No plausible conclusions about the refractive-index variation can therefore be drawn in the phase-separation region. The refractive-index measurements were performed in parallel with the light-scattering measurements, so that the scattering rate and refractive index were determined at identical measurement temperatures. The refractive index n is required to determine the scattering wave vector q:

Here, λ is the wavelength of the incident light and θ is the scattering angle.

The objective of dynamic light scattering is to determine the diffusion coefficient D from the measured data and the relationships known from theory (see Section 3.2), and then to determine the correlation length ξ using the viscosity measured independently.

5.5.1 Scattering rates I

Figures 5.12 and 5.13 show how the scattering rates depend on the scattering angle. The measurements at 90° are relevant for evaluating the diffusion coefficient D. Two temperatures are shown as examples: one far from the critical point and one close to it, illustrating the pronounced angular sensitivity and dependence on scattering volume.

The curves differ not only in their scattering rates but also in their shapes. The approximately parabolic dependence near the critical point must be attributed to the critical behaviour of the solution. This phenomenon is described using Ornstein–Zernike theory (see [52]).

5.5.2 Diffusion coefficient D and correlation length ξ

The Rayleigh linewidth Γ_R and diffusion coefficient D are related by: