Raindrops Can Launch Microplastics Into the Air, High-Speed Cameras Show

Rain on the sea seems calm, but when you watch it in slow motion with a high-speed camera, each drop looks like a tiny explosion. A drop hits the water, forms a crater, a ring of spray appears, and a thin column of water shoots up from the center. Engineers from the University of Oxford, University College London (UCL), and the National Autonomous University of Mexico (UNAM) used these slow-motion images to see if rainfall could help spread plastic pollution.^[1]

The researchers used high-speed cameras to watch water droplets hit a surface covered with tiny plastic particles. They published their findings in the Journal of Colloid and Interface Science.^[2] They discovered that microplastics can get into the air in two ways: from the first splash and from a thin jet of water that shoots up when the crater left by the drop closes.^[3] This upward jet is called a Worthington jet. In their preprint, the team explains that it forms when the cavity made by the drop closes and pushes momentum upward along the center.^[4] In some tests, a single droplet that was thrown up carried about 100 microplastic particles.^[5]

How microplastics get into the air

The most surprising finding was how important the surface of each particle was. “What struck me most was how changing just the surface properties of the particles could completely change what happened to them when a drop hit the water,” said lead author Muhammad Hamza Iqbal, a PhD researcher at UCL^[6].

The preprint talks about rafts made of fluorescent polyethylene, plain polyethylene, and glass particles, with some treated to be very water-repellent.^[7] These super water-repellent particles acted differently. Some formed tiny droplets covered in a plastic shell, called liquid marbles, which do not easily mix back into the water. This means the plastic can stay spread out for longer.

This research comes from Professor Alfonso Arturo Castrejón-Pita’s Fluid Dynamics Laboratory at Oxford, which studies drops and liquid jets.^[8] “Microplastics are now everywhere and understanding how they spread is hugely important if we are to track them and perhaps eventually contain them,” he said^[9].

The researchers are clear about what their study shows and what it does not. Their experiments were done in controlled lab conditions, so more work is needed to see how important this process is in the ocean. Real rain falls at different speeds, real waves stir the water, and real plastic is much messier than the clean lab particles. Still, the study gives a clear view of the mechanism, which is the first step before measuring it in nature.

There is an interesting connection for people who like making things. The same physics of drops and jets is important for inkjet and 3D printing, which are also studied in the lab.^[10] If you understand how a drop acts when it hits a surface, you can use that knowledge to print circuits or track pollutants. If you are unsure which type of engineering is right for you, our guide to engineering disciplines is a good place to begin.

The big question is about scale. No one knows yet how much plastic rain lifts off the oceans each year or how far those particles travel in the air. To answer this, scientists will need to take measurements at sea and use models that combine lab physics with real wind, waves, and weather.

What this means for you

Fluid dynamics engineers design experiments like these and build the high-speed camera setups to film them. Environmental engineers track where pollution goes after it leaves the water. Many fluid dynamics experts begin with a mechanical engineering degree, and our guide to mechanical engineering careers, degrees, and vocational qualifications explains the different paths, including apprenticeships.

Image: University of Oxford Department of Engineering Science