Under a midnight moon, Luigi Garlaschelli peered out over graves. He was scouting for glowing balls of light known as will-o’-the-wisps. Like a ghostbuster, Garlaschelli, a chemist formerly at the University of Pavia in Italy, wore a device to vacuum up the wisp for study — should one appear.
Sadly, one didn’t. But for hundreds of years, others’ tales have recounted these bluish lights floating above swamps and cemeteries. While folklore links the lights to spirits trying to lead travelers astray, scientists think rotting plants or creatures release a methane-filled gas that, under certain conditions, can burn slowly with a cool, blue flame.
But what ignites that flame is a mystery. Now, a team of chemists have turned up an unexpected new clue: microsparks of electricity.
In lab experiments, bubbling methane and air through water created itsy-bitsy zaps of lightning, the team reports September 29 in Proceedings of the National Academy of Sciences. Those flashes might be enough to ignite swamp gas and let it bloom into ethereal will-o’-the-wisps. What’s more, chemists might one day harness this microlightning as an environmentally friendly trigger to drive important chemical reactions.
Previously, chemist Richard Zare and colleagues showed that tiny water droplets can create electrical sparks. The team wondered: Could tiny bubbles do the same? And if so, might these zaps create will-o’-the-wisps by igniting the methane gas seeping out of swamps?
Zare put this question to Yu Xia, a researcher in his lab at Stanford University. Xia, now at Jianghan University in Wuhan, China, put together a team to design and 3-D print a nozzle to make tiny bubbles in water, mimicking gas seeping from a swamp.
Any zaps between bubbles would probably not be visible to the unaided eye, so the researchers set up two instruments to help: a photon counter, to alert them to any reaction that emitted particles of light, and a high-speed camera to capture any flashes.
As the chemists began bubbling air into the water, the counter began tallying emitted photons. Microlightning was happening, but too faint for the camera to see.
A bubbled mix of methane gas and air released more photons than before, and the water heated up. But still, no visible microlightning.
The team adjusted the nozzle to make some bubbles even smaller and closer together. The researchers’ earlier work with water droplets had shown that, in a spray of water, smaller drops are negatively charged, and larger ones are positively charged. If the same was true of the gas bubbles, the crowded jostling could send electrons leaping between differently charged bubbles, creating a flash of light. Sure enough, success: The camera captured tiny zaps of lightning.
This clearly shows microbubbles can discharge electrical zaps, says Antonio Pavão, a chemist at the Federal University of Pernambuco in Recife, Brazil. Yet he’s not convinced it answers the mystery of the wisp: A swamp is quite different from this lab setup. And if the new idea is correct, he thinks people should still be reporting them. But there’s a lack of modern-day sightings, his research shows. Long ago, travelers carried lanterns at night. Maybe, Pavão says, the open flame in those lanterns sparked swamp gas to ignite.
Garlaschelli, now retired, finds bubble zaps igniting swamp gas an intriguing idea. But it doesn’t fully answer the mystery of will-o’-the-wisps for him either. One issue: The new experiments were underwater. Will-o’-the-wisps, in contrast, are flames that hover in air.
Even if the source of these lights is not yet solved, Garlaschelli sees benefits in the new work. It’s a promising step forward in the field of triboelectricity, he says. This triboelectric effect creates electric charge from friction, like static electricity, or when objects come into close contact and are separated. For instance, he says, “it might be used to trigger chemical reactions that would otherwise require much harsher conditions.”
Xia agrees. “These tiny droplets and bubbles,” he says, “are important for their role in environmental chemistry.” They might help explain, for instance, “how pollutants behave and transform in the atmosphere.” Zaps from microbubbles might also be harnessed to help break down pollutant gases, he says. Such sparks might even find use, Xia says, as a greener path to “improving everyday technologies.”