The samples.

The samples we used are monodisperse colloids made of polystyrene spheres suspended in water. In order to avoid sedimentation, we used a mixture of water and heavy water with a weigh fraction of about 0.5. The diameters of the two colloids we used are $5.2\mathrm{\mu m} \pm 0.5\mathrm{\mu m}$ and $10.0\mathrm{\mu m} \pm 0.3\mathrm{\mu m}$, whose polydispersity is negligible. The diameters are quite large, since NFS gives advantages with respect to classical LS for small wavevectors.

The colloids we measured are held in a cell with plane parallel windows. The diameter is about $4\mathrm{cm}$, since the sample and the beam intensity must be uniform on a length $D$, where $D$ is given by Eq. (3.64). The thickness is about $2\mathrm{mm}$. We selected the tickness and the particle concentration in order to have a suitable attenuation of the main beam, about 1%. For ONFS measurements, the thickness of the cell and the volumetric particle density are enough to fulfill Eq. (3.55).

The liquid is held between the two windows by an O-ring; the parallelism between the windows is not critical, nor the optical quality of them. Since the measured scattered light comes from different regions of the sample, we must provide that it is homogeneous. This implies that the thickness must be uniform, but an optical quality allignment is far beyond what is needed.

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