Water droplet evaporation on patterned surfaces: Experimental data and a phenomenological model

The Wenzel and Cassie-Baxter models are widely used to design surfaces with tailored wettability, yet their validity and limitations remain debated. In this work, we experimentally investigate droplet evaporation on microstructured PDMS substrates, including isotropic micropillar arrays and anisotropic stripe patterns with varying roughness. On fine pillared surfaces, droplets initially adopt the Cassie-Baxter state before transitioning to the Wenzel state, while on coarser patterns they enter the Wenzel state directly. Analysis of contact line pinning and depinning at pillar edges reveals that droplets can wet pillar sidewalls rather than only the pillar tops. On pillared substrates, measured contact angles in both states agree closely with predictions from the classical Wenzel and Cassie-Baxter equations, reaffirming their applicability. On striped surfaces, the Wenzel model reliably predicts contact angles along the stripe direction, though deviations arise perpendicular to the stripes. We introduce a force-balance model for the Cassie-Baxter to Wenzel transition, which accurately captures the transition for the current experiments and those from the literature for substrates of various geometries.