Scientists have developed an ultrathin coating that can make copper surfaces transfer heat during condensation up to 5.5 times more effectively than conventional copper.
The technology could eventually improve the efficiency of power plants, desalination systems, industrial heat exchangers and electronic cooling.
The research, led by scientists at the Korea Advanced Institute of Science and Technology (KAIST), focuses on a simple but important process: condensation. The findings were published in Nature Communications.
Condensation happens when water vapor cools and turns into liquid water, such as the droplets that appear on the outside of a cold drink. The same process plays an important role in industry, including converting steam back into water in power plants and removing heat from equipment.
One problem is that condensed water can remain on a metal surface. Small droplets eventually merge into a continuous film, which acts as a barrier that slows heat transfer. It works somewhat like layers of clothing that reduce the movement of heat away from the body.
A more efficient alternative is “dropwise condensation.” Instead of forming a continuous film, water collects into separate droplets that quickly leave the surface. New droplets can then form in the newly exposed areas, allowing heat to move more efficiently.
Creating the ideal surface for this process has been difficult. Rough surfaces provide many locations where droplets can begin forming, but they can also trap those droplets. Smooth surfaces allow water to move away more easily but offer fewer places for droplets to form.
The KAIST researchers found a way to address both problems separately.
They created an extremely thin polymer coating using a process called initiated chemical vapor deposition. When the coating was made thinner, tiny polymer clusters appeared across its surface.
Normally, these nanoscale structures might be considered imperfections or defects. But the researchers discovered that they were useful because they provided sites where water droplets could start forming.
About three times as many droplets formed on the thin polymer films compared with thicker versions.
The scientists then used heat treatment to reduce the force holding the droplets to the surface. This allowed droplets to detach while they were still relatively small.
The result was a surface that could encourage many droplets to form while also allowing them to leave quickly. As soon as one droplet departed, the exposed area became available for another droplet to form, creating a continuous cycle that improved heat transfer.
To test the technology under more realistic conditions, the researchers applied the coating to copper tubes similar to those used in condensers.
The coated tubes achieved a maximum condensation heat-transfer coefficient of about 88 kilowatts per square meter per degree Kelvin. That was up to 5.5 times the performance of conventional copper covered by a water film and more than 50% better than a conventional water-repelling coating.
The coating could have applications wherever condensation and heat transfer are important. More efficient condensers could reduce energy use in power plants and industrial systems, while the technology might also improve desalination and atmospheric water collection. Better heat removal could also help cool increasingly powerful electronic devices.
The researchers say the coating can be made extremely thin and uniform, even on objects with complicated shapes.
That could make the approach particularly useful for real-world heat exchangers, where efficiently moving heat—and quickly getting water out of the way—can make a major difference.
