Droplet impact on a mesh wetted underneath

Droplet impacts on meshes are ubiquitous in applications where the mesh becomes wetted following even a single impact. The wetting morphology is diverse due to the mesh's leakage characteristics, but its effects on impact outcomes are yet to be explored. Here, droplet impact on a mesh wetted underneath was investigated using high-speed photography, with special attention paid to the influence of the height of the hanging droplet, the mesh size, and the Weber number. It was found that the threshold Weber number corresponding to the generation of secondary droplets initially increased and then decreased as the height of the hanging droplet increased and decreased as the mesh pore size increased. A semi-empirical model was proposed, capable of reproducing the nonmonotonic dependence between the threshold Weber number and the height of the hanging droplet. Slightly above the threshold Weber number, only one secondary droplet was generated, whose diameter increased with the hanging droplet's height, and its magnitude was similar to the impacting droplet. A further increase in the Weber number resulted in additional secondary droplets with smaller diameters. However, the diameter of the first secondary droplet was always of the same order of magnitude as the impacting droplet. The spray efficiency increased with an increase in the Weber number and the height of the hanging droplet, even well above 1, indicating that the impact can decrease the liquid mass that remains attached underneath the mesh.