Making A Splash

High-speed video shows how far one drop of water really goes.
Karin Heineman, Executive Producer

(Inside Science) -- A single drop of water, a few millimeters wide, probably goes unnoticed by most people. But when that tiny drop lands on a surface, a leaf, a blade of grass, or even your head, it doesn’t stop there. That one drop then produces more drops and more drops, and so on. Scientists say this spray can contribute to the spread of infectious disease on almost everything from humans to plants to ponds and lakes.

“The transport of pathogens occurs in droplets typically, whether we're talking about somebody sneezing, coughing, whether we're talking about sprays in irrigation on crops, whether we're talking about impacts on contaminated water or lakes. Ultimately, everything ends up being transported in the droplets.” said Lydia Bourouiba of the Massachusetts Institute of Technology.

Using high-speed cameras, scientists at MIT looked at single splashing droplets -- in just a few milliseconds, hundreds of smaller droplets are formed and are ejected into the air.

“Throughout the breakup process, secondary drops are being generated, and the time variation of that process is very important to understand to know what ultimately the final total drops that are emitted during that whole process is going to look like,” said Bourouiba.

Bourouiba and her team developed computer algorithms to measure the droplet splashes and patterns. They tracked the thickness of a droplet’s rim perimeter as it splashes and forms a sheet expanding away and up from a range of surfaces. This process can predict the number, size and speed of smaller droplets that are ejected from the rim and into the air.

“Some of the interesting results we found were also not only continuing to be applicable to not just water, but all sorts of other fluids, including some soapy fluids and some mucous salivary fluids,” said Bourouiba.

The results can also be used to model and control the physics of spray coatings, like pesticides that splash back up from crop leaves or raindrops that may pick up and spread diseases as they impact contaminated surfaces. Previous research has shown that splashes of rain can send contaminated droplets into the air and onto other plants. That might help explain why farmers notice that plant disease outbreaks often follow a rainstorm.

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Karin Heineman is the executive producer of Inside Science TV.