Development of Uniform Quantum Dot Coating Technology for Coffee Ring-Free Displays

KAIST Professor Hyungsoo Kim's Team "Application of Flexible Display Devices"

Coffee ring stains. Image courtesy of KAIST

[Asia Economy Reporter Kim Bong-su] The Korea Advanced Institute of Science and Technology (KAIST) announced on the 3rd that Professor Kim Hyungsoo's research team from the Department of Mechanical Engineering has developed a uniform coating technology that prevents coffee ring stains. This technology can uniformly coat quantum dots for displays and can be applied to flexible display devices.

When a drop of coffee dries on a solid surface, coffee ring stains remain due to the relative evaporation rate differences on the droplet surface. This phenomenon is called the coffee ring effect. The evaporation of droplets is directly related to the challenge of uniformly coating functional flexible materials in technologies such as inkjet printing. Recently, inkjet printing technology has been utilized beyond simple pattern printing to convergent production systems for next-generation energy and display-related electrical and electronic devices.

Until now, in the scientific community, various methods using surfactants or inducing partial surface tension changes to utilize the Marangoni effect have been introduced to control the coffee ring pattern of droplets and achieve uniform drying marks. The Marangoni effect refers to the phenomenon that occurs when the surface tension magnitude is not uniform along the interface between different liquids. The well-known "tears of wine" phenomenon is a representative example.

Professor Kim Hyungsoo has been researching effective methods to control the coffee ring effect since his postdoctoral research at Princeton University. In 2016, he elucidated the unique drying phenomenon of whiskey and conducted groundbreaking research to eliminate the coffee ring effect. However, while the coffee ring effect at the droplet contact line can be reduced, the effect still persists.

In response, researcher Pyeon Jeongsoo from Professor Kim's team developed a method to completely eliminate the coffee ring by temporarily sealing the space where the droplet evaporates. This technology effectively mixes two liquids with different evaporation rates and utilizes the fact that the molar molecular weight of the more volatile liquid evaporating first is greater than that of air. The volatile vapor trapped in the sealed space continuously induces solutal Marangoni effects, completely eliminating the coffee ring.

Professor Kim Hyungsoo stated, "By thoroughly understanding evaporating substances and optimizing the evaporation system using mass transfer mechanisms, it is possible to achieve uniform patterning of functional devices such as quantum dots for display materials and perovskite for solar panel materials through inkjet printing technology capable of mass production." He added, "The technology has been patented, and research is underway to apply it to flexible display devices."

The research results were invited to the special issue "2021 Soft Matter Emerging Investigator Special Issue" organized by the internationally prestigious academic society, the Royal Society of Chemistry, and were published as the cover paper in the June 7th issue of the renowned journal Soft Matter.

This content was produced with the assistance of AI translation services.