我们对水的经验认知是：水在0℃会结冰，但在某些条件下，水实际上可以在更低的温度范围内维持液体状态。对科学家来说，这个低温的极限是零下38℃，低于这个温度水一定会结冰。而最近发表的一项研究中，研究人员将这个极限推进到了零下44℃。
Researcher pushes limit of when water will freeze
Though it is one of the great mysteries of science, the transformation of water into ice often escapes people's minds as it is just assumed that's what happens. But how and why it happens is the subject of intense scrutiny by ice scientists like Hadi Ghasemi, Cullen Associate Professor of Mechanical Engineering at the University of Houston. In order to watch the process of crystallization of water into ice at the molecular level, Ghasemi is reporting the best look yet at the process: water-ice phase transformation down to 2 nm (nanometers) in diameter.
水会结冰似乎是一件理所当然的事情，但其实水结冰的过程如何发生、为什么会发生，仍是科学界的一大谜团，也是休斯敦大学机械工程学副教授哈迪·卡西米（Hadi Ghasemi）等研究“冰”的科学家密切关注的问题。为了研究在分子水平上水结晶成冰的过程，卡西米在直径2纳米的尺度下对水到冰的相变过程进行了观察。
Then when Ghasemi examined these tiny particles, he made another discovery. He could break the limit of when water freezes and maintain the tiny droplets as liquid by putting them in contact with soft interfaces, like gels or lipids.
卡西米在研究这些小水滴时有了一个新发现：让这些小水滴与诸如凝胶或脂类的柔性界面接触，就可以打破水结冰的温度限制，使它们在极低的温度下仍保持液态。
"We found that if a water droplet is in contact with a soft interface, freezing temperature could be significantly lower than hard surfaces. Also, a few-nanometer water droplet could avoid freezing down to -44 C if it is in contact with a soft interface," Ghasemi reports in Nature Communications.
这项研究近日发表于《自然·通讯》（Nature Communications）杂志。卡西米说：“我们发现，水滴在接触柔性界面时，比接触硬性界面时的冰点更低。一些纳米尺度的水滴在接触柔性界面时，可以在低达-44℃的温度下保持不结冰。”
The limit of freezing temperature of a water droplet is -38 C. That is, any water droplet will freeze at some temperature between 0 C to -38 C. Below this temperature, freezing has been inevitable, until now.
此前，水的冰点的极限是-38℃，也就是说，无论在任何情形下，水滴都会在0℃~-38℃之间的某个温度下结冰，而低于-38℃时水滴必然会结冰。现在，这个纪录被打破了。
The process of freezing such a tiny water droplet plays a critical role in the survival of animals in cold environments as a frozen water droplet inside a cell leads to the rupture of the cell and death. The process also plays a key role in climate prediction, cloud conditions, cryopreservation of organs and technologies exposed to icing conditions such as aircraft and wind turbines.
微小水滴的结冰过程对动物在寒冷环境下的生存有重要的影响，因为细胞内的水滴凝固会导致细胞破裂和死亡。此外，微小水滴的结冰过程，对气候预测、云层状况、低温冻存器官以及发展飞机和风力涡轮机等面临结冰挑战的技术，都发挥着关键作用。
"Experimental probing of freezing temperature of few nanometer water droplets has been an unresolved challenge. Here, through newly developed metrologies, we have been able to probe freezing of water droplets from micron scale down to 2 nm scale," said Ghasemi.
卡西米说：“通过实验探测几纳米尺度水滴的冰点，一直是个悬而未决的难题。在我们的最新研究中，通过新开发的测量技术，我们能够研究的结冰过程，已经从微米尺度缩小到2纳米的尺度。
Previously Ghasemi created an ice-repelling material for aerospace applications using a new concept called stress localization. His current findings contribute to a greater understanding of natural phenomena and provide guidelines for further design of anti-icing systems for aviation, wind energy and infrastructures and even cryopreservation systems.
此前，卡西米利用应力局部化（stress localization）的新概念发明了一种应用于航空航天领域的抗冰材料。而他这次的发现有助于加深我们对水结冰这种自然现象的理解，可以为设计航空设备、风能开发的基础设施以及低温贮存系统的抗冰系统提供参考。