| Scattering methods as small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) are very well suited to study fluctuations of density and concentration in condensed matter. Polymer networks swollen by a suitable solvent provide a good example for such systems. If the contrast between the network and the solvent is high enough, minute deviations of the equilibrium density and composition can be studied with utmost precision. Thus, SANS and SAXS have been used to study critical phenomena in swollen networks. Aqueous solutions of polyelectrolytes provide another example. Here careful studies can reveal the difference between the meanfield distribution of the counterions and their fluctuations. In our contribution we shall highlight recent work on this subject at the Helmholtz-Zentrum Berlin. In particular, we discuss work done on colloidal microgels done by a combination of SANS and SAXS. A second example related to polyelectrolyte will be given as well. Moreover, work in progress on studies of fluctuation-induced forces by SAXS will be discussed. |
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