Microscopic visualization of heterogeneous nucleation of water vapor on convex and concave particles

Heterogeneous nucleation of water vapor on fine particles affects droplet formation in key processes such as atmospheric physics, gas purification, crystallization, and particle measurement. Understanding and predicting the preferential nucleation sites on microscale particles, especially those with complex geometries such as convex and concave surfaces, remains a major challenge. In this work, the nucleation process on convex spherical particles is first visualized. Particle gap, i.e. particle concavity, will preferentially nucleate. A planar gap model is constructed to explain the reason why convex particles are more prone to nucleate at the gap compared to the particle surface. The influences of the gap angle and the contact angle on nucleation are analyzed. The smaller the gap angle, the smaller the contact angle, and the lower the nucleation energy barrier, making nucleation more likely to occur. Compared to using fractal theory to only obtain the nucleation energy barriers, this model can be used to predict the preferential nucleation sites of micrometer sized convex spherical particles. Importantly, to address the issue of whether all concavity will preferentially nucleate, the nucleation on micron-sized concave spherical particles is then visualized. And the nucleation energy barriers of concave cavities and particle surfaces with and without considering line tension are analyzed. It is found that when the particle radius and cavity radius are large, their energy barriers are almost the same. Water vapor is more likely to nucleate simultaneously inside the cavity and on the particle surface. When the particle radius and cavity radius are small, considering the line tension, the energy barrier inside the cavity is greater than that on the particle. Contrary to what is believed, water vapor is more likely to nucleate on the particle rather than in the cavity.