Guillaud Emmanuel, Merabia Samy, de Ligny Dominique, Joly Laurent
Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, LYON, France.
Institute of Glass and Ceramics, Department of Material Science, University of Erlangen-Nürnberg, Martensstr. 5, 91058 Erlangen, Germany.
Phys Chem Chem Phys. 2017 Jan 18;19(3):2124-2130. doi: 10.1039/c6cp07863j.
During the last few decades, many experimental and numerical studies have tried to understand the special dynamics of water at low temperatures by measuring structural relaxation times or shear viscosity, but their conclusions strongly depended on the chosen observable and on the range of temperatures considered. Moreover, recent work [J. Chem. Phys. 2013, 138, 12A526] showed that viscosity and relaxation times could decouple at low temperature in a model binary mixture, raising questions on their equivalence to study supercooled water. Here we used molecular dynamics simulations with the promising TIP4P/2005f water force field to investigate the behavior of both the shear viscosity and the relaxation times of water in a large range of temperatures, in order to get a consistent picture of the dynamics of supercooled water. We show that the TIP4P/2005f model reproduces accurately the experimental values of both the viscosity and the diffusion coefficient over a very large range of temperatures. Focusing first on the structural relaxation dynamics, we observe a decoupling between the so-called α- and β-relaxation times of water at ca. 350 K, suggesting a supercooled-like dynamics over a very large domain of temperatures. By computing shear viscosity over this domain, we compare the accuracy of several phenomenological laws for low temperature dynamics of water to describe both viscosity and α-relaxation time. Unlike what is usually admitted, our tests suggest those quantities are not coupled at low temperatures, and thus should not be considered equivalent. In particular, deviations from the Stokes-Einstein relation appear at lower temperatures for the viscosity than for the α-relaxation time. These results open new perspectives to understand the dynamics of supercooled water and show the performance of the TIP4P/2005f force field to characterize it.
在过去几十年中，许多实验和数值研究试图通过测量结构弛豫时间或剪切粘度来理解低温下水的特殊动力学，但它们的结论在很大程度上取决于所选择的可观测值以及所考虑的温度范围。此外，最近的工作[《化学物理杂志》2013年，第138卷，12A526]表明，在一个模型二元混合物中，粘度和弛豫时间在低温下可能会解耦，这引发了关于它们在研究过冷水时是否等效的问题。在这里，我们使用具有前景的TIP4P/2005f水势场进行分子动力学模拟，以研究在大范围温度下水的剪切粘度和弛豫时间的行为，以便获得过冷水动力学的一致图像。我们表明，TIP4P/2005f模型在非常大的温度范围内准确再现了粘度和扩散系数的实验值。首先关注结构弛豫动力学，我们观察到在约350 K时水的所谓α弛豫时间和β弛豫时间之间出现解耦，这表明在非常大的温度范围内存在类似过冷水的动力学。通过计算该温度范围内的剪切粘度，我们比较了几种描述水低温动力学的唯象定律在描述粘度和α弛豫时间方面的准确性。与通常的认识不同，我们的测试表明这些量在低温下并不耦合，因此不应被视为等效。特别是，粘度偏离斯托克斯 - 爱因斯坦关系出现在比α弛豫时间更低的温度下。这些结果为理解过冷水的动力学开辟了新的视角，并展示了TIP4P/2005f势场在表征过冷水动力学方面的性能。
