Dynamic Anomaly Research Articles

Dynamic Anomalies and their Relation to the Glass Transition: A Neutron Scattering Study of the Glass Forming Van der Waals Liquid Ortho‐terphenyl

AbstractNeutron scattering experiments on the molecular glass former ortho‐terphenyl reveal a dynamic anomaly at a temperature Tc ≈ 290 K well above the calorimetric glass temperature Tg = 243 K. Close above Tc the density autocorrelation function ΦQ(t) shows a two step decay over 4–5 decades in time. The slower component obeys the time‐temperature superposition principle. Its line shape can be well parametrized by a Kohlrausch law and is strongly temperature dependent as its relaxation time scales with the shear viscosity. Thus this component is identified with the structural relaxation (α‐process). The faster component (β‐process) is much less temperature dependent. Its line shape factorizes in a purely Q‐dependent and a purely time‐dependent part and its time scale follows a power law with a critical exponent a. The crossover region between the α‐and ß‐process follows a power law (von Schweidler law) with a critical exponent b. Approaching Tc from below the plateau value separating the α and the β process (= Debye‐Waller factor) shows a critical decrease and bends over to a weak temperature dependence at Tc. These findings support several predictions made by the mode coupling theory of the glass transition which relates these dynamic anomalies to remnants of an ideal glass transition.

Dynamic anomaly in the glass transition region of orthoterphenyl

We report on incoherent and coherent neutron scattering results in the supercooled liquid and the glassy regime of the van der Waals fluido-terphenyl using the backscattering and spin echo technique, respectively. A critical comparison of both techniques is presented. The data are analysed in the time domain assuming that microscopic correlation times (τ) scale with the viscosity η according to τ(T)∼η(T)/T. With this assumption we obtain an agreement with several predictions of mode coupling theory: the existence of a critical temperatureT c is shown, independently for both incoherent and coherent data, by a cusp in the temperature dependence of the Debye-Waller factorf Q (T). BelowT c fQ(T) follows the predicted $$\sqrt {T_c - T} $$ behaviour yielding also theQ-dependence of the critical parametersf Q c andh Q . AboveT c the α-process or structural relaxation can be well parametrized by a stretched exponential function. In accordance with theory the structural relaxation curves can also be described by a power law with a critical (von Schweidler) exponentb=0.525±0.10.

Large‐scale relative dynamic topography of the Southern Ocean

The large‐scale or climatic distribution of relative dynamic topography south of 30°S using a selective set of hydrographic stations indicates significant deviations from a purely latitudinal dependence of the isopleths. The geostrophic flow relative to 1000‐dbar level shows the Antarctic Circumpolar Current to vary greatly in its intensity and latitude during its course around Antarctica. High‐intensity flow occurs over the northern slope of the midocean ridge from 105°E eastward to the fracture zones near 145°W, significant perturbations being introduced in the vicinity of the Macquarie Ridge and Campbell Plateau. A strong expression of the Antarctic Circumpolar Current is found in the Drake Passage downstream of weak baroclinicity in the southeast Pacific. East of the Drake Passage strong meridional trends of the dynamic height isopleths occur with significant latitudinal offset (about 1100 km) of the main axis of geostrophic flow. In the southwest Indian Ocean, maximum baroclinicity is associated with the Agulhas Return Current near 40°S rather than the more continuous branch of Circumpolar flow to the south near 50°S. The two axes may combine east of Kerguelen Island. The dynamic height anomaly to the south of the main circumpolar current indicates the presence of weak baroclinic gyres in the western Weddell Basin and to the north and northeast of the Ross Sea and much meridional trend of the isopleths over the south eastern flank of the Kerguelen Plateau. North of the main circumpolar flow dynamic height anomaly also indicates weak baroclinicity, though a relatively strong axis of northward geostrophic flow occurs over the eastern flank of Campbell Plateau.