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 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 towards studying the characteristics of capillary waves at low Weber number droplet interactions with liquid pool. Experiments are carried out for 6 different pool heights (h = 1.4 to 12 mm), encompassing thin film, shallow pool and deep pool regimes. Water is chosen as the fluid of interest for droplet as well as for pool liquid. The study is focused on droplets impinging on liquid pool at low Weber number range (1 to 100). In order to characterize the post-impact perturbations in the liquid, background oriented schlieren has been employed, which offers real time, non-intrusive whole field measurements of perturbations in the liquid pool. Measurements from BOS are validated against the side view projection of the impact. The transient variations of the air-water interface for different pool regimes and Weber numbers are delineated. Results evince the formation of secondary wave at the impact followed by formation of primary wave after the crater retraction. The wave formation was faster and had higher amplitude in thin liquid regime for droplet with same Weber number compared to the other regimes, but the perturbations were reduced through higher dissipation. The formation of 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.