Abstract: Heterogeneous condensation of water vapor on particle surfaces is a highly valuable industrial pretreatment technique for dust removal. Heterogeneous condensation of water vapor on particle surfaces can significantly improve the efficiency of dust removal. To study the dynamic behavior of droplets on the surfaces of particle during the process of heterogeneous condensation, the environmental scanning electron microscope (ESEM) is employed for visual observation of the heterogeneous condensation process of water vapor on particle surfaces. The evolution process of droplet mode is studied, and the contribution of line tension to the Gibbs free energy of the system during the condensation process and the influence of particle size and contact angle on line tension are analyzed. The results indicate that particles first nucleate to form a critical embryo droplet. This embryo droplet continues to grow and rapidly spreads to the equator of the particle. The droplet then remains on the equator, undergoing continuous condensation and growth, gradually transitioning to crossing the equator. With increasing condensation time, the droplet gradually envelops the particle. Once the particle is completely enveloped, the heterogeneous condensation transforms into homogeneous condensation and continuous growth. The entire condensation process can be divided into five modes: nucleation, diffusion, transition, encapsulation, and growth. The line tension of the five modes is analyzed, and it is found that when the droplet is in the upper hemisphere of the particle, the closer it is to the equatorial line, the greater the contribution of the line tension to the Gibbs free energy change of the system, which promotes its movement towards the equatorial line, and when it is in the lower hemisphere, it promotes its encapsulation of the particle. The effects of particle size and contact angle on the line tension are numerically studied, and it is found that the line tension increased with the increase of particle size. The influence of the intrinsic contact angle of the particle on the line tension is small, but the bigger the difference between the particle and the apparent contact angle, the greater the line tension.

Key words: fine particle, heterogeneous condensation, modality, environmental scanning electron microscope, line tension

摘要： 水汽在颗粒表面异质凝结作为一种极具应用价值的工业除尘预处理技术，可有效提高除尘效率。为研究水汽异质凝结过程中液滴在颗粒表面的动态行为，本工作基于环境扫描电子显微镜(ESEM)可视化观察了水汽在颗粒表面的异质凝结过程，研究了液滴模态的演变过程，分析了线张力在凝结过程中对系统吉布斯自由能的贡献以及颗粒粒径、接触角对线张力的影响。结果表明，颗粒首先会成核形成一个晶胚液滴，该晶胚液滴继续生长并快速扩散至颗粒赤道线上。然后液滴停留在赤道线上不断凝结增长，逐渐过渡到越过赤道线。随着凝结时间的增加，液滴会包裹颗粒，当颗粒被完全包裹后，由异质凝结转变为均质凝结并持续生长。整个凝结过程可以划分为五个模态：成核、扩散、过渡、包裹、生长。对五个模态的线张力进行分析，发现当液滴处于颗粒上半球时，越靠近赤道线，线张力对系统吉布斯自由能变化量的贡献越大，促进其向赤道线运动，而处于下半球时则会促进其对颗粒实现包裹。数值研究了颗粒粒径和接触角的变化对线张力的影响，发现随着颗粒粒径增大，线张力也增大。颗粒本征接触角对线张力的影响很小，但与表观接触角之间的差值越大，线张力越大。

关键词: 细颗粒, 异质凝结, 模态, 环境扫描电子显微镜, 线张力