Topography-Driven Liquid Repellency in PFAS-Free Spherical-Particle Coating

The development of PFAS-free (per- and polyfluoroalkyl substances), liquid-repellent surfaces require a fundamental understanding of how surface geometry governs wetting-state stability. However, the independent role of interparticle spacing remains poorly understood because it is often coupled with changes in particle size, surface chemistry, or hierarchical morphology. Here, we present a geometry-driven strategy to systematically investigate the influence of spacing using re-entrant spherical architectures with controlled spacing ratios. Monodisperse 1 μm silica particles were immobilized within an epoxy matrix, while sacrificial 1 μm polystyrene spheres served as inert spacers to precisely tune interparticle distance without altering particle size or surface chemistry. After spacer removal and conformal hydrophobic silanization, wetting behavior was evaluated using ethanol-water mixtures with surface tensions ranging from 71.93 to 22.28 mN m-1. The apparent contact angle exhibits a nonmonotonic dependence on spacing ratio for liquids with surface tensions above approximately 25 mN m-1, reflecting a critical trade-off between reduced solid-liquid contact fraction, which enhances repellency, and diminished Cassie-Baxter state stability, which promotes wetting transition. As surface tension decreases, the optimal spacing ratio shifts toward smaller values, indicating increasingly stringent geometric requirements for sustaining the suspended wetting state. A theoretical framework integrating the Cassie-Baxter and Wenzel models with a breakthrough-pressure criterion quantitatively captures the experimental trends. These findings establish predictive design rules for geometry-controlled wetting and provide a scalable pathway for engineering high-performance, PFAS-free liquid-repellent coatings.