Cactus Needles Inspired A Super-Efficient New Way To Clean Up Oil Spills
Cactus needles are not just for warding off animals trying get a taste of the plant's moist flesh. They also use a trick from physics to help them draw water from the air to keep the plant hydrated in its harsh desert environment.
Cactus needles are shaped like cones with — as many unfortunate victims have found — a sharp tip facing outward. The spines widen as they get closer to the body of the plant.
Physics in action
Once a needle catches a drop of water from accumulated molecules in the air, its shape uses a property of water to its advantage to move it. The bonds between water molecules exert a strong inward pressure — they all want to be together. This is what creates the spherical shape of a drop of water. When water hits the side of a needle, the curved shape of the needle forces the water to bend and droop out of that perfect spherical shape.
Meanwhile, an opposing force within the droplet — surface tension — is trying to pull the drop back into shape.
These dueling forces cause the droplet to slide up the side of the cone toward the base, where the flatter surface allows the drop to return to a more circular shape.
There, the needle has a more porous surface that absorbs the water into the plant.
See the cone in action here:
After making this realization, the researchers retooled this process in the hopes of using it to separate dispersed oil droplets from water — a major barrier to effectively cleaning up ocean oil spills.
We traditionally think of oil and water as immixable, but oil droplets can disperse in water and other liquids to the point that they are very difficult to collect. Salad dressings are often based in part on dispersed oil drops.
Separating oil and water
Dispersed oil droplets have a tendency to maintain a perfectly round shape, similar to the water on the cactus needle. So the researchers made cones out of substances that would attract oil, but not water — a substance called polydimethylsiloxane (PDMS), which both attracts oil and is easy to mold.
They formed the PDMS into tiny spikes less than one millimeter long, placed them in rows on a plastic sheet, and then sprayed them with mixtures of oil and water.
The simple setup caught 99% of the oil that hit it. Oil droplets continuously attached to the cones, slid up their sides and coalesced at the plastic sheet.
The results were reported in the August 6 issue of the journal Nature Communications.
In theory, oil spill cleanup could make use of a similar setup to the one pictured to the right. Oil-contaminated water could be fed across large plates covered in PDMS cones which could separate out the oil, and suction on the back of the plates would vacuum it up, leaving clean water.
The toxic results of spilled oil
A single oil spill can be a major environmental catastrophe. The Deepwater Horizon oil rig owned by BP leaked over 200 million barrels of oil — more than half the amount imported to the U.S. in a single day — into the Gulf of Mexico in 2010.
It seems like that would be pretty easy to clean up: Send a cargo ship with a suction device to the area and suck up the oil. But in reality the oil forms droplets smaller than the width of a human hair.
Because of that, these large spills can have devastating effects — the Deepwater Horizon spill contaminated almost 700 miles of coastline and killed at least 8,000 marine birds, sea turtles, and marine mammals.
These events are devastating, and we haven't found a good way to clean up oil spills. Strangely, cleanup efforts often involve the use of dispersants — which spreads the oil out in the ocean in hopes of diluting it and reducing its toxicity and therefore the harm it does to the immediate ecosystem.
But a recent study, published in the journal Environmental Pollution, showed the chemicals used to disperse oil during the Deepwater Horizon spill actually raised the toxicity of the local environment more than 50 times, and generated 35 million gallons of dispersed oil — more than the daily OPEC nation exports.
A problem without a solution
There are a variety of cleanup methods currently used to contain and clean up an oil spill, which include burning the oil, deploying vast lines of absorbent material to contain or soak up the oil, or dumping oil-eating microorganisms into the water.
While these methods work reasonably well under certain circumstances, none are particularly effective on oil that has dispersed.
By collecting these droplets with synthetic PSDM cactus needles, cleanup time could be dramatically reduced and the devastating impact of oil spills decreased.
Of course, some outside experts are skeptical this technology in its current state will actually work in a disaster.
Igor Mezic, a professor of mechanical engineering and oil spill expert at UC Santa Barbara, who didn't work on the study, estimated that "a 0.3-by-0.3 meter array of oil-attracting needles could clean 1 liter of oil-contaminated ocean water per second" — not enough, he told Science News, to clean up large oceanic oil spills.
The researchers agree that the technology requires further testing and scale-up to be able to handle environmental disasters like the Deepwater Horizon spill.
But if successful, this new technology is a phenomenal example of using lessons from nature to solve problems.
"This excellent piece of work provides a perfect example of first describing an interesting biological system and then taking it one step further by solving an engineering problem," Joanna Aizenberg of Harvard University, who was not involved with the study, told the BBC. "It shows not only how we can learn from nature but also how to apply that knowledge in bio-inspired design."