Transverse dynamics in fluids: how a collective excitation sets in

The transverse-current autocorrelation function CT(Q, t), obtained by molecular dynamics simulation of a dense Lennard-Jones fluid, has been analyzed in the wavevector range between the hydrodynamic region and Qp/2 (where Qp is the position of the main peak in the static structure factor). The exponential expansion theory (EET) provides a full and accurate account of CT(Q, t), and reveals a varied dynamical behaviour. We identify and accurately locate the Q value at which a shear wave begins to propagate, and show how this phenomenon is described as the continuous transition of a harmonic oscillator from the overdamped to the underdamped state. Moreover, this analysis reveals the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher Q values, a similarly smooth over- to underdamped transition [1]. [1] E. Guarini et al., Phys. Rev. E (2023), in press