Researchers looking for a better way to clean up oil spills are taking a cue from the humble cactus. A new study shows that synthetic needles based on those of the desert plant can take up oil droplets from the ocean much as the cactus takes up water from the air.
Cactus needles have a curious effect on water. When tiny water droplets in the air land on them, the needles’ conical shape distorts them, nudging them into a clamlike shape. Because water droplets like to be circular, the clam-shaped beads tend to have surface tension (known as Laplace pressure) that forces them back into the circular shape. This tension pushes the droplets from the tip of the needle toward the cactus plant at the base of the needle, where the needle’s surface is less curved. The cactus plant typically has pores at the base of the needle to take up the liquid.
Oil spills also produce tiny droplets. When petroleum leaks from a ship or a deep-water drilling operation, “it tends to break up into tiny droplets that don’t all end up on the surface of the ocean,” says Thomas Azwell, an environmental scientist at the University of California (UC), Berkeley, who was not involved in the work. That's because seawater chemically transforms the oil into a denser type of oil, which disperses into micrometer-sized droplets that are too heavy to float. Current cleanup technologies—such as mechanical skimmers or membrane filters, which are maneuvered with boats and oil tankers—mainly remove oil from the surface but miss those denser droplets, Azwell says.
To tackle the problem, Lei Jiang and his colleagues at the Chinese Academy of Science in Beijing created an array of conical copper and synthetic needles similar in shape to cactus needles of up to 0.5 millimeters in length. They then submerged them in a mixture of silicone oil and water that they blasted with ultrasonic sound waves to generate micrometer-sized oil droplets. Using microscopic video recordings, the team showed that underwater oil droplets flowed continuously along the needles at a rate of 2 millimeters per second, similarly to the way water moves along cactus needles, the team reports  online today in Nature Communications.
Azwell praises the advance as a cheap and easy alternative to current oil cleanup methods. He envisions that arrays of the needles could be submerged several meters underwater to collect oil droplets, which could then be sucked up by a tanker. But he says it's unclear how changes in oil consistency might impact the needles' effectiveness in a real spill.
Igor Mezić, a professor of mechanical engineering and oil spill expert at UC Santa Barbara, further cautions that the technology might not be seaworthy. He estimates that a 0.3-by-0.3 meter array of oil-attracting needles could clean 1 liter of oil-contaminated ocean water per second—too little, he says, to feasibly clean up large areas of the ocean.