South Korean Team Turns Coating 'Defects' Into a 5.5x Heat Transfer Boost

Researchers at KAIST, a South Korean science and technology university, announced on August 23 that a joint team led by professors from its mechanical engineering and chemical and biomolecular engineering departments created a new coating technology that helps water droplets form more easily and detach more quickly during condensation. Faster droplet formation and removal can improve energy efficiency in power plants and desalination facilities, and enhance cooling for electronic devices.

Condensation, which occurs when water vapor turns into liquid, plays a role in converting steam back into water at power plants, producing fresh water from seawater, and carrying heat away from electronics. On ordinary metal surfaces, small droplets often merge into a continuous film of water that acts as a barrier to heat flow. A more efficient process, called dropwise condensation, happens when water stays in individual droplets that repeatedly form and detach, repeatedly exposing fresh surface area.

Previous surface designs faced a trade-off: rough surfaces give droplets more places to form but can trap them, while smoother surfaces let droplets detach more easily but provide fewer formation sites. The team addressed this by using nanoscale polymer aggregates, previously treated as unwanted defects, as nucleation sites for droplets. They produced the coating using a technique called initiated chemical vapor deposition (iCVD), which deposits gas-phase precursors to form an extremely thin polymer layer; making the film thinner produced roughly three times as many droplets, and a heat treatment then weakened the force holding droplets to the surface so they could detach before growing large.

Applied to copper tubes commonly used in condensers, the coating reached a maximum condensation heat transfer coefficient of approximately 88 kW per square meter per kelvin, about 5.5 times greater than a conventional copper surface covered by a water film, and more than 50% better than a conventional hydrophobic coating. "This research is meaningful because it uses nanostructures previously regarded as defects as features that help droplets form," said Professor Youngsuk Nam, who led the mechanical engineering side of the project.