Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States.
J Am Chem Soc. 2021 Feb 10;143(5):2272-2284. doi: 10.1021/jacs.0c10663. Epub 2021 Jan 28.
Contact freezing is a mode of atmospheric ice nucleation in which a collision between a dry ice nucleating particle (INP) and a water droplet results in considerably faster heterogeneous nucleation. The molecular mechanism of such an enhancement is, however, still a mystery. While earlier studies had attributed it to collision-induced transient perturbations, recent experiments point to the pivotal role of nanoscale proximity of the INP and the free interface. By simulating the heterogeneous nucleation of ice within INP-supported nanofilms of two model water-like tetrahedral liquids, we demonstrate that such nanoscale proximity is sufficient for inducing rate increases commensurate with those observed in contact freezing experiments, but only if the free interface has a tendency to enhance homogeneous nucleation. Water is suspected of possessing this latter property, known as surface freezing propensity. Our findings therefore establish a connection between the surface freezing propensity and kinetic enhancement during contact nucleation. We also observe that faster nucleation proceeds through a mechanism markedly distinct from classical heterogeneous nucleation, involving the formation of hourglass-shaped crystalline nuclei that conceive at either interface and that have a lower free energy of formation due to the nanoscale proximity of the interfaces and the modulation of the free interfacial structure by the INP. In addition to providing valuable insights into the physics of contact nucleation, our findings can assist in improving the accuracy of heterogeneous nucleation rate measurements in experiments and in advancing our understanding of ice nucleation on nonuniform surfaces such as organic, polymeric, and biological materials.
接触冻结是大气冰核形成的一种模式，其中干冰成核粒子 (INP) 与水滴之间的碰撞导致异质成核速度显著加快。然而，这种增强的分子机制仍然是一个谜。虽然早期的研究将其归因于碰撞诱导的瞬态扰动，但最近的实验指出 INP 和自由界面的纳米级接近度起着关键作用。通过模拟 INP 支持的两种模型类四面体液体的纳米薄膜内的冰异质成核，我们证明这种纳米级接近足以诱导与接触冻结实验中观察到的速率增加相当的速率增加，但前提是自由界面具有增强均相成核的趋势。水被怀疑具有后一种性质，称为表面冻结倾向。因此，我们的研究结果在接触成核过程中的表面冻结倾向和动力学增强之间建立了联系。我们还观察到，更快的成核通过一种与经典异质成核明显不同的机制进行，涉及到沙漏形晶核的形成，这些晶核在任一侧界面形成，并且由于界面的纳米级接近度和 INP 对自由界面结构的调制，形成晶核的自由能更低。除了为接触成核的物理性质提供有价值的见解外，我们的研究结果还可以帮助提高实验中异质成核速率测量的准确性，并加深我们对非均匀表面（如有机、聚合物和生物材料）上冰核形成的理解。
