Drop-on-Drop Impact Dynamics on a Superhydrophobic Surface

Investigation of multiple droplet interactions is vital due to its importance in various applications, including sprays. In this context, the interaction of a sessile droplet (droplet #1) interaction with an oncoming droplet (droplet #2), in a vertically aligned, drop-on-drop configuration, on a superhydrophobic surface is investigated, both experimentally and computationally. Three droplet impact regimes, with low We number of the impacting droplet (i.e., We_droplet #2 ∼ 1.0) is observed, viz., gentle merging, late merging, and droplet bounce-off without merging. Complementary simulation of the gentle merging regime shows that a very high-pressure gradient is generated between the neck and the bulk region immediately after the contact, which acts as a driving force for the subsequent rapid evolution of the droplet shape. With a larger We number (We_droplet #2 ∼ 8.2 and 14.8), there is always a unique outcome of the impact experiments. The two droplets merge, spread, recede, and bounce-off; there are multiple instances of rebound of the combined droplet mass from the surface. The estimated shear stress emanating during the impact and subsequent dynamics is large enough to cause deterioration of chemically coated superhydrophobic surfaces. The difference in the shape of moving droplet #2 at the instance of impact for We_droplet #2 ∼ 1, as compared to We_droplet #2 ∼ 8.2 and 14.8 is also highlighted. Important time scales during coalescence and beyond are the inertial-capillary time scale (∼16 ms) and viscous-capillary time scale (∼8 s). We also highlight the role of the droplet #1 deposition boundary condition, influence of the We number of droplet #2, and substrate wettability on drop-on-drop interactions, which has direct relevance to the transport mechanisms happening in various engineering applications like spraying pesticides, spray cooling, and rain interactions, etc.