Results.

Although low angle light scattering techniques are very suitable to study long range correlated fluids, their sensitivity is hampered by the divergence of stry light at small wave vectors. The data presented in [22,23] were collected by using the shadowgraph projection technique. Shadowgraphy has traditionally been used to obtain a qualitative mapping of inhomogeneities in the index of refraction. However, very recently the technique has been reintroduced as a powerful quantitative tool to assess the features of long wavelength fluctuations in fluids [15,16].

The main problem of shadowgraph is the oscillatory behaviour of its transfer function: see Eq. (3.37). The scattering intensities around the zeroes of the transfer function cannot be measured: for example, in Fig. 3 of [22], the values of $S\left(q\right)$ are missing for $q\approx 2\cdot 10^4\mathrm{m}^{-1}$. Moreove, the region in which the zeroes are too frequent cannot be considered in the data analysis. The overall wavevector range covered about one decade.

We measured the scatterig intensities at different times after the beginning of the diffusion process. The power spectra measured with SNFS are shown in Fig. 9.3. They show the $q^{-4}$ divergence and the saturation for small wavevectors. The roll off wavevector is about $10^{4}\mathrm{m}^{-1}$, and is compatible with the value given by Eq. (9.2)

Figure 9.3: Measured power spectrum of non equilibrium fluctuations in the free diffusion process of glycerol in water.

The data cover about two decades in wavevectors: about ten times the range covered with shadowgraph. The quality must be compared with data shown in Fig. 2 of [13], obtained with SALS on a similar, but quite peculiar system: the wavevector range covered by SNFS is slightly more wide. Moreover, it should be noticed that SALS gives no reliable results for the present system, that is for the nonequilibrium fluctuations in the free diffusion of simple fluids, due to stray light, since the scattering is weak and the wavelength associated to the process is quite long. The results we present are the best obtained for such a system up to now.

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