On the shape of droplets confined on non-wetting surfaces

Solid-liquid interactions on textured surfaces dictate droplet evolution, producing diverse morphologies that serve as sensitive probes of wetting physics. Precise control of liquid morphology is essential to applications in microfluidics, printing, and functional coatings. While conventional theory effectively quantifies apparent contact angles, a fundamental gap persists in explaining and predicting the global droplet shape selection.

Droplet spreading on non-wetting micropillar surfaces was induced by geometrical confinement via controlled variation of the droplet thickness. Under confinement, the droplet underwent a distinct symmetry-breaking transition from a circular to a polygonal morphology below a critical thickness, while remaining in the ultralow-adhesion Cassie-Baxter wetting state throughout the transition.

We identified a critical thickness governing droplet morphology under confinement. The symmetry-breaking transition arises from direction-dependent pressure barriers imposed by the micropillar array, revealing a previously unreported mechanism that cannot be predicted by classical wetting theories. We further developed a simple theoretical model demonstrating that the critical thickness follows a universal relation controlled primarily by the pillar spacing and advancing contact angle. These findings clarify how geometric confinement selects droplet symmetry on non-wetting textured surfaces and establish general design principles for programmable droplet manipulation in micro- and nanoscale systems.