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How do you put out a fire with no extinguisher? By being really, really loud.
The Defense Advanced Research Projects Agency, better known by its acronym DARPA, has been developing what it's called IFS or Instant Fire Suppression, that is, some kind of method for instantly putting out a fire in an enclosed space, like a Navy ship, for instance. Though it's commercially viable yet, it looks like they've succeeded.
To do it, they had to come down on one side of an ongoing debate: Is fire a plasma? The typical understanding of fire from the perspective of a chemical reaction, fire is not plasma, at least not normal fires. They simply don't get hot enough to ionize molecules or atoms. Or so you'd think.
DARPA researchers thought of fire as a cold plasma - the kind that makes your plasma screen TV so fancy and clear. Once they understood that, they narrowed their thinking down to two two ways to disrupt fire based on the physics of plasmas rather than the chemistry of combustion: Sound and electromagnetic fields.
DARPA used extremely loud sounds at a particular frequency to both thin the boundary between flame and air, reducing combustion, and also increase the vaporization rate of the fuel, which thins the flames further and lowers the temperature of the fire. Eventually, the blaze completely disappears.
Though less spectacular, the DARPA also used electromagenetic waves to (sort of) blow out small gas flames by passing a glass-coated electrode over them. "Put simply, the electric field creates an ionic wind that blows out the flame.
However, the ionic wind has it's limits. "This same approach was not able to suppress a small heptane pool flame."
Though making these techniques actually useful, it is interesting to think about not only how this could change firefighting, but how it could be reverse-engineered to actually increase combustion rather than stop it.
"We have shown that the physics of combustion still has surprises in store for us. Perhaps these results will spur new ideas and applications in combustion research," said DARPA program manager Matthew Goodman.
