Yang Zhang
Department of Nuclear, Plasma, and Radiological Engineering, Department of Materials Science and Engineering, Department of Electrical and Computer Engineering, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign
Long wavelength longitudinal phonons can propagate in liquids, but whether transverse phonons exist in liquids has been debated since the 1970s. Recent experimental and computational studies of the collective modes in liquids suggested that transverse acoustic excitations not only exist in liquids but also contribute to the density fluctuations characterized by the dynamic structure factor. However, the classical hydrodynamic theory, which is only valid in long wavelengths and low frequencies, fails to explain these observations. Herein, we extend the hydrodynamic theory by incorporating viscoelasticity and anisotropy, as a result of breaking continuous symmetry at short distances. Consequently, transverse acoustic excitations emerge from the viscoelasticity of liquids, and deviation from isotropic symmetry causes a coupling mechanism between the longitudinal and transverse modes, which altogether contribute to the density fluctuations. This approach not only provides an inverse method to examine the density and current correlation functions in liquids beyond the hydrodynamic regime but also serves as a generalized hydrodynamic theory for viscoelastic materials. Furthermore, we found that the Ioffe-Regel delocalization point of these phonon modes coincides with the onset of super-Arrhenius transport and dynamic heterogeneity, and the breakdown of Stokes-Einstein relation. Therefore, we interpreted phonon delocalization as the microscopic driving force of the strongly-correlated behavior of supercooled liquids.