Molecular origins of bulk viscosity in liquid water
Molecular origins of bulk viscosity in liquid water
复制标题
液态水中体积粘度的分子起源
DOI：
10.1039/d0cp01560a
复制
发表时间：
2020
影响因子：
3.3
通讯作者：
Nickels, Jonathan D.
中科院分区：
文献类型：
--
作者：
Yahya, Ahmad;Tan, Luoxi;Perticaroli, Stefania;Mamontov, Eugene;Pajerowski, Daniel;Neuefeind, Joerg;Ehlers, Georg;Nickels, Jonathan D.
水分子的快速平衡波动与流变响应密切相关;分子运动重置局部结构和应力，视为流量和体积变化。在水或氢键液体的情况下，一般来说，由于强的方向性相互作用使理论模型复杂化，并且需要清楚地观察相关平衡运动的时间尺度和性质，因此这种关系是一个重要的考虑因素。最近的工作已经说明了短距离亚皮秒运动的时间尺度和液态水中剪切粘度响应的隐含时间尺度的一致性。在这里，中子和光散射方法被用来实验说明体积粘度的时间尺度，并提供相关的分子弛豫的描述。布里渊散射已被用于建立体粘度的时间尺度;借用麦克斯韦方法，体粘度ε与体模量K的比率产生弛豫时间τB，其在280 K至303 K的温度范围内出现在1-2 ps的量级。非弹性中子散射随后被用来描述水和重水在分子尺度上的运动，提供相干和非相干散射数据。在水的非相干散射光谱中，水质子在1-2 ps时间尺度上的旋转（或者描述为局部化）运动是明显的，而在第一尖锐衍射峰的长度尺度上来自D2 O的相干光谱描述了水的微观密度波动，证实了水结构在1-2 ps的可比时间尺度上的弛豫。这三个时间尺度的重合提供了本体粘性响应的机械描述，其中局部结构由于1-2 ps量级的旋转/局部运动而重置，比与剪切粘度相关的弛豫慢大约三倍。在这种方式中，我们表明，剪切粘性响应是最密切相关的水网络连接的变化，而体积粘性响应与局部密度波动。
The rapid equilibrium fluctuations of water molecules are intimately connected to the rheological response; molecular motions resetting the local structure and stresses seen as flow and volume changes. In the case of water or hydrogen bonding liquids generally, the relationship is a non-trivial consideration due to strong directional interactions complicating theoretical models and necessitating clear observation of the timescale and nautre of the associated equilibrium motions. Recent work has illustrated a coincidence of timescales for short range sub-picosecond motions and the implied timescale for the shear viscosity response in liquid water. Here, neutron and light scattering methods are used to experimentally illustrate the timescale of bulk viscosity and provide a description of the associated molecular relaxation. Brillouin scattering has been used to establish the timescale of bulk viscosity; and borrowing the Maxwell approach, the ratio of the bulk viscosity, ζ, to the bulk modulus, K, yields a relaxation time, τB, which emerges on the order of 1–2 ps in the 280 K to 303 K temperature range. Inelastic neutron scattering is subsequently used to describe the motions of water and heavy water at the molecular scale, providing both coherent and incoherent scattering data. A rotational (alternatively described as localized) motion of water protons on the 1–2 ps timescale is apparent in the incoherent scattering spectra of water, while the coherent spectra from D2O on the length scale of the first sharp diffraction peak, describing the microscopic density fluctuations of water, confirms the relaxation of water structure at a comparable timescale of 1–2 ps. The coincidence of these three timescales provides a mechanistic description of the bulk viscous response, with the local structure resetting due to rotational/localized motions on the order of 1–2 ps, approximately three times slower than the relaxations associated with shear viscosity. In this way we show that the shear viscous response is most closely associated with changes in water network connectivity, while the bulk viscous response is associated with local density fluctuations.