Droplet impact on small droplet above hydrophobic microstructure surface

The behavior of an interface is influenced not only by the material and surface pattern but also by the contact conditions. Increasing the Weber number (We) increases the likelihood of droplet splashing, which generates smaller droplets that alter contact conditions. This, in turn, affects the coalescence and morphological evolution of larger impacting droplets. However, the influence of these small droplets on coalescence dynamics under varying Weber numbers and contact sequences remains unclear. To determine the coalescence threshold and deformation characteristics under soft and wettable hard surface conditions, we conducted high-speed imaging experiments of a large droplet colliding with a small droplet resting above a hydrophobic surface. The results indicate that the threshold for droplet coalescence occurs at an eccentricity distance of 1.2 mm, with the effect of the Weber number (1 ≤ We ≤ 9) being limited. Upon contact, the small droplet influences the maximum spreading ratio and contact time by increasing the volume of the impacting droplet, altering its impact direction, hindering spreading, and changing its own morphology. When the impacting droplet contacts the substrate before the small droplet, there is a linear relation between contact time and eccentricity distance. From this, we propose a relationship considering the effects of eccentricity distance and Weber number on contact time. Beyond these effects, the small droplet also influences the transition between symmetric and asymmetric structures of the rebounding droplet, thereby regulating the rebound height.