Effect of interface shape on advancing and receding fluid-contact angles around spherical particles.

The angle of contact between a solid surface and a fluid interface plays a key role in wetting and is therefore a focus in studies of a wide range of natural phenomena and fluidic technologies. The contact angle ranges between two values, a maximum (advancing) angle and a minimum (receding) angle. These limiting angles are thought to be properties of the fluids and of the chemistry or topography of the solid. By contrast, we find that the value of the receding angle can be significantly reduced by altering the interface shape. Using millimeter-sized spheres coated with polydimethylsiloxane and pulled through an air-water interface, we observe that the receding angle decreases from 101 ± 1° at a planar interface to as low as 80 ± 1° at saddle- or cylinder-shaped interfaces. The angle decreases smoothly with the deviatoric curvature of the interface (a measure of the shape anisotropy) and is linked to a non-circular contact line.