On the interfacial dynamics and capillary waves during impingement of a drop on liquid pool: A background-oriented schlieren study at low Weber numbers

Understanding the dynamics of a droplet impinging on a liquid pool and the associated phenomena have been of interest due to its prevalence in nature as well as in technical applications. This paper aims toward studying the characteristics of the capillary waves generated due to the low Weber number droplet interactions with the liquid pool. In this direction, experiments have been carried out for six different pool heights varying from h = 1.4 to 12 mm, encompassing thin liquid film, shallow pool, and deep pool regimes. Due to its wide usage, water has been chosen as the fluid of interest for droplet as well as for pool liquid. The study is focused on droplets impinging on the liquid pool at low Weber number ranging from 1 to 100. In order to characterize the post-impact perturbations in the liquid, background oriented schlieren (BOS) technique has been employed which offers real-time, non-intrusive whole-field measurements of the perturbations in the liquid pool. Measurements from BOS have been validated against the side-view projection of the impact. The transient variations of the air–water interface for different pool regimes and Weber numbers have been delineated. Results evince the formation of secondary wave at impact followed by the formation of primary wave after the crater retraction. The wave formation was faster and had higher amplitude in thin liquid regime for droplets with the same Weber number compared to the other regimes, but the perturbations were reduced through higher dissipation. The formation of the Worthington jet was seen in shallow and deep pool regimes for droplets with higher Weber number (We = 100), and its effect on the capillary wave is also discussed.