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3 Experimental Methods

1) + Dl, where Dl is the sound attenuation constant, |⃗q| = q and the sound velocity cs = 1

m(∂p/∂n)S (m denotes mass). Eq.(3.15) is a heuristic formula for the spectrum of light scattered by simple fluid and can be considered as a relation between Eq.(3.10), with the static and dynamic structure factor and the thermodynamic parameters of a fluid. The shape of the spectrum is shown in Fig.(3.16). The frequency dependence in Eq.(3.15) is that of a Lorentz function:

Another important expression for the evaluation of DLS data is given by the Einstein-

Figure 3.2: Spectrum of Scattered Light: Spectrum for the light scattered by thermal fluc- tuations in liquids according to Eq.(3.15).

Stokes relation [50], [51], which relates the diffusion coefficient D, the shear viscosity ηs and to the radius r of a particle:

Eq.(3.17) holds for simple fluids. In the case of multi-component critical systems the diffusion has to be considered within the framework of the mode-coupling theory. Mode- coupling theory yields a similar expression for the critical part of mutual diffusion coeffi- cient:

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