Showing papers by "Sylwester J. Rzoska published in 2010"

Evidence for critical-like behavior in ultraslowing glass-forming systems.

Abstract: Glass transition constitutes one of main problems of condensed matter physics and material engineering that remains unsolved. The common acceptance of the Vogel-Fulcher-Tammann (VFT) equation for portraying the primary relaxation time or shear viscosity indicated a possible phase transition, hidden below the glass transition temperature (T(g)). Recently Hecksher [Nat. Phys. 4, 737 (2008)] delivered strong empirical arguments that VFT description lacks a direct experimental basis and thus theories not predicting a dynamic divergence should be focused on. We present clear evidence for a superiority of critical-like divergent equation τ(T)=τ(0)(T-T(C))(-ϕ) and T(C)

Microscopic structures and dynamics of high- and low-density liquid trans-1,2-dichloroethylene

Abstract: We present a study of the dynamics and structural changes for trans-1,2-dichloroethylene between high- and low-density liquids using neutron-scattering techniques diffraction, small-angle neutron scattering, and time of flight spectroscopy and molecular-dynamics simulations. We show that changes in the short-range ordering of molecules goes along with a change in the molecular dynamics: both structure and dynamics of the highdensity liquid are more cooperative than those of the low-density liquid. The microscopic mechanism underlying the cooperative motions in the high-density liquid has been found to be related to the backscattering of molecules due to a strong correlation of molecular ordering.

Dielectric and mechanical relaxation in isooctylcyanobiphenyl (8*OCB)

Abstract: The dynamics of isooctylcyanobiphenyl (8*OCB) was characterized using dielectric and mechanical spectroscopies. This isomer of the liquid crystalline octylcyanobiphenyl (8OCB) vitrifies during cooling or on application of pressure, exhibiting the typical features of glass-forming liquids: non-Debye relaxation function, non-Arrhenius temperature dependence of the relaxation times, ??, a dynamic crossover at T ~ 1.6Tg. This crossover is evidenced by changes in the behavior of both the peak shape and the temperature dependence of ??. The primary relaxation time at the crossover, 2?ns at ambient pressure, is the smallest value reported to date for any molecular liquid or polymer. Interestingly, at all temperatures below this crossover, ?? and the dc conductivity remain coupled (i.e., conform to the Debye?Stokes?Einstein relation). Two secondary relaxations are observed in the glassy state, one of which is identified as the Johari?Goldstein process. Unlike the case for 8OCB, no liquid crystalline phase could be attained for 8*OCB, demonstrating that relatively small differences in chemical structure can effect substantial changes in the intermolecular potential.

Consistency of the Vogel — Fulcher — Tammann (VFT) Equations For The Temperature-, Pressure-, Volume-and Density- Related Evolutions of Dynamic Properties in Supercooled and Superpressed Glass Forming Liquids/Systems

Abstract: A consistent set of temperature- (T), pressure- (P), volume- (V) and density- (ρ) related VFT-type equations for portraying the evolution of the structural relaxation time or viscosity is presented, namely: $$\begin{array}{rcl} \tau \left(P \right) & = & \tau _0 \,\exp \left[ {{{D_P \left({P - P_{SL} } \right)}/{\left({P_0 - P} \right)}}} \right] \\ \tau \left(T \right) & = & \tau _0 \,\exp \left[ {{{D_T \left({T_{SL} - T} \right)\left({{{T_0 }/{T_{SL} }}} \right)}/{\left({T - T_0 } \right)}}} \right],\\ \tau \left(\rho \right) & = & \tau _0 \,\exp \left[ {{{D_\rho \left({\rho - \rho _{SL} } \right)}/{\left({\rho _0 - \rho } \right)}}} \right]\\ \end{array}$$ andτ (V) = τ 0 exp[D T (V SL — V)(V 0 V SL )/(V — V 0)], where T 0,P 0,V 0 and ρ0 are VFT estimates of the ideal glass loci and T SL , P SL , V SL and ρ SL are estimates of the location of the absolute stability limit, partially hidden in the negative pressures domain (P<0). For these equations prefactors are well defined via ρ 0 = ρ (T SL ,P SL , V SL ,ρ SL ), ie. they are linked to the absolute stability limit loci (gas-liquid spinodal). Noteworthy is their smooth transformation into VFT-type equations, used so far, on approaching the glass transition, and into Arrhenius-type equations remote from the glass transition, on approaching the absolute stability limit. The latter may suggest the re-examination of experimental data suggesting the VFT-to-Arrhenius crossover far away from the glass transition. Novel VFT counterparts also lead to the consistent set of fragility strength coefficients (D T ,D P ,D V ,D ρ) and fragilities associated with the slope (steepness index) at appropriate “Angell plot” counterparts.

Anomalous Decoupling of the dc Conductivity and the Structural Relaxation Time in the Isotropic Phase of a Rod-Like Liquid Crystalline Compound

Abstract: Recently, the isotropic phase of rod-like liquid crystalline compounds is advised as an experimental model system for studying complex glassy dynamics One of unique phenomena occuring close to the glass temperature, for the time scale 10−7±1 s T I−N is shown The link between this behavior and the FDSE is suggested, namely: S = ϕ′/ϕ Finally the call for further pressure studies is formulated

Viscosity, relaxation time, glass temperature, melting temperature and fragile-to-strong transition parameterizations at extreme pressures in soft-matter systems

Abstract: This contribution presents the extended, pressure-related Vogel–Fulcher–Tammann equation applied to portray the pressure evolution of viscosity η (P) and the related dynamic properties, such as the primary relaxation time τ (P), in soft-matter systems as well as the modified Simon–Glatzel-type equation for describing pressure dependences of the glass temperature T g (P), the melting temperature T m (P) and the fragile-to-strong dynamical transition in confined water T d (P). Both equations are capable of penetrating the negative pressure (isotropically stretched liquid) domain, and at very high pressures are capable of the inverse behavior. They have the following forms: (i) η (P)=η0 exp [D P Δ P/(P 0−P)]=η0 exp [(D P P−D P P SL)/(P 0−P)], where P 0 is the estimate of the ideal glass temperature, P SL is for the stability limit at negative pressures and D P denotes the pressure fragility strength coefficient and (ii) , where and are the reference temperature and pressure,−π is the negative pressure asympt...

Transformation of the Strongly Hydrogen Bonded System into van der Waals one Reflected in Molecular Dynamics

Abstract: Dielectric relaxation studies on disaccharides lactose and octa-O-acetyl-lactose are reported. The latter is a hydrogen bonded system while the former is a van der Waals glass former. The transformation between them was arranged by substituting hydrogen atoms in lactose by acetyl groups. Hereby the influence of differences in bounding on dynamics of both systems is discussed. We showed that the faster secondary relaxation (labeled γ) in octa-O-acetyl-lactose has much lower amplitude than that of lactose. The relaxation time and activation energy remain unchanged in comparison to the γ- relaxation of lactose. We did not observe the slow secondary relaxation (labeled β), clearly visible in lactose, in its acethyl derivative. Detailed analysis of the dielectric spectra measured for octa-O-acetyl-lactose in its glassy state (not standard change in the shape of the γ- peak with lowering temperature) enabled us to provide probable explanation of our finding. No credible comparative analysis of the α- relaxation process of the lactose and octa-O-acetyl-lactose are presented, because loss spectra of the former carbohydrate were affected by the huge contribution of the dc conductivity. Notwithstanding, one can expect that octa-O-acetyl-lactose has lower glass transition temperature and steepness index than lactose.

New Proposals for Supercritical Fluids Applications

Abstract: Supercritical fluids (SCF) constitute a promising platform for future technological and environmental applications. The basic advantage of this method is their enormous selectiveness matched with a broad-range tuning of useful properties. So far, SCF technologies have made use of the vicinity of the gas — liquid critical point in a one component system, such as CO2. For specific applications an additional component may be used to reach the desired technological target. In this report novel possibilities associated with supercriticality near the liquid — liquid critical point in binary solutions of limited miscibility and in a one component fluid are presented. The latter is discussed for a weakly supercooled nitrobenzene and trans-1,2-dichlorethylene. For the critical consolute point the possibility of obtaining arbitrary values and sings of the pressure shift of the critical temperature dT C /dP are stressed.

About the shape of the melting line as a possible precursor of a liquid-liquid phase transition

Abstract: Several simple, non-mesogenic liquids can exists in two or more different liquid forms. When the liquid-liquid line, separating two liquid forms, meets the melting line, one can expect some kind of break on the melting line, caused by the different freezing/melting behaviour of the two liquid forms. Unfortunately recently several researchers are using this vein of thinking in reverse; seeing some irregularity on the melting line, they will expect a break and the appearance of a liquid-liquid line. In this short paper, we are going to show, that in the case of the high-pressure nitrogen studied recently by Mukherjee and Boehler, the high-pressure data can be easily described by a smooth, break-free function, the modified Simon-Glatzel equation. In this way, the break, suggested by them and consequently the suggested appearance of a new liquid phase of the nitrogen might be artefacts.

Erratum: Microscopic structures and dynamics of high- and low-density liquid trans-1,2-dichloroethylene [Phys. Rev. B 81, 092202 (2010)]

Abstract: We present a study of the dynamics and structural changes for trans-1,2-dichloroethylene between high and low density liquids using neutron scattering techniques (diffraction, small angle neutron scattering and time of flight spectroscopy) and molecular dynamics simulations We show that changes in the short range ordering of molecules goes along with a change of the molecular dynamics: both structure and dynamics of the high density liquid are more cooperative than those of the low density liquid The microscopic mechanism underlying the cooperative motions in the high density liquid has been found to be related to the backscattering of molecules due to a strong correlation of molecular ordering

The link between the pressure evolution of the glass temperature in colloidal and molecular glass formers

Abstract: Recently, Voigtmann [Phys. Rev. Lett. 101, 095701 (2008)] suggested the existence of the universal “generic steep“ behavior for the glass pressure vs. temperature dependence in molecular glass formers. We indicate that such behavior disappear when the absolute stability limit in the negative pressures domain is taken as the reference. It is a parasitic artifact of omitting the stability limit in the negative pressures domain for the log-log scale plot. Results presented suggest a totally common pattern for the evolution of the glass temperature in colloidal fluids and molecular liquids. However, for molecular liquids both positive and negatives pressures domains have to be taken into account. Consequently the pattern for colloidal glass formers introduced by Sciortino “One Liquid, Two Glasses“ [Nature Materials 1, 1-3 (2002)] may appear to be valid also for molecular glass formers.