Here D_T is the thermal diffusion coefficient; the remaining definitions of D_S, the magnitude q of the scattering vector and the sound velocity v_S are given in the original expression. A closer examination of Eq. (3.61) reveals a structure closely resembling a Lorentzian curve with centre frequency ω₀:

This analogy is illustrated in Fig. 3.3. Three Lorentzian curves can clearly be distinguished. The central, principal curve at ω₀ is called the Rayleigh line. In this region the magnitudes of the momenta involved in scattering remain unchanged. Its half-width is given by:

Here Λ_T is the thermal conductivity. Thermodynamically, this means that density fluctuations caused by entropy fluctuations propagate through the liquid under investigation. They are described by the diffusion equation. This connection is particularly important for the present study of a two-component system. Concentration fluctuations Δc, which increase near the critical point, can be investigated through the half-width. Another important relation was established by Stokes and Einstein:

This establishes a direct connection between the diffusion coefficient and shear viscosity, temperature and the radius r of suspended particles in the liquid. Einstein idealized these particles as spheres of radius r, allowing the relationship to be applied to a binary solution.

7. Fick’s second law. 8. Albert Einstein (1879–1955), physicist.

Original mathematical typography retained. .