When astronauts travel deeper into space, every drop of fuel becomes critical. Keeping those fuels stable for months or even years is one of the biggest challenges facing future missions to the moon, Mars, and beyond. Now, University of Florida researchers have uncovered something surprising about how boiling liquids behave in space — and it might help solve that challenge.
The research, led by Youngsup Song, an assistant professor of mechanical and aerospace engineering, found that boiling liquids actually work better in reduced gravity — at least up to a point. That was the opposite of what scientists expected.
"Boiling plays a critical role" in two major space travel challenges, Song explained. Electronics in spacecraft need cooling, and enhanced boiling helps carry heat away. But rocket fuels like liquid hydrogen and liquid oxygen are the opposite — every bubble that forms means fuel escaping into space, a problem called "boil-off."
Engineers have long assumed that reduced gravity makes boiling less efficient, because bubbles normally rely on buoyancy — their own rising force — to float away from heated surfaces. In space, without gravity, scientists expected bubbles would just stick around and break the cooling process.
"Because we lose buoyancy in reduced gravity, we expected boiling would become less effective across the board," Song said.
To test this, Song's team boiled liquid nitrogen — an extremely cold liquid often used in space technology — on atomically smooth silicon dioxide surfaces during parabolic flight campaigns, which create short bursts of reduced gravity. The surfaces were smoother than anything used in previous experiments, removing microscopic scratches and pits that could interfere with the results.
The findings, published in the journal npj Microgravity, showed something unexpected. Reduced gravity improved heat removal under certain conditions. The team believes this happens because bubbles stay stuck to heated surfaces longer in space, creating an ultra-thin liquid layer between the bubble and the surface that actually speeds up heat transfer.
But there is a catch. While boiling became more effective, the maximum heat a surface could safely handle before boiling became unstable dropped by about 65 percent.
"What we found is that up to a point, boiling actually got more effective, which was the opposite of what we expected," Song said. "The catch is that the safety limit drops sharply. Both halves matter if you're designing real hardware."
The research points toward potential solutions for the boil-off problem. If engineers can understand exactly how surfaces influence boiling, they could design storage tanks that keep rocket fuels stable for long missions.
"That means we can tune the surface features, like structures and chemistry, to suppress boiling," Song said. "We want to engineer surfaces to see if we can delay boiling in the storage tank. That is related to a zero boil-off storage tank."
The project brought together University of Florida graduate students Mohammad S. Reza, Philip Ignatoff, and Jimmy Almacddissi, along with NASA researcher Jason Hartwig from Glenn Research Center. Song and his team hope future spaceflight testing will build on these findings, bringing us one step closer to the fuels and systems needed for long-duration space exploration.