Clusters Stagnating During Condensation: Metastable Material or a Separate State of Matter?

Abstract

Condensation stagnation, that is, the existence of a delay between nucleation and growth during gas-phase particle formation, is a crucial phenomenon that affects associated processes and manifests itself as the presence of supercritical clusters in resulting particulates. In previous work, this unique phenomenon was experimentally demonstrated in a magnesium (Mg) particle flame, and an empirical explanation for the mechanisms responsible for the phenomenon was provided. The occurrence of stagnant clusters was revealed using specially designed sampling. The analysis of collected products, however, could not provide details about their evolution, so no information on the cluster's lifespan (i.e., stagnation time) was available. In the current manuscript, the results of spectroscopic studies of the Mg particle flame in air are reported. The recently developed advanced processing analysis of pyrometry data makes it possible to identify the light emission signature of stagnant clusters and, accordingly, estimate their lifespan, that is, the time during which clusters have physical properties (e.g., emissivity) different from those of mature nano-oxides. The obtained time on the order of 10 ms seems to be sufficiently long, which allows one to consider stagnant clusters as a separate state of matter, and not as a metastable metal oxide material.