Under a midnight moon, Luigi Garlaschelli peered out over graves. This University of Pavia chemist was scouting for glowing balls of light. They’re known throughout the world by a number of names. One of the most common is will-o’-the-wisp. Like a ghostbuster, Garlaschelli wore a device to vacuum up a wisp for study — should one appear.
To his disappointment, none did. But throughout history, plenty of other people have reported seeing these ghostly glows. For hundreds of years, tales have recounted bluish lights floating above swamps and cemeteries.
With limited success, Garlaschelli and others have searched to understand what might cause them. Now, a team of chemists have turned up an unexpected new clue to their possible source: micro-sparks of electricity.
And there may be additional payoff for this research. Those itsy-bitsy zaps might offer chemists a green trigger to drive useful chemical reactions.
Looking for the spark
Most folk tales describe will-o’-the wisps as something to be feared.
Folklore tends to attribute the eerie lights — also known as fool’s flame, ignis fatuus or jack o’ lanterns (yes, that’s where carved Halloween pumpkins get their name) — to a spirit or fairy that’s trying to mislead a traveler. Their light can flicker, hover and glide. Those brave enough to touch one have would have found it cool to the touch.
Science has offered other explanations. The most common is swamp gas released by rotting plants (or decaying flesh). Swamp gas is mostly methane, a flammable hydrocarbon. And under certain conditions, methane can burn slowly with a cool blue flame.
Unable to capture a will-o’-the-wisp, Garlaschelli went back to his lab in Italy. There, he experimented with compounds given off by rotting plants or creatures. With the right combo, he got a mix of gases to ignite on its own. But it still wasn’t quite right. It burned with a green flame, not a blue one.
Back in 1776, another chemist at the same university studied ghost lights. This Alessandro Volta proposed that some spark ignited the swamp gas. Now, a California team has found data to support Volta’s idea.
Micro-lightning from methane bubbles
Earlier this year, chemist Richard Zare’s team at Stanford University showed that tiny water droplets can create even tinier electrical sparks. 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 proposed this to Yu Xia, a researcher in his lab. Xia, now at Jianghan University in Wuhan, China, put together a team to test that. First, they designed and 3-D printed a nozzle. It would 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 they set up two instruments to help. The most sensitive one was a photon counter. It would alert them to any reaction that emitted light, even dim light. A high-speed camera would capture sparks strong enough to make visible flashes.
As the chemists began bubbling air into the water, the photon counter gave a signal. This meant micro-lightning was happening. But the camera didn’t pick up any flashes.
Next, they bubbled a mix of methane gas and air through the water. Now, more photons emerged than before and the water heated up. But still, no flashes.
So the team adjusted their nozzle to make bubbles of a different size. To see a spark, the bubbles had to be very tiny and crowded together. Over and over they did this, hundreds of times — until finally, zap! The camera caught micro-sparks of lightning.
The team described that success Sept. 29 in Proceedings of the National Academy of Sciences.
Where are the wisps now?
This clearly shows micro bubbles can discharge electrical zaps, says chemist Antonio Pavão. He works at the Federal University of Pernambuco in Recife, Brazil. Yet he’s not convinced the experiment explains ignis fatuus. A swamp is quite different from this lab setup, he points out.
Pavão has done his own research on will-o’-the-wisps. He has focused on a lack of modern-day sightings. Why don’t people see swamp lights anymore, he wonders? If the new theory is correct, he thinks people should still be reporting them.
Long ago, travelers carried lanterns at night. Maybe, Pavão says, those lanterns sparked swamp gas to ignite.
Garlaschelli, now retired, also 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. The new experiments were done in the water, he notes. Will-o’-the-wisps, in contrast, are flames that hover in air.
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A current-day ghost-light sighting would be ideal. Perhaps, Garlaschelli says, someone could replicate the conditions. They might try burying a rotting pig carcass (which would give off gas) and look for a will-o’-the-wisp to emerge.
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. For instance, “it might be used to trigger chemical reactions that would otherwise require much harsher conditions,” he suggests.
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 micro-bubbles might also be harnessed to help break down pollutant gases, Xia says. Or even find use as a greener path to “improving everyday technologies.”