Oscillation and recoil of single and consecutively printed droplets.

In this study, the recoil and oscillation of single and consecutively printed drops on substrates of different wettabilities are examined using a high speed camera. The results show that, for a droplet impact on a dry surface at Weber number ~ O (1), both inertia and capillary effects are important in the initial spreading regime before the droplet starts to oscillate. For a substrate of higher wettability, drop oscillation decays faster due to a stronger viscous dissipation over a longer oscillation path parallel to the substrate. It is also found that when a drop impacts on a sessile drop sitting on a hydrophobic substrate, the combined drop recoil twice resulted from the coalescence of the two drops, whereas no recoil is observed for the impact of a single drop on a dry surface under the same condition. Furthermore, a single-degree-of-freedom vibration model for the height oscillation of single and combined drops on a hydrophobic substrate is established. For the condition considered, the model predictions match well with the experiments. The results also show the extent to which the increase in the liquid viscosity facilitates oscillation damping and the quantitative extension of the oscillation time of a combined drop compared to a single drop.