Structural origin of the strong effect of attraction on bulk viscosity in simple liquids

The effects of pairwise attraction and three-body interactions (Axilrod-Teller-Muto triple-dipole potential) on bulk viscosity, compared with shear viscosity, were investigated using molecular dynamics simulations, with particular focus on the underlying structural dynamics. Our previous study [H. Kobayashi et al., J. Chem. Phys. 162, 164507 (2025)] showed that bulk viscosity of simple liquids is highly sensitive to intermolecular potentials. Weakening the pair attraction reduces the bulk viscosity, whereas three-body interactions also decrease it by effectively weakening the net attraction through their long-range repulsive character. In this study, we investigated the mechanism by which attraction affects the bulk viscosity. Analysis of the time correlation function of pressure fluctuations, Cζ(t), showed that it is well described by a Gaussian plus double-exponential model. The results revealed that attraction enhances the slow relaxation component represented by the double-exponential modes, which is the primary factor governing the magnitude of the bulk viscosity. Stress-structure coupling analysis further revealed that this slow relaxation mode originates from the strong coupling between pressure fluctuations and two-body density fluctuations in the low-q region. This represents the coupling between pressure fluctuations and long-range density fluctuations. Moreover, attraction enhances both the strength and the relaxation time of this coupling. The observed changes in the intermediate scattering function F(q, t) indicate that this enhancement originates from structural changes caused by attraction. In contrast, shear stress does not couple with two-body density fluctuations in this region. As a result, the low-q structural changes induced by attraction have little influence on shear viscosity.