Saito Shinji
Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan and The Graduate University for Advanced Studies (SOKENDAI), Myodaiji, Okazaki, Aichi 444-8585, Japan.
J Chem Phys. 2024 May 21;160(19). doi: 10.1063/5.0209713.
When a liquid is rapidly cooled below its melting point without inducing crystallization, its dynamics slow down significantly without noticeable structural changes. Elucidating the origin of this slowdown has been a long-standing challenge. Here, we report a theoretical investigation into the mechanism of the dynamic slowdown in supercooled water, a ubiquitous yet extraordinary substance characterized by various anomalous properties arising from local density fluctuations. Using molecular dynamics simulations, we found that the jump dynamics, which are elementary structural change processes, deviate from Poisson statistics with decreasing temperature. This deviation is attributed to slow variables competing with the jump motions, i.e., dynamic disorder. The present analysis of the dynamic disorder showed that the primary slow variable is the displacement of the fourth nearest oxygen atom of a jumping molecule, which occurs in an environment created by the fluctuations of molecules outside the first hydration shell. As the temperature decreases, the jump dynamics become slow and intermittent. These intermittent dynamics are attributed to the prolonged trapping of jumping molecules within extended and stable low-density domains. As the temperature continues to decrease, the number of slow variables increases due to the increased cooperative motions. Consequently, the jump dynamics proceed in a higher-dimensional space consisting of multiple slow variables, becoming slower and more intermittent. It is then conceivable that with further decreasing temperature, the slowing and intermittency of the jump dynamics intensify, eventually culminating in a glass transition.
当一种液体在不引发结晶的情况下迅速冷却至其熔点以下时，其动力学显著减慢，而结构没有明显变化。阐明这种减慢的起源一直是一个长期的挑战。在此，我们报告了一项关于过冷水动态减慢机制的理论研究，过冷水是一种普遍存在但非同寻常的物质，其具有因局部密度波动而产生的各种异常性质。通过分子动力学模拟，我们发现跳跃动力学（即基本的结构变化过程）随着温度降低而偏离泊松统计。这种偏离归因于与跳跃运动相互竞争的慢变量，即动态无序。对动态无序的当前分析表明，主要的慢变量是跳跃分子第四近邻氧原子的位移，这种位移发生在由第一水化层之外分子的波动所创造的环境中。随着温度降低，跳跃动力学变得缓慢且间歇。这些间歇动力学归因于跳跃分子在扩展且稳定的低密度区域内被长时间捕获。随着温度持续降低，由于协同运动增加，慢变量的数量增多。因此，跳跃动力学在由多个慢变量组成的高维空间中进行，变得更慢且更间歇。可以想象，随着温度进一步降低，跳跃动力学的减慢和间歇性加剧，最终导致玻璃化转变。
