:It was shown recently [1] that the structural α-relaxation time τ of supercooled o-terphenyl depends on a single control parameter Γ, which is the product of a function of density E(ρ), by the inverse temperature T -1 . We extend this finding to other fragile glassforming liquids using light-scattering data. Available experimental results do not allow to discriminate between several analytical forms of the function E(ρ), the scaling arising from the separation of density and temperature in Γ. We also propose a simple form for τ(Γ), which depends only on three material-dependent parameters, reproducing relaxation times over 12 orders of magnitude. I-IntroductionThe steady increase over more than 12 orders of magnitude of the shear viscosity η, or of the structural α-relaxation time τ measured by dielectric or light-scattering spectroscopic methods, is among the spectacular features that accompany the liquid-glass transition [2]. The latter can take place in a large variety of liquids ranging from mineral oxides, salts, organic polymers, metallic alloys, to simple molecular liquids. Under cooling, these liquids avoid crystallisation at the melting temperature T m and become a glass at a lower temperature T g .For low molecular weight glassforming materials, a glass is usually defined as a liquid whose relaxation times τ are larger than 10 2 seconds, a phenomenon that takes place at some temperature T g . In spite of its frequent occurrence and of continuous theoretical and experimental efforts over the last fifty years, this transition remains one of the least understood phenomena in condensed matter physics. In particular, one still does not fully grasp how and on which parameters, usually called control parameters, these relaxation times τ depend [3,4]. Obviously, in simple glassforming colloids the only control parameter is the density. Conversely, would the structural relaxation of a liquid at equilibrium be only due to jumps over energy barriers that are independent from temperature and density, τ might have a simple Arrhenius behaviour and the only control parameter would be temperature. The Angell plot [2], which represents the logarithm of relaxation times τ (or viscosity η) as a function of orders of magnitude, they showed that the data can be scaled using control parameters of the same form as in OTP, n becoming material dependent. Previously, a scaling was shown to hold in glycerol by . Also ref. [24] showed that the domain of density explored in the experiments was too small to ascertain the analytical form of E(ρ Table 1. At each pressure and temperature, the corresponding density can be obtained from the Tait interpolation formula, whose form and parameters are also collected in Table 1. Table 2 as well as that obtained from unpublished PCS data on BMPC (1,1-bis(p-methoxyphenyl)cyclohexane ) [25]. We add also the n value obtained by the scaling of the dielectric α-relaxation time in KDE (cresolphtaleine-dimethyl ether) [26].The values of n vary from 3.8 to 7.5.We note that similar result...

Transverse Brillouin spectra of orthoterphenyl are measured in the (250-305 K; 0.1-100 MPa) temperature-pressure range, which corresponds to the supercooled phase of this organic glass former. We show that the analysis of these spectra combined with an extrapolation of the reorientation times under pressure leads to an estimate of the static shear viscosity in a pressure range whose validity extends beyond the range of the Brillouin measurements. The relative contributions of temperature and of density to the change of this reorientation time measured along an isobar are extracted from our results in a large temperature range extending from the liquid to the low temperature supercooled state. They appear to be always of the same order of magnitude. It is also shown that in the range of the experiment, the orientational time is depending on a unique parameter built on temperature and density.

Articles you may be interested inDensity and confinement effects of glass forming m-toluidine in nanoporous Vycor investigated by depolarized dynamic light scattering A dynamic light scattering study of the hypersonic relaxation in liquid toluene Dynamics of liquid-glass transition in propanol AIP Conf. Proc. 469, 525 (1999); 10.1063/1.58534 Crystalization in the glass-forming materials: [KNO 3 ] X [Ca(NO 3 ) 2 ] 1−X and Brillouin scattering study of glass transition in X=0.64 mixture AIP Conf.An experimental study of the glass transition of meta-toluidine combining several light scattering techniques was performed. The structural relaxation time is measured in depolarized geometry from the glass transition temperature up to well above the melting point and found to vary over 13 time decades. An analysis by means of the idealized Mode Coupling Theory shows that, as found in other aromatic liquids, experimental results obtained in depolarized light scattering can be described by this theory above T c in a two-decade frequency range. The polarized Brillouin doublet, measured in the backscattering geometry between 176 K and 300 K, is also analyzed. None of the sets of parameters we obtained in fitting those spectra could fulfil all the requirements of this Mode Coupling Theory.

We discuss the hydrodynamic equations which describe the shear dynamics of a liquid composed of anisotropic molecules, both in its normal and its supercooled phases. We use these equations to analyze 90 • depolarized light scattering experiments performed in the supercooled phase of a glass forming liquid, metatoluidine, and show that the information extracted from this analysis is consistent with independent shear viscosity measurements performed on that liquid in the same temperature range. PACS. 66.20.+d Viscosity of liquids; diffusive momentum transport -78.35.+c Brillouin and Rayleigh scattering; other light scattering -64.70.Pf Glass transitions

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