Engineers at the University of California San Diego are pushing the limit of how fast they can remove heat from a flat surface. In a recent study published in Nano Letters, researchers invented a nanoporous membrane that can effectively cool a record high heat flux of 1,850 watts per square centimeter—equivalent to the heat from 14,230 suns shining down on a fingernail. This heat flux off a flat surface is more than ten times higher than the previous record.
Applications for this discovery include more efficient cooling systems for high-powered electronics and lasers. For example, a high-powered laser can generate a heat flux of over 1,000 watts of power per square centimeter; if the heat is not properly managed, then the laser material can shatter like glass from thermal stresses.
To combat high heat fluxes, the new membrane has a thin film of water on its surface that quickly absorbs heat energy and boils away to cool the membrane. This film is continuously replenished by pushing water up through the membrane’s nanosized pores back onto the surface to let off heat as it boils away the water.
“It was rather by accident,” says Renkun Chen, professor of mechanical and aerospace engineering at the UC San Diego Jacobs School of Engineering, when describing how his team accomplished this impressive feat. They initially began by studying a well understood cooling phenomenon—evaporation.
Evaporation is what keeps marathon runners cool. Their liquid sweat absorbs their body heat and transforms into vapor, which rises and takes the heat away from the body. Engineers have been working on new ways to use evaporation to similarly cool down high-powered electronics faster and more efficiently.
Chen and first author Qingyang Wang, a mechanical engineering Ph.D. student at UC San Diego, started by designing porous membranes that could efficiently evaporate water. These membranes are made of thin sheets of aluminum oxide with tiny pores 200 nanometers in diameter (about 450 times thinner than human hair). Confining water to such small corridors allows more efficient evaporation, the same way a drop of water evaporates quicker than a spilled bottle of water.
The UC San Diego engineers were tweaking and testing the design of their nanoporous membrane when they found an anomaly in their data. They observed that the heat fluxes, or the rates of heat flow through the surface, were too high to be explained by efficient evaporation. It took two years to figure out that a new phenomenon was at work: “thin-film boiling,” a term that Chen’s team coined to describe the thin liquid layer that was relentlessly regenerating on top of their membrane surface before being boiled away.
“There was no such thing as thin film boiling,” said Chen. “People didn’t know it, we didn’t know it. We were so focused on making the best evaporation device that we couldn’t understand what was going on, but the data were very interesting. We were seeing heat fluxes much higher than previously reported.”
Microscopic bubbles were the hint that the researchers were measuring boiling instead of evaporation. Their experimental setup involved first heating the membrane and then pushing pressurized water up through nanosized pores and onto the surface, where the water would form a thin, bubbling layer. Heat energy is being transferred to this thin layer of water, causing it to boil away. The key to keeping the membrane cool was maintaining a thin layer of liquid at the surface by continuously pushing water through the pores.
The thin film of water is crucial for efficient boiling because it puts a hard cap on how large bubbles can grow. Making bubbles smaller increases the density of bubbles leaving the membrane, ultimately leading to more heat being whisked away.
Moving forward, Chen’s group is pushing this research in a number of exciting directions. First, they will continue to push their record heat flux even higher by optimizing the geometry and materials used in their design. Second, they are applying their new understanding of thin film boiling to cool devices more efficiently, at rates above 1,000 watts per square centimeter. And last, they are exploring the fundamental science that governs the transition between evaporation and boiling in other liquids such as ethanol, which is the subject of another study from Chen’s team.