Yahya Ahmad, Tan Luoxi, Perticaroli Stefania, Mamontov Eugene, Pajerowski Daniel, Neuefeind Joerg, Ehlers Georg, Nickels Jonathan D
Department of Chemical and Environmental Engineering, University of Cincinnati, 2901 Woodside Drive. PO Box 210012, Cincinnati, OH 45221, USA.
Shull Wollan Center-a Joint Institute for Neutron Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Phys Chem Chem Phys. 2020 May 6;22(17):9494-9502. doi: 10.1039/d0cp01560a.
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.
水分子的快速平衡波动与流变学响应密切相关；分子运动重置局部结构和应力，表现为流动和体积变化。一般而言，对于水或氢键液体，由于强方向性相互作用使理论模型复杂化，且需要清晰观察相关平衡运动的时间尺度和性质，所以这种关系是一个需要深入考虑的问题。最近的研究表明，短程亚皮秒运动的时间尺度与液态水中剪切粘度响应的隐含时间尺度相吻合。在此，利用中子散射和光散射方法通过实验来说明体粘度的时间尺度，并对相关的分子弛豫进行描述。布里渊散射已被用于确定体粘度的时间尺度；借鉴麦克斯韦方法，体粘度ζ与体积模量K的比值得出弛豫时间τB，在280 K至303 K的温度范围内，τB约为1 - 2皮秒。随后，利用非弹性中子散射在分子尺度上描述水和重水的运动，提供相干和非相干散射数据。在水的非相干散射光谱中，水分子质子在1 - 2皮秒时间尺度上的旋转（也可描述为局域化）运动明显可见，而来自D2O在第一个尖锐衍射峰长度尺度上的相干光谱描述了水的微观密度涨落，证实了水结构在1 - 2皮秒的可比时间尺度上的弛豫。这三个时间尺度的吻合提供了体粘性响应的机理描述，局部结构因1 - 2皮秒量级的旋转/局域化运动而重置，其速度比与剪切粘度相关的弛豫慢约三倍。通过这种方式我们表明，剪切粘性响应与水网络连通性的变化最为密切相关，而体粘性响应与局部密度涨落相关。
