Making a splash: unraveling the impact of large water droplets

The simple process of a droplet falling into water is underpinned by very complex dynamics. Researchers at KAUST have taken a deep dive into this phenomenon, focusing on the impact of extremely large water droplets^[1].

Research has traditionally focused on small droplets, ones with diameters up to the capillary length, where surface tension dominates gravitational forces. Led by postdoctoral researcher, Sandip Dighe, under the supervision of renowned fluid dynamics researcher, Tadd Truscott, this study explores the impact of large water droplets, with diameters ranging from 1 to 6 centimeters.

The researchers found that when these large droplets fall into a pool of water, the shape of the droplet at impact greatly influences the depth and width of the cavity formed in the water. Large droplets amplify this behavior by flattening before impact. The maximum cavity depth is influenced by the velocity of impact and the ratio of the height and width of the droplet at impact.

“The impact of droplet shape on cavity formation provides a nuanced understanding of a phenomenon previously assumed to be primarily influenced by droplet size and velocity,” explains Dighe. “The observation of extreme droplet shapes and their impact behavior, such as the ‘bagging regime’ where droplets flatten significantly upon impact, adds another layer to our understanding of fluid dynamics.”

The study showed that cavity depth depends more on the droplet height than width, and the maximum cavity diameter is independent of the droplet height. More oblate droplets — those with a greater horizontal diameter — result in decreasing cavity depths for a fixed liquid volume. This is because an increase in horizontal droplet diameter results in a reduced impact energy flux and therefore reduced cavity depth.

Truscott emphasizes the importance of the discovery in the field of fluid dynamics: “This research enhances our understanding of natural phenomena such as raindrop impacts,” he says. “The turbulence generated on raindrop impact is instrumental in mixing ocean surface layers, and facilitating the distribution of heat, nutrients and gases. Such processes are integral to environmental science, climate models, marine ecosystems and fluid mechanics. So, gaining insights into droplet behavior improves our understanding in these areas too.”

The research also has potential applications in fields such as mechanical and marine engineering. “It could inform the design of water-based cooling systems, optimizing fuel injection systems or development of technologies for tackling oil spills,” says Dighe.