Abstract
Liquids are ubiquitous, but the atomic origin of viscosity remains unknown because of structural disorder and dynamic complexities. By using molecular dynamics simulation for liquid iron, we find a strong local correlation between the atomic-level pressure and the relaxation of atomic-level shear stress that is directly connected to viscosity. The results show that atomic sites under compression are more unstable against shear than those under tension, which causes fast relaxation in atomic-level shear stress. This result indicates spatial heterogeneities in the local viscosity in a liquid. We also find a temperature-independent relation between the local shear relaxation time and atomic-level volume strain, which suggests a universal structure-dynamics relationship in liquids. To explain our new findings, we discuss the relationships between the atomic-level pressure and various other parameters, such as the atomic-level shear stress, von Mises stress, local coordination number, and mean-squared displacements of atoms.
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