Understanding vapor nucleation on the molecular level: A review

Gas phase nucleation is a universal phenomenon that plays a crucial role in both natural settings and industrial processes. Despite more than 100 years’ work to decipher this highly dynamic process, our understanding of nucleation remains incomplete. In particular, the formation, decay, and structures of the initially formed nucleating clusters critically determine the overall behavior of the nucleation systems, yet they are elusive to conventional experimental methods that only detect grown particles. Filling this gap of understanding calls for the ability to probe nucleation on a molecule-by-molecule basis to retrieve direct evidence of nucleation pathways and the temporal evolution of the nucleating clusters. In this review, we summarize recent modelling and experimental efforts to meet this demand. The techniques discussed here have offered unprecedented molecular level insight into the nucleation process, but they are still subject to constraints in terms of applicable scope and measurement uncertainties. We shall highlight both the advantages and limitations of each technique, with the hope that this review can lead to novel applications and further improvements of the introduced methods.