*Review* **Research on Mesoscale Nucleation and Growth Processes in Solution Crystallization: A Review**

**Xiaowei Wang 1,2,†, Kangli Li 1,3,†, Xueyou Qin 1,2, Mingxuan Li 1,2, Yanbo Liu 1,2, Yanlong An 4, Wulong Yang 4, Mingyang Chen 1,2,\*, Jinbo Ouyang 5,\* and Junbo Gong 1,2**


**Abstract:** In recent studies, the existence of mesoscale precursors has been confirmed in crystallization. Different from the classical crystallization theory, which only considers the sequential attachment of basic monomers (atoms, ions, or molecules), the nonclassical crystallization process involving precursors such as prenucleation clusters, nanoparticles, and mesocrystals is more complicated. The mesoscale structure is important for the quantitative description and directional regulation of the solution crystallization process. It is necessary to explore the mechanism by the mesoscale scientific research methods on the base of traditional chemical engineering and process system engineering research methods. Therefore, the paper reviews several representative nonclassical nucleation and growth theories, mainly including two-step nucleation theory, prenucleation clusters theory, particle agglomeration theory, amorphous precursor growth theory, particle attachment growth theory and mesocrystal growth theory. Then, the mesoscale structure and its spatiotemporal dynamic behavior are discussed, and the application of the EMMS model in the nucleation and growth process is analyzed. Finally, we put forward our views on the prospect of the paradigms and theoretical innovations of using mesoscale methods in crystal nucleation and growth.

**Keywords:** nucleation; growth; mesoscale; precursor; EMMS model

https://doi.org/10.3390/cryst12091234 Academic Editor: Heike Lorenz

**Citation:** Wang, X.; Li, K.; Qin, X.; Li, M.; Liu, Y.; An, Y.; Yang, W.; Chen, M.; Ouyang, J.; Gong, J. Research on Mesoscale Nucleation and Growth Processes in Solution Crystallization: A Review. *Crystals* **2022**, *12*, 1234.

Received: 23 July 2022 Accepted: 29 August 2022 Published: 1 September 2022

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