KAIST (President Choongsik Bae) announced on August 23 that a joint team led by Professor Youngsuk Nam from the Department of Mechanical Engineering and Professor Sung Gap Im from the Department of Chemical and Biomolecular Engineering created the technology by carefully controlling the thickness and structure of an ultrathin polymer coating. The coating encourages more droplets to appear as water vapor condenses while also making it easier for those droplets to leave the surface.

Why Condensation Matters for Heat Transfer

Condensation occurs when water vapor changes into liquid water. A familiar example is the layer of droplets that appears on the outside of a cold drink. In industry, condensation plays an important role in converting steam back into water at power plants, producing fresh water from seawater, and carrying heat away from electronic equipment.

For these systems to work efficiently, condensed water must be removed from the surface quickly. On ordinary metal surfaces, small droplets often merge into a continuous film of water. That film acts as an additional barrier to heat flow, reducing heat transfer efficiency in much the same way that multiple layers of winter clothing slow the movement of heat away from the body.

A more efficient process occurs when water remains in individual droplets that repeatedly form and detach. This behavior is called dropwise condensation. Because the droplets leave instead of forming a continuous layer, fresh areas of the surface are repeatedly exposed, allowing heat to move through the surface more effectively.

The Trade-Off Between Droplet Formation and Removal

Previous surface designs have struggled with an important limitation. Rough surfaces provide more places for droplets to begin forming, but those same structures can trap the droplets and make them difficult to remove. Smoother surfaces allow droplets to slide or detach more easily, but they provide fewer sites where new droplets can form.

That creates a basic trade-off between nucleation, the initial formation of droplets, and droplet mobility.

The researchers addressed this problem by taking advantage of nanoscale polymer aggregates that had previously been treated as unwanted 'defects' in polymer coatings. They produced the coating using initiated chemical vapor deposition (iCVD), a technique that deposits gas-phase precursors onto a surface to form an extremely thin polymer layer.

When the researchers made the polymer film thinner, many small polymer aggregates appeared across the surface. Instead of removing these structures, the team used them as nucleation sites where water droplets could begin forming. Thin polymer films produced approximately three times as many droplets as thicker films.

Helping Droplets Form and Leave Faster

The team then introduced a heat treatment that weakened the force holding droplets to the coated surface. This allowed the droplets to detach more easily, often before they had time to grow very large.

The two adjustments addressed different parts of the condensation process. Reducing the polymer film thickness increased the number of locations where droplets could form, while thermal treatment made it easier for those droplets to leave the surface. By controlling these effects separately, the researchers were able to overcome the usual conflict between creating more droplets and removing them quickly.

Once a droplet leaves, another can form in the newly exposed space. The process is similar to a vacant seat being filled as soon as someone gets up. The more frequently droplets appear and depart, the more often the surface is refreshed, allowing heat to move through it more efficiently.

Heat Transfer Performance Increased Up to 5.5 Times

To test the technology under conditions closer to real-world applications, the researchers applied the polymer coating to copper tubes commonly used in condensers.

The maximum condensation heat transfer coefficient, which measures a surface's ability to transfer heat, reached approximately 88 kW·m^-2·K^-1. That represented heat transfer performance up to approximately 5.5 times greater than a conventional copper surface covered by a water film.

The new coating also delivered more than 50% better performance than a conventional hydrophobic coating surface.

Rather than relying only on smooth or water-repelling surfaces, the researchers deliberately made use of small surface 'defects.' Their results showed that nanoscale particles once viewed as imperfections to be eliminated could instead provide useful sites for droplet formation. The finding led the team to a new strategy for designing condensation surfaces.

Potential Uses in Energy, Water, and Electronics

If the coating can be adopted in power plants or industrial heat exchangers, it could improve energy efficiency by allowing heat to move more effectively. The technology could also improve water collection in desalination and water-harvesting devices, while faster heat removal could provide better cooling for electronic equipment.

Professor Nam said, "This research is meaningful because it uses nanostructures previously regarded as defects as features that help droplets form. We have presented a new method for improving heat transfer efficiency by separately controlling droplet formation and removal."

He added, "Because this technology can form extremely thin, uniform coatings even on surfaces with complex shapes, we expect it to be used in various energy and environmental applications, including industrial heat exchangers."

Jun Soo Kim, a researcher in the Department of Mechanical Engineering, and Minjeong Kang, a researcher in the Department of Chemical and Biomolecular Engineering, co-authored the study as first authors. The results were published online in the international journal Nature Communications on July 16.

This research was supported by the Mid-Career Researcher Program (Ministry of Science and ICT and the National Research Foundation of Korea), the SME Technology Innovation Development Program (Ministry of SMEs and Startups and the Korea Technology and Information Promotion Agency for SMEs), and the Deep-Tech Startup Activation Support Program (Ministry of Science and ICT and Commercialization Promotion Agency for R&D Outcomes, COMPA).