Thermal diffusivity of fluids by dynamic light scattering

The thermal diffusivity a is the transport property for which the development of dynamic light scattering (DLS) is probably most advanced and where measurements can be carried out routinely over a wide range of temperatures and pressures for the liquid phase and, due to the lower signal levels, in an extended vicinity of the critical point for the vapor phase. What makes the measurement of a particularly interesting is the fact that it is hardly possible by any other than light scattering techniques to measure this property directly, with a comparable accuracy, and in macroscopic thermodynamic equilibrium. Other methods access the thermal conductivity λ = a ρ cp, which is related to a by density ρ and isobaric heat capacity cp, and are subject to the use of macroscopic gradients. 2. Method When a fluid sample in macroscopic thermodynamic equilibrium is irradiated by coherent laser light, light scattered from the sample can be observed in all directions. The underlying scattering process is governed by microscopic fluctuations of temperature, pressure, and of species concentration in mixtures. The relaxations of these statistical fluctuations follow the same rules that are valid for the relaxation of macroscopic systems. Thus, the decay of temperature fluctuations is governed by a. In light scattering experiments the above-mentioned equalization processes result in a temporal modulation of the scattered light intensity. Information about these processes can be derived from a temporal analysis of the scattered light intensity using photon correlation spectroscopy (PCS). For a pure fluid under heterodyne conditions, where the scattering light is superimposed with stronger coherent reference light, the time-dependent intensity correlation function for the analysis of the temperature fluctuations is described by [1]