CRITICAL-BEHAVIOR IN THE PRESENCE OF A DISORDERED ENVIRONMENT

We report details of experimental studies of the effect of dilute silica networks on critical phenomena for two binary fluid mixtures: lutidine-water (LW) and isobutyric acid-water (IBAW). For both mixtures, the primary effect of the silica is to induce a time-independent perturbation of the mixtures' concentration, making it spatially nonuniform. We interpret this as the static response of the critical system to the spatial nonuniformities of the silica concentration. We have measured this response by light scattering and find it is both temperature and concentration dependent, becoming strongly so in the vicinity of the consolute points of the mixtures. We observed no critical fluctuations for the LW-gel system. For the IBAW-gel system, time-dependent scattering was observed, and the temporal autocorrelation function of the scattered intensity revealed three regimes. Well away from the consolute point, the decay was exponential. By taking the effect of the time-independent response of the mixture to the silica gel into account, we found that the decay rates were comparable to those of the pure system. Correlation functions measured closer to the consolute point contained a significant nonexponential component for sufficiently large values of the scattering wave vector. This component is well fitted by either a stretched exponential or an activated form. From the amplitude of the normalized autocorrelation function we deduce that the critical fluctuations are suppressed in amplitude relative to those of the pure system as the consolute point is approached. Sufficiently near the phase boundary of the pure system, a very slowly decaying mode was also observed in the autocorrelation function. This mode died away hours after the small temperature change that induced it. A further temperature change toward the two-phase region induced it agian, while a change in the opposite direction did not. We interpret this behavior as resulting from a phase separation process. IBAW-gel samples held deep in the two-phase region of the pure system for months ordered macroscopically, proving that this system has long-range order in the presence of such silica networks. © 1995 The American Physical Society.