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Recently, Wang Jianjun's group from the Institute of Chemistry of the Chinese Academy of Sciences collaborated with Professor Zhou Xin from the University of Chinese Academy of Sciences to creatively detect the size-matching signals of ice nucleation and nanoparticles by using fixed-size nanoparticles to detect tiny transient critical nuclei Existence and characteristics. By studying the relationship between the size of nanoparticles and their ability to promote the nucleation of ice crystals, researchers have found that the nucleation of ice can be effectively promoted only when the size of the nanoparticles is larger than a certain threshold value, while nanoparticles of smaller sizes can hardly help Ice nuclei are formed. Moreover, the size threshold phenomenon of nanoparticle size in promoting ice nucleation ability is common, and it is simply inversely proportional to the supercooling temperature, and it hardly depends on the type and structure of the nanoparticle. The experimental results are completely consistent with the predictions of the critical nucleus and free energy calculations of classical nucleation theory. For the first time, the existence of critical ice nucleus and its dependence on the size and subcooling temperature have been experimentally confirmed.

Phase changes such as water icing at low temperatures are very common and are a basic problem in scientific research. At the same time, they have important application value in the fields of chemical industry, low temperature biology, and materials science. Gibbs and others proposed the "classical nucleation theory" of phase transition based on the principle of thermodynamics nearly a hundred years ago, and believed that phase transitions such as water freezing need to undergo a nucleation process. For example, when water is too cold to form small ice nuclei, the phase transition can occur spontaneously only when the ice nuclei formed occasionally exceed a critical size, that is, a critical nuclei are formed.

In recent decades, with the development of microscopic detection technology, many inferences of classical nucleation theory have been confirmed, but some results beyond their expectations have also been found. In particular, the existence of the core concept of critical nucleation, the critical nucleus, has not been able to give direct experimental evidence, thus hindering further understanding of the important physical phenomenon of phase transformation into nucleus in nature, and even causing macro-inference The practicality of the concept of the critical kernel at the micro level is also questionable. Specific to the icing phase transition of water, due to the special complexity of water molecules and its close relationship with our lives, people have been looking forward to detecting critical ice cores for a long time, but after many efforts, they cannot give clear results. A fundamental unresolved issue in the field.

This research has effectively clarified the general doubts about the effectiveness of the "classical nucleation theory" in describing the critical nuclear characteristics at the atomic scale in recent decades, and deepened the micro-mechanism of the important phase transition phenomenon of water icing. understanding. This work is of great significance to the mechanism of water icing, the phase transition phenomenon, and the macro and micro relationships in statistical physics. At the same time, this strategy of detecting critical ice nuclei can also be used for critical nuclear detection of other phase transition nuclear processes, which may improve the understanding of the entire phase transition nuclear field. In addition, the results of this study indicate that designing a patterned surface that is equivalent to the critical ice core size can achieve the purpose of efficiently controlling the formation of ice crystals, and provides new ideas for the design of anti-icing coatings.

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