A Model for the Two-Phase Behavior of Fluids in Dilute Porous Media

Experiments show that the coexistence region of a vapor-liquid system or binary liquid mixture is dramatically narrowed when the fluid is confined in a dilute porous medium such as a silica aerogel. We propose a simple model of the gel as a periodic array of cylindrical strands, and study the phase behavior of an Ising system confined in this geometry. Our results suggest that the coexistence region should widen out at lower temperatures, and that the narrowness observed near the critical point may be a fluctuation-induced effect

Translational and Rotational Diffusion Constants of Tobacco Mosaic Virus from Rayleigh Linewidths

The translational and rotational diffusion constants of tobacco mosaic virus (TMV) have been determined from homodyne and heterodyne measurements of the spectrum of laser light scattered from dilute aqueous solutions of TMV. Our results for the translational and rotational constants respectively, reduced to 20°C, are: D(T) = 0.280 ± 0.006 × 10(-7) cm(2)/sec, and D(R) = 320 ± 18 sec(-1). We include a theoretical derivation of the spectrum of light scattered from rod-shaped molecules which reproduces results obtained previously by Pecora, but which is specialized at the outset to the problem of dilute solutions so that simple single-particle correlation functions may be utilized. An analysis of the photocurrent spectrum for both the homodyne and heterodyne detection schemes is given. Various data reduction schemes utilized in the analysis of our spectra are described in some detail, and our results are compared with values of the diffusion constants obtained from other experiments

Light-scattering study of the liquid-glass transition in propylene carbonate

Depolarized light-scattering spectra of propylene carbonate were obtained in the frequency range 0.2 GHz 4 THz at temperatures from 350 to 135 K. Analysis of the resulting susceptibility spectra revealed reasonable agreement with the predictions of the idealized mode coupling theory, yielding critical exponents a=0.29, b=0.50, and an exponent parameter λ=0.78±0.05. A scaling analysis demonstrated critical slowing down of the scaling frequencies from both above and below TC, with the scaling frequency extrapolating to zero at TC=187±5 K. The α-relaxation peak was fit to a Kohlrausch function, which gave β=0.77±0.05 for 210≤T≤350 K with no sign of systematic temperature dependence. An extended mode coupling theory analysis was also carried out which corroborated the results obtained with the idealized version. Polarized Brillouin spectra were also obtained, and were analyzed with a generalized hydrodynamics approach using low-frequency sound velocity values determined separately in an ultrasonics experiment. The results indicate that the Cole-Davidson function is not an adequate model for the structural relaxation due to the neglect of β-relaxation processes, so that this analysis cannot provide a meaningful estimate of TC. Furthermore, the α-relaxation time associated with Brillouin scattering was found to be about five times shorter than that probed by depolarized light scattering

Light scattering spectroscopy of the liquid-glass transition : comparison with idealized and extended mode coupling theory

Light scattering experiments were performed on several fragile glass-forming materials at temperatures ranging from above the melting temperature Tm to below the calorimetric glass transition temperature T in the frequency range 0.2 GHz to 4000 GHz. The experimental susceptibility spectra x(w) exhibit both the form and the scaling properties predicted by the idealized mode coupling theory. At low frequencies and temperatures the spectra deviate from the predictions due to the neglect of ergodicity-restoring activated transport (hopping) processes. We have therefore reanalyzed the data using the extended version of the theory, including activated transport

Tests of the mode-coupling theory for a molten salt

A recently reported analysis (Cummins et al., 1993, Phys. Rev. E, 47, 4223) of depolarized light and neutron scattering experiments in CaKNO3 using the mode-coupling theory is extended. The temperature dependence of the critical amplitudes found in this analysis is explained