Abstract

Heterogeneous condensation of supersaturated water vapor on submicron particles is an important phase transition process. Particle surface topography has a great influence on the whole condensation process. When water vapor condenses on a smooth SiO₂ particle, only one embryo drop occurs randomly on the particle surface of a certain edge. By contrast, on rough particles with irregular surfaces, the number of embryo drops is arbitrary. The word “rough” mentioned in this article refers to the change of macro roughness caused by the change of surface morphology, rather than micro-scale surface roughness. Furthermore, the location of the embryo drop is no longer at any random position, but at the protuberance of the particle’s surface. The concept of Gibbs free energy change is presented to interpret the formation of the embryo drop. Fractal dimension is then proposed as a parameter by which to characterize irregular structure of rough particles’ surface. The number of nucleation sites in the condensation process is greatly affected by the surface morphology of SiO₂ particles. The experimental results show that nucleation sites increase significantly with increasing surface fractal dimensions. The views of the apparent contact angle and the minimum nuclear radius are proposed to interpret nucleation site distributions. Moreover, the critical temperature of the cold stage required to activate the condensation process is shown to increase with increasing surface fractal dimension. Irregular structure of rough particles’ surface is helpful in reducing the value of the critical supersaturation.

Copyright © 2021 American Association for Aerosol Research

摘要

过饱和水蒸气在亚微米颗粒上的非均相冷凝是一个重要的相变过程。粒子表面形貌对整个凝聚过程有很大的影响。当水蒸气凝结在光滑的 SiO _{2 上时}颗粒，在某一边缘的颗粒表面上随机出现一个胚滴。相比之下，在表面不规则的粗糙颗粒上，胚滴的数量是任意的。本文中的“粗糙”一词是指由于表面形貌的变化而引起的宏观粗糙度的变化，而不是微观尺度的表面粗糙度。此外，胚胎滴的位置不再是任何随机位置，而是在粒子表面的突起处。提出吉布斯自由能变化的概念来解释胚胎滴的形成。然后提出分形维数作为表征粗糙颗粒表面不规则结构的参数。缩合过程中成核位点的数量受SiO ₂表面形貌的影响很大粒子。实验结果表明，随着表面分形维数的增加，成核位点显着增加。提出了表观接触角和最小核半径的视图来解释成核位点分布。此外，激活冷凝过程所需的冷阶段的临界温度显示出随着表面分形维数的增加而增加。粗糙颗粒表面的不规则结构有助于降低临界过饱和度值。

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