Michelangelo had to battle paint pouring onto his face while painting The Creation of Adam on the Sistine Chapel ceiling. He described the work as closer to torture than painting. KAIST researchers have now developed a new technology that can hold up that “falling paint.”

KAIST said on Wednesday that a research team led by Professor Hyeong-su Kim (김형수) of the Department of Mechanical Engineering reinterpreted, through interfacial fluid dynamics, “gravitational instability,” the root cause of liquid films that pour downward under gravity, and presented a way to control it using a small amount of volatile liquid.

When paint is applied to a ceiling, a thin liquid film forms but becomes increasingly unstable over time and eventually falls. This is easy to see in everyday life. When water vapour condenses on a bathhouse ceiling, a thin layer of water forms and, over time, clumps into droplets and falls. Water droplets that form on the inside ceiling of a refrigerator also start as a thin layer but gradually grow and then pour downward.

Such collapse of liquid on an upper surface due to gravity is called Rayleigh-Taylor instability. It has been regarded as an unavoidable natural phenomenon as long as gravity exists.

The research team proposed mixing a small amount of volatile liquid into an upside-down hanging liquid film. As the volatile component evaporates, the concentration distribution on the liquid surface changes, creating differences in surface tension. Surface tension is the force that pulls a liquid surface inward on itself, keeping droplets round.

When surface tension differs, the side with higher tension pulls on the side with lower tension, generating flow along the surface. This is called the Marangoni effect. The team found through experiments and theory that this surface flow holds up liquid that would otherwise fall, suppressing gravity-driven instability.

For example, if pepper powder is sprinkled evenly over water and a drop of detergent is placed in the middle, the pepper is instantly pushed to the edges. That is because surface tension where the detergent touches becomes weaker than the surrounding area, and the stronger outer tension pulls the liquid. This creates flow along the surface, carrying the pepper particles with it.

In this study, evaporation of the volatile liquid created the same kind of surface tension difference. But instead of pushing pepper away, it worked by pulling the liquid upward and suppressing the force that would make it fall.

Experiments showed that under certain conditions the liquid film remained stable despite gravity. Under some conditions, the team also observed new behaviour in which droplets did not fall and the liquid film oscillated periodically. The results show that gravitational instability can be actively controlled without external energy input, using only natural processes of liquid composition and evaporation.

The technology is expected to enable thinner and more uniform liquid films in precision coating, electronic circuit printing and additive manufacturing processes. It could also allow stable coating on inclined surfaces and be extended to fluid control technology for 3D printing processes and space environments.

Professor Kim said, “This study is meaningful in that it showed gravitational instability can be actively controlled without external energy by using natural processes of liquid composition and evaporation.” He said, “This principle could be extended not only to coating, printing and additive processes but also to fluid control technology in space environments.”

The study listed Min-woo Choi (최민우), a combined master’s and doctoral student in mechanical engineering, as first author. It was published online in the international journal Advanced Science on Jan. 29 and was also selected as a frontispiece paper. The research was supported by the National Research Foundation of Korea’s individual basic mid-career programme and the KAIST UP programme.