Water, Water, Everywhere, but How Does It Flow?

Scientists use new X-ray technique to see how water moves at the molecular level.

Image courtesy of Takeshi Egami, Oak Ridge National Laboratory and the University of Tennessee

The Science

Water is the most abundant substance on earth, but we do not fully understand it. How do water molecules interact? How does bonding between water molecules influence its viscosity (resistance to flow)? For the first time, scientists have used X-ray scattering to observe the real-space, real-time motions of water molecules. The data show that water molecules move in a correlated manner with respect to one another. The scientists used the correlations between water molecules to determine its viscosity.

The Impact

It’s now possible to know how a liquid will flow, or not, based on the molecules involved. This novel X-ray scattering approach can connect molecular-level dynamics and bonding directly to the macroscopic properties of liquids. Scientists and engineers can use the technique to improve liquid-based electronics, such as batteries. Also, they can use the technique to improve the properties of materials used as lubricants.

Summary

Viscosity is one of the most fundamental physical properties of a liquid. Yet, its atomic and molecular origins are not well understood. Recently, researchers have used new instrumentation developed for inelastic X-ray scattering studies of materials to determine the molecule-molecule correlation of water molecules, in real space and real time. The researchers related these interactions to water’s viscosity. Scientists have used inelastic X-ray and neutron scattering techniques to observe the behavior of materials at atomistic length and time scales. The scattering data are typically presented in momentum and energy space. Then, the data are brought to real space and real time in the form of the Van Hove function (see figure). In this study, researchers looked directly at inelastic X-ray scattering data of water in real space and real time and measured the time-dependent molecular motion. Using the Van Hove function, they determined the time required to change the neighbor of a water molecule (which is directly related to its viscosity) was less than a picosecond. The team found that the time scale was directly related to the Maxwell relaxation time and viscosity, which was predicted by an earlier simulation. This demonstration shows that it is possible to determine rheological properties of various liquids from direct measurements of the molecular dynamics.

Contact

Takeshi Egami
Oak Ridge National Laboratory and the University of Tennessee
egami@utk.edu

Funding

Department of Energy, Office of Science, Office of Basic Energy Sciences and Japan Synchrotron Radiation Research Institute (synchrotron radiation beamline)

Publications

T. Iwashita, B. Wu, W.R. Chen, S. Tsutsui, A.Q.R. Baron, and T. Egami, “Seeing real-space dynamics of liquid water through inelastic X-ray scattering.” Science Advances 3, e1603079 (2017). [DOI: 10.1126/sciadv.1603079]