Droplet-droplet interaction and coalescence mechanism of water droplets on textured oil-impregnated surfaces

Droplet-droplet interaction and coalescence mechanism on lotus leaf-inspired textured hydrophobic surfaces has been studied extensively for decades and the governing physics is well understood. The fundamental physics, however, changes on state-of-the-art micro/nanotextured oil-impregnated surfaces due to the presence of wetting ridge that forms due to the imbalance of forces at the contact line. In this study, using high-speed visualization, we show that water droplets coalesce in two stages: surface tension-mediated droplet-droplet interaction (pre-coalescence attraction) followed by coalescence. The two stages of coalescence appear as peaks in the velocity and acceleration of the droplets. We modeled the coalescence mechanism using the standard damped mass-spring system by treating the water droplets as squishy spheres. Our results show that the period of oscillation of the droplets scales with the droplet radius to the 3/2 power. We also modeled the time required to squeeze out the oil separating the droplets after the initial attraction, which scales with the oil thickness to the -3/2 power. This work provides new understanding and unveils the fundamental physics that governs droplet dynamics on textured oil-impregnated surfaces.