NEUTRON-SCATTERING STUDIES OF BINARY-MIXTURES IN SILICA-GELS

We report the results of neutron-scattering experiments designed to probe structure in binary mixtures confined in silica gels over the wavelength range 60-2000 . In what would be the one-phase region of the pure system, the scattering can be fit very well to the sum of three contributions: critical fluctuations of the mixture, preferentially adsorbed fluid, and the silica gel itself. By interpreting the adsorption as the response of the order parameter to an imposed field due to the gel, we find that the q dependence of the response is consistent with linear response theory on all length scales accessible in these experiments, although the amplitude of the response does not diverge as strongly as does the order-parameter susceptibility. In what would be the two-phase region of the pure system, we observe changes in the scattering that are consistent with the existence of large, slowly coarsening domains. The results are discussed in the context of the random-field Ising model and its magnetic realizations. © 1995 The American Physical Society.