Dong-A University Professor Heonsang Lee's Team Solves 130-Year-Old Mystery of “Interfaces in Miscible Fluids”

Derivation of the "Co-Evolving Free-Energy Variational Principle"

Enables Quantitative Prediction of Interfacial Lifetimes

A new physical theory has been proposed by researchers at Dong-A University to explain the phenomenon in which the interface between two fully miscible fluids persists for a certain period after contact, even though the fluids are expected to mix completely.

Dong-A University (President Lee Hae-Woo) announced on the 28th that the research findings of Professor Heonsang Lee's team in the Department of Chemical Engineering were published on the 23rd in 'Communications Physics,' an international journal in the field of physics under the Nature Portfolio.

'Communications Physics' is an international journal covering all areas of pure and applied physics, and its impact factor stands at 5.8, ranking it in the top 25% (Q1) of its field.

Professor Heonsang Lee's team derived a 'co-evolving free-energy variational principle,' a theoretical framework that explains the formation, persistence, and collapse of the interface between fully miscible fluids after contact. Based on this theory, the research team also succeeded in quantitatively predicting the lifetime of the interface.

The title of the paper is 'The universal law behind long-lived interfaces in fully miscible fluids.'

The phenomenon of contact between fully miscible fluids appears across a wide range of natural and engineered systems, including the interior of living cells, the human body, industrial chemical processes, and marine environments. Similar interfacial problems also occur in material systems such as semiconductors and electrodes, where ions and electrons are distributed collectively, similar to fluids.

However, the specific physical mechanism by which fluids maintain an interface for some time after contact, rather than mixing immediately, has remained unresolved for over 130 years since J.D. van der Waals presented his interfacial theory in 1893.

In this study, the researchers approached the problem using the concepts of dual pairing between 'free energy landscapes' and 'interfacial concentration fields,' as well as 'self-consistent co-evolution.'

Whereas previous theories described the phenomenon sequentially, either as predetermined landscapes dictating fluid flow or as fluid flow altering the landscape and these changes influencing the flow again, the research team’s co-evolutionary theory approaches it from the perspective that the landscape and fluid flow form a single state and evolve simultaneously.

With this co-evolutionary structure as a basis, the researchers provided a unified theoretical framework that connects the interface's formation, persistence, thickening process, and eventual collapse. They also demonstrated that the lifetime of the interface can be quantitatively predicted even between fluids with different chemical properties.

Dong-A University explained that the significance of this research lies in going beyond applying existing theories to specific phenomena, by presenting a mathematical and physical framework that integrates various physical phenomena under a single principle. In particular, the university highlighted that this basic-theoretical research proposes a 'variational first principle' that explains how nature chooses and changes states based on a single fundamental criterion.

Professor Heonsang Lee stated, "This research takes the design and control principles of soft matter and precision chemical processes to a new level, and it provides a foundation for eventual optimization and industrial application. For nearly 30 years, I have adhered to the principle of publishing work that takes a step forward each year, rather than simply increasing the number of quantitative research results. As a result, we have now achieved the meaningful outcome of proposing a general physical law, in the form of a variational first principle, that can serve as an engineering foundation—beyond models limited to specific phenomena."

He added, "I hope that Dr. Kyungmin Choi and Dr. Ashar Husein, who have demonstrated exceptional ability as joint researchers, will continue to develop as outstanding early-career scientists. While artificial intelligence is useful in finding solutions to problems posed by humans, creativity in redefining existing conceptual frameworks and formulating new problems is still the core role of human researchers."

Dr. Kyungmin Choi, who participated in this research, is currently continuing his work at the Korea Institute of Science and Technology (KIST), while Dr. Ashar Husein is working as a postdoctoral researcher in the BK21 Program at Dong-A University.

Professor Lee heads the BK21 Program at the Department of Chemical Engineering at Dong-A University, leading research and talent development centered on precision chemical materials and precision soft matter for physical AI. Covering both theory and experiments in the fields of soft matter and advanced materials, he has published his findings in international journals such as 'Nature Communications' and 'Science Advances.' More recently, his research scope has expanded to the establishment of universal physical principles governing nonequilibrium fluids and interfaces.

(From left) Heonsang Lee, Professor of Chemical Engineering at Dong-A University; Kyungmin Choi, PhD; Hussain Ashar, PhD.

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