Unusual features of long-range density fluctuations in glass-forming organic liquids: A Rayleigh and Rayleigh-Brillouin light scattering study - art.no. 061503

A new feature of glass-forming liquids, i.e., long-range density fluctuatio ns of the order of 100 nm, has been extensively characterized by means of s tatic light scattering, photon correlation spectroscopy and Rayleigh-Brillo uin spectroscopy in orthoterphenyl (OTP) and 1,1-di(4'-methoxy-5' methyl-ph enyl)-cyclohexane (BMMPC). These long-range density fluctuations result in the following unusual features observed in a light scattering experiment. w hich are not described by the existing theories: (i) strong q-dependent iso tropic excess Rayleigh intensity, (ii) additional slow component in the pol arized photon correlation function, and (iii) high Landau-Placzek ratio. Th ese unusual features are equilibrium properties of the glass-forming liquid s and depend only on temperature, provided that the sample has been equilib rated long enough. The temperature-dependent equilibration times were measu red for BMMPC and are about 11 orders of magnitude longer than the a proces s. It was found that the glass-forming liquid OTP may occur in two states: with and without long-range density fluctuations (''clusters"). We have cha racterized the two states by static and dynamic light scattering in the tem perature range from T-g to T-g + 200 K. The relaxation times of the a proce ss as well as the parameters of the Brillouin line are identical in both OT P with and without clusters. The a process (density fluctuations) in OTP wa s characterized by measuring either the polarized (VV) or depolarized (VH) correlation function, which are practically identical and a-independent. Th is feature, which is commonly observed in glass-forming liquids, is not ful ly explained by the existing theories.