Viscosity and elementary excitations in molecular liquids

A liquid can change its shape easily and flow under stress. The flow behaviors are often characterized by the viscosity of the liquid, but the microscopic origins remain elusive. Previous studies on simple liquid metals showed that at high temperatures an elementary excitation, defined by a creation or annihilation of atomic bonds, determines the Maxwell relaxation time, that is, the viscosity divided by high frequency shear modulus. In this study, we report an extension of this concept to more complex molecular liquids, in which molecules consists of several atoms, and show similarities and differences between simple liquids and molecular liquids, including liquid silica, water, and silicon. We found that the appropriate choice of high frequency shear modulus is necesarry in estimating the Maxwell relaxation time. In addition, we discuss the role of shear modulus on structural relaxation through complex shear moduli.