Ice nucleation is a crucial process in nature and industries; however, the role of the free surface of water in this process remains unclear. To address this, we investigate the microscopic freezing process using brute-force molecular dynamics simulations. We discover that the free surface assists ice nucleation through an unexpected mechanism. The surface-induced negative pressure enhances the formation of local structures with a ring topology characteristic of Ice 0-like symmetry, promoting ice nucleation despite the symmetry differing from ordinary ice crystals. Unlike substrate-induced nucleation via water-solid interactions that occurs directly on the surface, this negative-pressure-induced mechanism promotes ice nucleation slightly inward the surface. Our findings provide a molecular-level understanding of the mechanism and pathway behind free-surface-induced ice formation, resolving the longstanding debate. The implications of our discoveries are of substantial importance in areas such as cloud formation, food technology, and other fields where ice nucleation plays a pivotal role.

冰成核是自然界和工业中的一个关键过程；然而，水的自由表面在此过程中的作用仍不清楚。为了解决这个问题，我们使用强力分子动力学模拟研究微观冷冻过程。我们发现自由表面通过一种意想不到的机制协助冰成核。表面引起的负压增强了具有类似冰0对称性的环形拓扑特征的局部结构的形成，尽管其对称性与普通冰晶不同，但仍促进了冰成核。与直接发生在表面上的水-固相互作用引起的基底诱导成核不同，这种负压诱导机制会促进冰在表面稍微向内成核。我们的研究结果提供了对自由表面诱导冰形成背后的机制和途径的分子水平理解，解决了长期存在的争论。我们的发现对于云形成、食品技术和其他冰核发挥关键作用的领域具有重要意义。