Water freezes below 0 degrees Celsius, which is called a phase transition. The phase transition of water freezing into ice is an extremely common phenomenon in nature. It not only subtly affects the climate, geology and life on earth but also plays a vital role in the chemical industry, low-temperature biology, materials science and other fields. Physicists have always been focusing on this topic, but the microscopic mechanism of water freezing into ice has not determined for a century.

Gibbs, one of the founders of statistical physics, proposed the "classical nucleation theory" of phase transitions a hundred years ago. He believed that the phase transition of water freezing into ice entails undergoing a nucleation process: small ice nuclei are formed when the temperature of the water is too low, but these small ice nuclei are unstable, and their size fluctuates with time. A long time later, after the size of an ice core accidentally increases beyond a critical value, the ice core will grow spontaneously and rapidly, and the phase transition of water freezing into ice occurs. The formation of critical-size ice nuclei (critical nuclei) is crucial during this process. Because the critical nucleus has a small size (several nanometers), a short lifetime (less than nanoseconds), and an rare event occurs occasionally, there has been no direct evidence experimentally to confirm its existence and measure its characteristics.

Nature journal is the world's earliest international sci-tech journal and one of the most authoritative scientific journals in the world, with a high reputation recognized in the academic field. Since its inception in 1869, the journal's mission is to report major discoveries and breakthroughs in the scientific world, and it requires innovativeness in scientific research results. Professor Zhou Xin is the co-corresponding author of the paper. He was elected to the "Hundred Talents Program" of the UCAS in 2012, and was appointed as a professor with a joint appointment at the Wenzhou Institute of UCAS in May this year. He was formerly the assistant researcher of Los Alamos National Laboratory in the United States; head of independent research group of Korea Asia-Pacific Theoretical Physics Center; adjunct professor of Physics Department in Pohang University of Science and Technology. He published nearly 70 research papers in top international journals such as Nature.

"Empirically obtain the micro-mechanism of water freezing—supercooled water first forms a nano-sized critical ice core, and then the ice core rapidly grows into macro ice." Zhou Xin said that they leveraged nanoparticles (graphene oxide nanosheets) as probes to capture the critical nucleation signal. By studying the relationship between the size of the nanoparticles and their ability to promote ice nucleation, it is found that only when the size of nanoparticles is larger than a special value can they effectively promote ice nucleation, while smaller nanoparticles can hardly help ice nucleation. The actual experimental results obtained are highly consistent with theoretical expectations and theoretical calculations. It is verified that in the phase transition of water freezing into ice, the core concept of classical nucleation theory lies in the effective nanoscale, rather than the kinds and materials of nanoparticles being used.

Zhou Xin stated that this research could provide essential theoretical guidance for the application of artificial ice control. He specifically pointed out that by regulating the formation and growth of ice crystals, the recovery efficiency of frozen preserved cell tissues could be improved, so could the freshness of refrigerated foods.

Since its inception, the Wenzhou Institute of UCAS has fully embarked on building high-end talent teams and reached cooperation intentions with a number of academicians. It has established 16 scientific research teams and introduced more than 10 post-doctors and more than 30 outstanding doctoral masters from Harvard University and Cambridge University; Next year, it is anticipated to possess a scientific research team of not less than 200 people.

Wenzhou institute is working hard to develop key technology research and development projects in the field of life and health, and construct a full-chain achievement transformation model that integrates incubation, acceleration, pilot test, and industrialization. At present, 5 representative achievements have entered the stage of marketization and have introduced to 6 key projects of UCAS, and a myriad of cooperation agreements have been reached with related enterprises in Wenzhou.

In September next year, after the new building in the high-tech zone of the Wenzhou National Self-Contained District is completed and put into use, the Wenzhou Institute exert all of its effort in building four innovation centers such as medical biomaterials and medical intelligent equipment and bolster the joint development of research and development platforms and related institutions with the UCAS Research center.