing the dynamics of rolling and impacting caustic drops. As degradation intensifies, rolling drops decelerate due to increasing retention forces, while impacting drops rebound less due to increasing surface-induced energy dissipation. Rolling and impacting drops resolve retention forces and energy at resolutions of ≈0.1 µN and ≈1 µJ, respectively. These techniques are benchmarked on three metal oxide-based superhydrophobic surfaces: a commercial silica-based surface (Glaco), a perfluoroalkylated silica-based surface, and a newly developed fluoro-free, hydrocarbon-functionalized silicate-titania surface. Despite similar apparent wetting properties (contact angle, CA > 150°, sliding angle, SA < 10°), these surfaces degrade at rates that differ by up to an order of magnitude in both differential retention forces and energy dissipation under identical caustic exposure. Accordingly, these methods provide a robust, generalizable, high-precision platform for advancing the design of chemically-durable (super) liquid-repellent materials.