Transverse Waves in Complex Systems
In glass forming liquids formed of anisotropic molecules or polymers , which possess segments with anisotropic units, the striking feature is the observation of propagating transverse waves in the fluid phase provided the frequency of the probe is high enough, usually some GHz. We have investigated the continuous transformation of these transverse modes from a diffusive to a propagative behaviour upon cooling, or increasing pressure, using depolarised Rayleigh-Brilloin spectroscopy. Spectra were recorded with a Tandem Fabry- Perot interferometer. The results are theoretically understood considering a set of coupled equations of motions for the mass density and the mean local orientation of the molecules taking into account a rotation-translation coupling. One result of the theory is the determination of the zero shear viscosity from the inelastic light scattering spectra performed at temperatures and pressures at which propagative modes are present. We have measured spectra for poly(methylphenylsiloxane) PMPS as a function of temperature and pressure and can calculate the pressure dependence of the viscosity, which is difficult to obtain otherwise.
-8 -6 -4 -2 0 2 4 6 8
0,01 0,1
Frequency / GHz
Counts /sweep
17.6 °C 24.3 °C 11.9 °C 8.1 °C 4.3 °C
The figure shows the change of the transversal mode in PMPS with temperature at a pressure of 600bar. The spectra were obtained with the Tandem Fabry-Perot interferometer at a scattering angle of θ=90° and a finess of 130.
H. Kriegs, G. Meier, J. Gapinski, A. Patkowski, J.Chem.Phys. 128, 014507(2008)
