Transition from hydrodynamic to viscoelastic propagation of sound in molten RbBr.
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.
ISIS Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom.
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Demmel
Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg, Germany.
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Szubrin
Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg, Germany.
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Pilgrim
CNR-IOM c/o OGG Institut Laue-Langevin, 38042 Grenoble, France.
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De Francesco
CNR-IOM c/o OGG Institut Laue-Langevin, 38042 Grenoble, France.
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Formisano