Transition from hydrodynamic to viscoelastic propagation of sound in molten RbBr

Abstract

Inelastic neutron scattering was applied to measure the acoustic-type excitations in the molten alkali halide rubidium bromide. For molten RbBr neutron scattering is mainly sensitive to the number density fluctuation spectrum and is not influenced by charge fluctuations. Utilizing a dedicated Brillouin scattering spectrometer, we focused on the small-wave-vector range. From inelastic excitations in the spectra a dispersion relation was obtained, which shows a large positive dispersion effect. This frequency enhancement is related to a viscoelastic response of the liquid at high frequencies. Towards small wave vectors we identify the transition to hydrodynamic behavior. This observation is supported by a transition of the sound velocity from a viscoelastic enhanced value to the adiabatic speed of sound for the acoustic-type excitations. Furthermore, the spectrum transforms into a line shape compatible with a prediction from hydrodynamics.

The hydrogen-bond collective dynamics in liquid methanol

“…In conclusion, we here observed that methanol displays in the whole Q range a viscoelastic behaviour characterised by propagating sound modes which are increasingly damped with growing Q , until they become overdamped in a rather narrow range around the position of the main peak in the static structure factor Q p , where the sound propagation is arrested. This behaviour, already known to be a common property of a large variety of liquids45464955 described by exactly the same S VE ( Q, ω ), is also shared by methanol. In fact, if the acoustic mode dispersion ω s ( Q ) of methanol is compared to that of its non-hydrogen-bonded analog, i.e.…”

Sodium ion self-diffusion in molten NaBr probed over different length scales

“…That model delivers a dynamic scattering function for the collective and for the self-dynamics of a liquid. This so-called viscoelastic model describes the collective dynamics reasonably well, which was recently demonstrated in a data analysis of the collective dynamics of molten RbBr [21]. Also the incoherent model was successfully applied in the description of self-dynamics in monatomic liquids [6].…”

“…The most exciting prediction concerning the dynamics of binary ionic liquids was the existence of optic-type modes, which have been demonstrated in a pioneering MD simulation of a symmetric molten salt by Hansen and McDonald [18]. Due to these predictions in the following years significant experimental effort was put into the study of the collective particle dynamics of molten salts with neutrons [19][20][21] and with inelastic x-ray scattering [22][23][24][25][26][27].…”

Modelling of structure and dynamics of molten NaF using first principles molecular dynamics

“…Calculated QENS observables compare well with experiments. Thus the current calculation can be a benchmark to get suitable force fields for these ionic melts, which might then be compared to measurements of the vibrational properties [43,44].…”