"‘Bulgyunil Diffusion’ 160-Year Puzzle Solved… KAIST Identifies Physical Cause"

The physical cause of the ‘uneven diffusion’ phenomenon, which had remained unsolved for over 160 years, has been identified in South Korea.

KAIST announced on the 2nd that a joint research team led by Professor Yongjeong Kim from the Department of Mathematical Sciences and Professor Myungchul Choi from the Department of Bio and Brain Engineering succeeded in presenting a new diffusion law explaining the fractionation phenomenon occurring in heterogeneous environments, along with its experimental verification.

(From left) Dr. Kim Ho-yeon (Department of Mathematical Sciences, currently a postdoctoral researcher at KAUST), Lee Geun-min (Ph.D. candidate, Department of Bio and Brain Engineering), Professor Choi Myung-chul (Department of Bio and Brain Engineering), Professor Kim Yong-jung (Department of Mathematical Sciences). Provided by KAIST

‘Diffusion’ refers to the macroscopic mass transfer phenomenon created by the random movement of microscopic particles. This concept arises not only in natural phenomena such as physics, chemistry, biology, and materials science, where randomness is the main cause, but also in most social phenomena including information, economics, and stock price fluctuations.

In 1905, Einstein explained diffusion by combining it with Brownian motion, describing it as the random walk of molecules. Since then, the theory of diffusion in homogeneous environments has been perfectly established.

However, Ludwig discovered in 1856 that in heterogeneous environments, substances do not mix by diffusion but rather undergo fractionation.

Since then, the scientific community has debated for over 160 years whether an additional advection phenomenon exists alongside diffusion to cause fractionation, or if it is solely due to the random movement of particles.

The joint research team proceeded under the assumption that ‘Einstein’s particle-based explanation’ could explain the fractionation phenomenon occurring in heterogeneous environments.

As a result, they mathematically demonstrated that when the random walk at the microscopic level is applied to a heterogeneous environment, the diffusion coefficient D separates into conductivity K and mobility M (D=KM), and fractionation occurs due to the mobility M. This clarified a process that was not physically intuitive through mathematical calculation and derivation.

The new diffusion law discovered by the joint research team is not composed of a single coefficient D like the existing diffusion law, but is determined by two coefficients, becoming a ‘two-component diffusion law.’

In particular, the joint research team emphasized that if the new diffusion law can perfectly explain the fractionation phenomenon, it proves that ‘no additional advection phenomenon exists, and fractionation occurs solely due to the random motion of particles.’

Due to the nature of diffusion, measuring data with the precision necessary to verify the fractionation phenomenon was a challenging aspect of the experiments conducted by the joint research team, and verifying this experimentally is considered a significant achievement of the study.

Professor Kim stated, "This study is an important discovery that proves particle fractionation is possible by diffusion alone in spatially heterogeneous environments," and added, "It is meaningful in that it accurately interprets phenomena that the existing diffusion law could not explain."

Professor Choi said, “The joint research team expects that these results will contribute to the development of new separation technologies in life sciences and materials science,” and “Furthermore, the diffusion law presented can be applied in various fields dealing with diffusion phenomena in heterogeneous environments.”

Meanwhile, the joint research team plans to conduct follow-up studies on fractionation phenomena caused by temperature heterogeneity and component heterogeneity within solids. The core goal is to demonstrate that various types of fractionation phenomena can be explained by the two-component diffusion law and to characterize their properties.