Water sprays formed by impinging millimeter-sized droplets on superhydrophobic meshes

Abstract

Impinging a millimeter-sized liquid droplet on a leaky substrate—such as a porous mesh—can cause the formation of many small droplets from the ligament fragmentation. Although this phenomenon has been widely considered as a desirable strategy to produce liquid sprays of monodisperse droplets, the underlying mechanism has not yet been completely elucidated, and the spray needs detailed characterization. Herein, we experimentally investigate the atomization phenomena occurring in the recoiling and spreading stages of impinging water droplets on superhydrophobic meshes. We show that the spray formed during droplet recoiling is stimulated by the longitudinally symmetric air cavity collapse on the superhydrophobic mesh, and thus the size of the spray formation area on the mesh is almost identical to the size of the simultaneously generated upward jet. By contrast, the water spray produced during droplet spreading is expelled under the action of the inertia-induced hydrodynamic pressure, and the size of spray formation area on the mesh exhibits a power-law dependence on the Weber number; yet, the pore geometry restricts it to take a constant value when the Weber number is sufficiently high. By performing statistical analyses on the spray droplet sizes, we further demonstrate that the mean sizes of spray droplets are mainly set by the mesh pore sizes, but the complex atomization dynamics leads to a broad size distribution, which is beyond the expectation.