Spatially Heterogeneous Wetting on Stochastic Nanoflake Surfaces

While Wenzel and Cassie-Baxter wetting on textured surfaces is well established, their spatial occurrence on stochastic nanotopographies remains less well understood. Here, boehmite nanoflake surfaces with systematically varied morphologies were used to examine local wetting-state occurrence by spatially resolved droplet adhesion measurements. The nanoflake structures were produced by hot-water treatment of atomic-layer-deposited aluminum oxide (Al2O3) films, with the initial film thickness used to vary the resulting nanotopography. The adhesion measurements on the hydrophobized nanoflake surfaces revealed a systematic change in local wetting behavior across the surface series: Wenzel and Cassie-Baxter states coexisted spatially at low and intermediate nanoflake coverage, and Cassie-Baxter wetting was observed at all investigated locations on higher-coverage surfaces under the applied measurement conditions. The occurrence of Cassie-Baxter wetting was quantified by the occurrence fraction, fCB, and its dependence on image-derived nanoflake coverage was described empirically using logistic regression. The results demonstrate that the evolution of stochastic nanoflake morphology is accompanied by a probabilistic change in local wetting-state occurrence and reveal an intermediate regime characterized by spatially heterogeneous Wenzel and Cassie-Baxter wetting.