In contrast, the team found that raindrops that splashed onto leaves covered with droplets, rather than a film, launched these drops far and wide. When a droplet hit this surface, it launched a crown-like spray of the filmy substance, though most of the spray stayed within the general vicinity. To observe such dynamic differences, the team first simulated rainfall over a flat surface coated with a thin film. “That can initially seem like a small difference, but when you look at the fluid dynamics of the fragmentation and resulting range of contamination around an infected leaf, it actually changes a lot of the dynamics in terms of the mechanism by which are emitted,” Bourouiba says. The team concluded that pathogens, in turn, must rest as droplets - not film - on a leaf’s surface. From these images, Bourouiba and Gilet noted that as water fell, leaves were unable to support a thin film, instead forming drops on their surface. The researchers captured the sequence of events as raindrops hit each leaf, using high-speed videography at 1,000 frames per second. The container was suspended several meters in the air, high enough for drops to reach terminal velocity - the speed of an actual raindrop upon impact. In initial trials, the researchers simulated rainfall by running water through a container pricked with tiny holes. They conducted hundreds of experiments for each type of foliage, using 30 examples of real plant foliage and 12 artificially engineered materials. The team ran experiments with dozens of types of common foliage, including ivy, bamboo, peppermint, and banana leaves. In their paper, Bourouiba and Tristan Gilet, of the University of Liege, first addressed a widely held assumption: that pathogens coat leaves in a thin film.

Raindrops images