Precipitation can descend at velocities of up to 25 miles per hour. When the droplets make contact with a puddle or pond, they can generate a crown-like splatter that, with sufficient force, can displace any surface particulates and propel them into the atmosphere.

Recently, scientists from MIT have captured high-velocity footage of droplets splashing into a deep basin to observe how the liquid behaves, both above and below the water’s surface, frame by frame at millisecond intervals. Their findings could aid in forecasting how splashing droplets from rain events and irrigation methods might affect watery surfaces and aerosolize surface elements, such as pollen in puddles or agricultural runoff chemicals.

The research team conducted experiments where they released water droplets of assorted sizes and from different heights into a water pool. Utilizing high-speed imaging, they recorded how the liquid body deformed as the impacting droplet contacted the surface of the pool.

Throughout all their experiments, they noted a shared splash progression: Upon hitting the pool, a droplet pressed downward beneath the surface to create a “crater,” or cavity. Almost simultaneously, a mass of liquid surged above the surface, creating a crown. Notably, the team observed that small, secondary droplets were expelled from the crown prior to the crown achieving its greatest elevation. This entire transformation occurs in mere fractions of a second.

Researchers have previously captured images of droplet splashes, such as the renowned “Milk Drop Coronet” — an iconic photograph of a milk drop in mid-splash, taken by the late MIT professor Harold “Doc” Edgerton, who developed a photographic technique to capture swiftly moving subjects.

This new effort signifies the first occasion that scientists have employed such high-speed visuals to simulate the entire splash dynamics of a droplet in a deep basin, integrating both the above and below-surface actions. The team has leveraged this imaging to collect new data essential for constructing a mathematical model that anticipates how a droplet’s shape will change and combine upon impacting the surface of a pool. They intend to utilize the model as a reference point to investigate how much a splashing droplet might lift and disperse particles from the water pool.

“The impacts of droplets on liquid surfaces are omnipresent,” remarks study author Lydia Bourouiba, a professor in MIT’s Civil and Environmental Engineering and Mechanical Engineering departments, and a core member of the Institute for Medical Engineering and Science (IMES). “Such impacts can generate countless secondary droplets that could carry pathogens, particles, or microbes present on the surfaces of affected pools or contaminated water bodies. This study is crucial for predicting droplet size distributions and potentially what those droplets might carry.”

Bourouiba and her students have published their findings in the Journal of Fluid Mechanics. Co-authors from MIT include former graduate student Raj Dandekar PhD ’22, postdoctoral researcher (Eric) Naijian Shen, and student mentee Boris Naar.

Above and below

At MIT, Bourouiba leads the Fluid Dynamics of Disease Transmission Laboratory, which is part of the Fluids and Health Network, where she and her team investigate the core physics of fluids and droplets in various environmental, energetic, and health-related contexts, including the spread of diseases. For their latest study, the team aimed to gain insight into how droplets affect a deep pool — a seemingly simple event that has proven challenging to accurately capture and analyze.

Bourouiba observes that recent advancements have been made in modeling how a splashing droplet evolves below the surface of a pool. When a droplet strikes a body of water, it breaks through the surface and pulls air down, forming a transient crater. Until now, researchers have concentrated mainly on the evolution of this underwater cavity, particularly concerning energy harvesting applications. The phenomena occurring above the water surface, and how a droplet’s crown evolves with the cavity beneath, remains less comprehended.

“The descriptions and insights of what occurs below and above the surface have been quite disconnected,” Bourouiba states, who believes that understanding this relationship could help predict how droplets disperse and launch substances, particles, and microbes into the atmosphere.

Splash in 3D

To investigate the interconnected dynamics between a droplet’s cavity and crown, the team set up a controlled experiment to release water droplets into a deeper water pool. For their study purposes, the researchers defined a deep pool as one that is sufficiently deep enough for a splashing droplet to remain a considerable distance from the pool’s bottom. They determined that a pool with a minimum depth of 20 centimeters was adequate for their experiments.

They adjusted each droplet’s size, averaging around 5 millimeters in diameter. Additionally, they dispensed droplets from various heights, resulting in different impact velocities, which averaged approximately 5 meters per second. Bourouiba mentions that these overall dynamics should resemble what takes place on the surface of a puddle or pond during a typical rainstorm.

“This captures the velocity at which raindrops descend,” she explains. “These would not be minuscule, misty droplets but rather rainstorm drops necessitating an umbrella.”

Employing high-speed imaging techniques inspired by Edgerton’s groundbreaking photography, the team recorded videos of droplets splashing in the pool at speeds of up to 12,500 frames per second. They subsequently applied proprietary image processing techniques to extract key metrics from the image sequences, such as the changing width and depth of the underwater cavity, as well as the developing diameter and height of the ascending crown. The researchers also successfully captured particularly challenging measurements of the crown’s wall thickness profile and internal flow — the cylindrical structure that rises out of the pool just prior to forming a rim characterized by a crown.

“This cylindrical-like wall of ascending liquid and its evolution over time and space is fundamental to everything,” Bourouiba states. “It is what connects the fluid from the pool to what will enter the rim and subsequently get ejected into the air through smaller, secondary droplets.”

The researchers transformed the image data into a set of “evolution equations,” or a mathematical framework that relates the various characteristics of an impacting droplet, including the width of its cavity and the thickness and speed profiles of its crown wall, and how these attributes evolve over time based on a droplet’s initial size and impact velocity.

This research was partially funded by the Department of Agriculture-National Institute of Food and Agriculture Specialty Crop Research Initiative; the Richard and Susan Smith Family Foundation; the National Science Foundation; the Centers for Disease Control and Prevention-National Institute for Occupational Safety and Health; Inditex; and the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.