Citroni Margherita, Fanetti Samuele, Falsini Naomi, Foggi Paolo, Bini Roberto
LENS-European Laboratory for Non-Linear Spectroscopy, 50019 Sesto, Florence, Italy;
Dipartimento di Chimica Ugo Schiff, University of Florence, 50019 Sesto, Florence, Italy.
Proc Natl Acad Sci U S A. 2017 Jun 6;114(23):5935-5940. doi: 10.1073/pnas.1620039114. Epub 2017 May 23.
How does a crystal melt? How long does it take for melt nuclei to grow? The melting mechanisms have been addressed by several theoretical and experimental works, covering a subnanosecond time window with sample sizes of tens of nanometers and thus suitable to determine the onset of the process but unable to unveil the following dynamics. On the other hand, macroscopic observations of phase transitions, with millisecond or longer time resolution, account for processes occurring at surfaces and time limited by thermal contact with the environment. Here, we fill the gap between these two extremes, investigating the melting of ice in the entire mesoscopic regime. A bulk ice I or ice VI sample is homogeneously heated by a picosecond infrared pulse, which delivers all of the energy necessary for complete melting. The evolution of melt/ice interfaces thereafter is monitored by Mie scattering with nanosecond resolution, for all of the time needed for the sample to reequilibrate. The growth of the liquid domains, over distances of micrometers, takes hundreds of nanoseconds, a time orders of magnitude larger than expected from simple H-bond dynamics.
晶体是如何熔化的？熔核生长需要多长时间？一些理论和实验工作已经探讨了熔化机制，这些工作覆盖了亚纳秒时间窗口，样品尺寸为几十纳米，因此适合确定该过程的起始，但无法揭示后续的动力学。另一方面，具有毫秒或更长时间分辨率的相变宏观观测，考虑了发生在表面的过程以及受与环境热接触限制的时间。在这里，我们填补了这两个极端之间的空白，研究了整个介观尺度下冰的熔化。一个块状的冰I或冰VI样品通过皮秒红外脉冲均匀加热，该脉冲提供了完全熔化所需的所有能量。此后，通过具有纳秒分辨率的米氏散射监测熔体/冰界面的演化，监测时间为样品重新平衡所需的全部时间。在微米尺度上，液域的生长需要数百纳秒，这一时间比简单氢键动力学预期的要大几个数量级。
