Dynamics of supercooled liquids and glasses: comparison of experiments with theoretical predictions

Glass spectrum, excess wing phenomenon, and master curves in molecular glass formers: A multi-method approach

The relaxation spectra of glass formers solely displaying an α-peak and excess wing contribution collected by various methods are reanalyzed to pin down their different spectral evolution. We show that master curve construction encompassing both α-peak and emerging excess wing works for depolarized light scattering (DLS) and nuclear magnetic resonance (NMR) relaxometry. It reveals the self-part of the slow dynamics' spectrum. Master curves are to be understood as a result of a more extensive scaling covering all temperatures instead of strict frequency-temperature superposition. DLS and NMR display identical relaxation spectra; yet, comparing different systems, we do not find a generic structural relaxation at variance with recent claims. Dielectric spectroscopy (DS) spectra show particularities, which render master curve construction obsolete. The DS α-peak is enhanced or suppressed with respect to that of DLS or NMR, yet, not correlated to the polarity of the liquid. Attempting to single out the excess wing from the overall spectrum discloses a stronger exponential temperature dependence of its amplitude compared to that below Tg and a link between its exponent and that of the fast dynamics' spectrum. Yet, such a decomposition of α-peak and excess wing appears to be unphysical. Among many different glasses, the amplitude of the excess wing power-law spectrum is found to be identical at Tg, interpreted as a relaxation analog to the Lindemann criterion.

Filamentous crystal growth in organic liquids and selection of crystal morphology

Filamentous crystals such as whisker crystals are often seen not only in metallic liquids, but also in organic liquids and solutions. They are interesting as reinforce materials. However, it remains challenging to induce filamentous crystals due to an incomplete understanding of the mechanisms behind their formation. In this paper, we investigate filamentous crystal growth in viscous organic liquids. It is found that filamentous crystals grow via an extraordinary dynamical path, where the molecules locally evaporate to bubbles and then redeposite to the tip of growing crystalline filaments. We also succeeded in controlling whether filamentous or faceted crystal growth is selected by inducing or suppressing the bubbles.

NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of ¹H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole-Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

Molecular dynamics simulations vs field-cycling NMR relaxometry: Structural relaxation mechanisms in the glass-former glycerol revisited

We combine field-cycling (FC) relaxometry and molecular dynamics (MD) simulations to study the rotational and translational dynamics associated with the glassy slowdown of glycerol. The ¹H NMR spin-lattice relaxation rates R₁(ω) probed in the FC measurements for different isotope-labelled compounds are computed from the MD trajectories for broad frequency and temperature ranges. We find high correspondence between experiment and simulation. Concerning the rotational motion, we observe that the aliphatic and hydroxyl groups show similar correlation times but different stretching parameters, while the overall reorientation associated with the structural relaxation remains largely isotropic. Additional analysis of the simulation results reveals that transitions between different molecular configurations are slow on the time scale of the structural relaxation at least at sufficiently high temperatures, indicating that glycerol rotates at a rigid entity, but the reorientation is slower for elongated than for compact conformers. The translational contribution to R₁(ω) is well described by the force-free hard sphere model. At sufficiently low frequencies, universal square-root laws provide access to the molecular diffusion coefficients. In both experiment and simulation, the time scales of the rotational and translational motions show an unusually large separation, which is at variance with the Stokes-Einstein-Debye relation. To further explore this effect, we investigate the structure and dynamics on various length scales in the simulations. We observe that a prepeak in the static structure factor S(q), which is related to a local segregation of aliphatic and hydroxyl groups, is accompanied by a peak in the correlation times τ(q) from coherent scattering functions.

Anomalous diffusion and non-monotonic relaxation processes in Ge-Se liquids

We investigate the dynamical properties of liquid GexSe100-x as a function of Ge content by first-principles molecular dynamic simulations for a certain number of temperatures in the liquid state. The focus is set on ten compositions (where x ≤ 33%) encompassing the reported flexible to rigid and rigid to stressed-rigid transitions. We examine diffusion coefficients, diffusion activation energies, glassy relaxation behavior, and viscosity of these liquids from Van Hove correlation and intermediate scattering functions. At fixed temperature, all properties/functions exhibit an anomalous behavior with Ge content in the region 18%-22%, and provide a direct and quantitative link to the network rigidity.

Depolarized light scattering spectra of molecular liquids: Described in terms of mode coupling theory

Depolarized light scattering spectra of eight molecular liquids as obtained from applying tandem-Fabry-Pérot interferometry and double monochromator are analyzed in the frame work of the mode coupling theory (MCT). The susceptibility spectra are fitted to the numerical solution of the schematic F12 model of MCT and the validity of the asymptotic laws is discussed. The model is able to quantitatively describe the spectra up to the boiling point, where the main (structural) relaxation and the contribution of the microscopic (vibrational) dynamics essentially merge, and down to the moderately super-cooled liquid where glassy dynamics establishes. The changes of the spectra with temperature are mapped to only two control parameters, which show a smooth variation with temperature. Strong correlation between experimental stretching parameters and extrapolated values from the model is found. The numerical solutions are extrapolated down to Tc, where the asymptotic scaling laws can be applied. Although the spectra apparently follow scaling relations, the application of the asymptotic laws usually overestimates Tc by up to 12 K. In all the cases, the experimental spectra are outside the applicability regime of the asymptotic laws. This is explained by more or less strong vibrational contributions. Within a phenomenological approach which extends the spectral analysis down to Tg and which allows for separating fast and slow dynamics, the strength of the fast dynamics 1 - frel is revealed. It shows the cusp-like anomaly predicted by MCT; yet, the corresponding critical temperature is significantly higher than that derived from the F12 model. In addition, we demonstrate that close to Tg, the susceptibility minimum is controlled by the interplay of the excess wing and the fast dynamics contribution.

Ions in glass-forming glycerol: close correlation of primary and fast β relaxation

We provide broadband dielectric loss spectra of glass-forming glycerol with varying additions of LiCl. The measurements covering frequencies up to 10 THz extend well into the region of the fast β process, commonly ascribed to caged molecular dynamics. Aside from the known variation of the structural α relaxation time and a modification of the excess wing with ion content, we also find a clear influence on the shallow loss minimum arising from the fast β relaxation. Within the framework of mode-coupling theory, the detected significant broadening of this minimum is in reasonable accord with the found variation of the α-relaxation dynamics. A correlation between α-relaxation rate and minimum position holds for all ion concentrations and temperatures, even below the critical temperature defined by mode-coupling theory.

Reorientational Dynamics of Organophosphate Glass Formers – a Joint Study by 31P NMR, Dielectric Spectroscopy and Light Scattering

We study molecular reorientation in the glass formers triethyl-, tripropyl-, and m-tricresyl phosphate by measuring 31P NMR spectra, relaxation (T 1 and T 2), stimulated echo decays and two-dimensional spectra over a large temperature range (130–370 K). The results are compared to those from dielectric spectroscopy (DS) and depolarized light scattering (LS). While the time constants τ α of the primary (α-) process in the range  × 10−11–100 s well agree, the stretching of the reorientational correlation function is probe dependent, i.e., the rank-two functions (NMR and LS) essentially agree whereas the rank-one function (DS) is less stretched. The very similar 2D spectra recorded as a function of mixing time demonstrate that the reorientational mechanism does not significantly vary among the super-cooled liquids. A model of combining large- and small-angle reorientation allows for reproducing the 2D spectra and may be taken as generic for the dynamics in viscous molecular liquids. Pronounced secondary (β-) processes do not only effect the NMR relaxation but can be identified directly in the time domain by the stimulated echo technique. This becomes possible due to its broad time window (10 μs–100 s). Thus, applying the different 1D and 2D techniques makes 31P NMR well suited to probe molecular reorientation over a wide dynamic range.

From boiling point to glass transition temperature: transport coefficients in molecular liquids follow three-parameter scaling

The phenomenon of the glass transition is an unresolved problem in condensed matter physics. Its prominent feature, the super-Arrhenius temperature dependence of the transport coefficients, remains a challenge to be described over the full temperature range. For a series of molecular glass formers, we combined τ(T) collected from dielectric spectroscopy and dynamic light scattering covering a range 10(-12) s < τ(T) < 10(2) s. Describing the dynamics in terms of an activation energy E(T), we distinguish a high-temperature regime characterized by an Arrhenius law with a constant activation energy E(∞) and a low-temperature regime for which E(coop)(T) ≡ E(T)-E(∞) increases exponentially while cooling. A scaling is introduced, specifically E(coop)(T)/E(∞) [proportionality] exp[-λ(T/T(A)-1)], where λ is a fragility parameter and T(A) a reference temperature proportional to E(∞). In order to describe τ(T) still the attempt time τ(∞) has to be specified. Thus, a single interaction parameter E(∞) describing the high-temperature regime together with λ controls the temperature dependence of low-temperature cooperative dynamics.

An upper limit to kinetic fragility in glass-forming liquids

The kinetic fragility of a liquid is correlated to the magnitude of enthalpy hysteresis in various glass-forming materials during thermal cycling across the glass transition. While the lower bound of liquid fragility is well known, there has been little research into the possibility of an inherent upper limit to fragility. In this paper, we present a theoretical argument for the existence of a maximum fragility and show that the correlation between fragility and enthalpy hysteresis allows for an empirical evaluation of the upper limit of fragility. This upper limit occurs as the enthalpy hysteresis involved in thermal cycling about the glass transition approaches zero, leading to m(max)≈175. This result agrees remarkably well with our previous estimate. The dynamics of maximum fragility liquids are discussed, and a critical temperature of ∼1.5 T(g) (where T(g) is the glass transition temperature) is revealed where a transition from nonexponential to exponential structural relaxation occurs.

Correlating the stretched-exponential and super-Arrhenius behaviors in the structural relaxation of glass-forming liquids

Following the report of a single-exponential activation behavior behind the super-Arrhenius structural relaxation of glass-forming liquids in our preceding paper, we find that the non-exponentiality in the structural relaxation of glass-forming liquids is straightforwardly determined by the relaxation time, and could be calculated from the measured relaxation data. Comparisons between the calculated and measured non-exponentialities for typical glass-forming liquids, from fragile to intermediate, convincingly support the present analysis. Hence the origin of the non-exponentiality and its correlation with liquid fragility become clearer.

Temperature development of glassy α-relaxation dynamics determined by broadband dielectric spectroscopy

We present the temperature dependence of α -relaxation times of 13 glass formers determined from broadband dielectric spectroscopy, also including data from aging measurements. The data sets partly cover relaxation-time ranges of up to 16 decades enabling a critical test of the validity of model predictions. For this purpose, the data are provided for electronic download. Here we employ these results to test the applicability of the Vogel-Fulcher-Tammann equation and a recently proposed new approach that was demonstrated to provide superior fits of a vast collection of viscosity data.

On the nature of the high-frequency relaxation in a molecular glass former: a joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering

Fast field cycling (1)H NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T(1)(omega) of the glass former glycerol in broad temperature (75-360 K) and frequency (10 kHz-30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility chi(")(omega) proportional, variantomegaT(1)(omega), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature T(g). This allows to combine NMR data of different temperatures into a single master curve chi(")(omegatau(alpha)) that extends over 15 decades in reduced frequency omegatau(alpha), where tau(alpha) is the structural alpha-relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At omegatau(alpha)<1, NMR susceptibility is significantly different from both the dielectric and light scattering results. At omegatau(alpha)>1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at omegatau(alpha)>>1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l=2) rather coincide with each other and are about three times more intense than the dielectric (l=1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below T(g), dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence.

Broadband dielectric spectroscopy on benzophenone: alpha relaxation, beta relaxation, and mode coupling theory

We have performed a detailed dielectric investigation of the relaxational dynamics of glass-forming benzophenone. Our measurements cover a broad frequency range of 0.1 Hz to 120 GHz and temperatures from far below the glass temperature well up into the region of the small-viscosity liquid. With respect to the alpha relaxation this material can be characterized as a typical molecular glass former with rather high fragility. A good agreement of the alpha relaxation behavior with the predictions of the mode coupling theory of the glass transition is stated. In addition, at temperatures below and in the vicinity of T(g) we detect a well-pronounced beta relaxation of Johari-Goldstein type, which with increasing temperature develops into an excess wing. We compare our results to literature data from optical Kerr effect and depolarized light scattering experiments, where an excess-wing-like feature was observed in the 1-100 GHz region. We address the question if the Cole-Cole peak, which was invoked to describe the optical Kerr effect data within the framework of the mode coupling theory, has any relation to the canonical beta relaxation detected by dielectric spectroscopy.

Dielectric spectroscopy in benzophenone: the beta relaxation and its relation to the mode-coupling Cole-Cole peak

We report a thorough characterization of the glassy dynamics of benzophenone by broadband dielectric spectroscopy. We detect a well-pronounced beta relaxation peak developing into an excess wing with increasing temperature. A previous analysis of results from Optical-Kerr-effect measurements of this material within the mode-coupling theory revealed a high-frequency Cole-Cole peak. We address the question if this phenomenon also may explain the Johari-Goldstein beta relaxation, a so-far unexplained spectral feature inherent to glass-forming matter, mainly observed in dielectric spectra. Our results demonstrate that according to the present status of theory, both spectral features seem not to be directly related.

Relaxation phenomena in poly(vinyl alcohol)/fumed silica affected by interfacial water

Interaction of poly(vinyl alcohol) (PVA) with fumed silica was investigated in the gas phase and aqueous media using adsorption, broadband dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC), infrared spectroscopy, thermal analysis, and one-pass temperature-programmed desorption (OPTPD) mass-spectrometry (MS) methods. PVA monolayer formation leads to certain textural changes in the system (after suspension and drying) because of strong hydrogen bonding of the polymer molecules to silica nanoparticles preventing strong interaction between silica particles themselves. This strong interaction promotes associative desorption of water molecules at lower temperatures than in the case of silica alone. Interaction of PVA with silica and residual water leads to depression of glass transition temperature (T(g)). There are three types of dipolar relaxations at temperatures lower and higher than the T(g) value. A small amount of adsorbed water leads to significant conductivity with elevating temperature.

Crossover behavior and multistep relaxation in a schematic model of the cut-off glass transition

We study a schematic mode-coupling model in which the ideal glass transition is cut off by a decay of the quadratic coupling constant in the memory function. (Such a decay, on a time scale tau I , has been suggested as the likely consequence of activated processes.) If this decay is complete, so that only a linear coupling remains at late times, then the alpha relaxation shows a temporal crossover from a relaxation typical of the unmodified schematic model to a final strongly slower-than-exponential relaxation. This crossover, which differs somewhat in form from previous schematic models of the cutoff glass transition, resembles light-scattering experiments on colloidal systems, and can exhibit a "slower-than- alpha " relaxation feature hinted at there. We also consider what happens when a similar but incomplete decay occurs, so that a significant level of quadratic coupling remains for t>>tau I . In this case the correlator acquires a third, weaker relaxation mode at intermediate times. This empirically resembles the beta process seen in many molecular glass formers. It disappears when the initial as well as the final quadratic coupling lies on the liquid side of the glass transition, but remains present even when the final coupling is only just inside the liquid (so that the alpha relaxation time is finite, but too long to measure). Our results are suggestive of how, in a cutoff glass, the underlying "ideal" glass transition predicted by mode-coupling theory can remain detectable through qualitative features in dynamics.

Raman scattering study on structural and dynamical features of noncrystalline selenium

We report on a detailed, temperature-dependent, off-resonant Raman scattering study of glassy and supercooled selenium. Raman spectra in the frequency regime of the first-order scattering (5-450 cm(-1)) have been recorded over a wide temperature range, i.e., 143-353 K. To facilitate the analysis, the spectra have intuitively been divided in three spectral regions. The analysis of the high frequency region (bond-stretching vibrational modes) yielded information on the rings-chains equilibrium. In particular, the polymer content was found to amount to more than 85% around the glass transition temperature, exhibiting a weak temperature dependence, which extrapolates nicely to the high-temperature dissolution data. The intermediate frequency range (representative of the medium-range structural order) was treated together with the low frequency regime (where low-energy excitations, i.e., the quasielastic line and the Boson peak are the dominant contributions) owing to their strong overlap. The study of the bond-bending regime revealed information which made it possible to clarify the role of ringlike and chainlike fragments incorporated in polymeric molecules. The temperature evolution of the Boson peak and the frequency dependence of the Raman coupling coefficient Comega were also determined. An attempt to decompose the partial contribution of the pure Boson peak to Comega revealed valuable information concerning the limiting (omega-->0) behavior of the coupling coefficient.

Slow dynamics of salol: a pressure- and temperature-dependent light scattering study

We study the slow dynamics of salol by varying both temperature and pressure using photon correlation spectroscopy and pressure-volume-temperature measurements, and compare the behavior of the structural relaxation time with equations derived within the Adam-Gibbs entropy theory and the Cohen-Grest free volume theory. We find that pressure-dependent data are crucial to assess the validity of these model equations. Our analysis supports the entropy-based equation, and estimates the configurational entropy of salol at ambient pressure approximately 70% of the excess entropy. Finally, we investigate the evolution of the shape of the structural relaxation process, and find that a time-temperature-pressure superposition principle holds over the range investigated.

Viscosity at the dynamic crossover in o-terphenyl and salol under high pressure

The viscosities of two prototypical glass formers, o-terphenyl and phenyl salicylate (salol), are shown to exhibit a change in their temperature and pressure dependences at a constant value of the viscosity. This is the first evidence of a dynamic crossover in the viscosity induced by pressure. The characteristic value associated with the change in dynamics is material dependent, but independent of temperature and pressure. These results are in accord with the previous finding, for other glass formers, that the dielectric relaxation time assumes a density-independent value at the dynamic crossover.

Diffusion in a metallic melt at the critical temperature of mode coupling theory

According to mode coupling theory, liquidlike motion becomes frozen at a critical temperature T(c) well above the caloric glass transition temperature T(g). Here, for the first time, we report on radiotracer diffusion in a supercooled Pd43Cu27Ni10P20 alloy from T(g) to the equilibrium melt. Liquidlike motion is seen to set in exactly above T(c) as evidenced by a gradual drop of the effective activation energy. This strongly supports the mode coupling scenario. Isotope effect measurements, which have never been carried out near T(c) in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.

Cohen-Grest model for the dynamics of supercooled liquids

Recent experiments have established that, at least for van der Waals glass formers, volume fluctuations contribute significantly to the slowing down of the dynamics near T(g). Accordingly, we use the Cohen-Grest (CG) free-volume model to analyze dielectric relaxation data for six van der Waals liquids. The CG equation accurately describes the structural relaxation times over broader ranges of temperature than the more common Vogel-Fulcher relation. Moreover, the CG equation requires two less adjustable parameters when the data span the Stickel temperature T(B) associated with a change in the dynamics. The characteristic temperature T0 of the CG model can be identified with T(B), suggesting that the crossover reflects onset of percolation of the free volume. The CG parameters used to fit the structural relaxation times allow the free volume per liquidlike molecule to be calculated. These results, however, are at odds with free-volume estimates extracted from pressure-volume-temperature data.

Nature of the divergence in low shear viscosity of colloidal hard-sphere dispersions

Measurements of the low-shear viscosity eta(o) with a Zimm-Crothers viscometer for dispersions of colloidal hard spheres are reported as a function of volume fraction phi up to 0.56. Nonequilibrium theories based on solutions to the two-particle Smoluchoski equation or ideal mode coupling approximations do not capture the divergence. However, the nonhydrodynamic contribution to the relative viscosity Deltaeta(o) is correlated over a wide range of volume fractions by the Doolittle and Adam-Gibbs equations, indicating an exponential divergence at phi(m)=0.625+/-0.015. The data extend the previously proposed master curve, providing a test for improved theories for the many-body thermodynamic and hydrodynamic interactions that determine the viscosity of hard-sphere dispersions.

Critical temperature T(c) and memory kernel in molecular-dynamics-simulated glass-forming Ni(0.2)Zr(0.8)

The mode-coupling theory (MCT) of dense liquids marks the dynamical glass transition by a critical temperature T(c) that is reflected in the temperature dependence of a number of physical quantities. Here, molecular-dynamics simulation data of a model adapted to Ni0.2Zr0.8 are analyzed to deduce T(c) from different quantities and to check the consistency of the estimated values. Analyzed are the critical temperature T(c) from (i). the nonvanishing nonergodicity parameters as asymptotic solutions of the MCT equations in the arrested state, (ii). the g(m) parameters describing the approach of the melt towards the arrested state on the ergodic side, (iii). the diffusion coefficients in the melt, and (iv). the alpha-relaxation time. The resulting T(c) values are found to agree within about 10%. In addition, the time dependent memory kernel is calculated from the MCT for the incoherent intermediate scattering function around T(c) and compared with the kernel obtained by inverting the molecular dynamics data for the corresponding correlator.

Calcium rubidium nitrate: mode-coupling beta scaling without factorization

The fast dynamics of viscous calcium rubidium nitrate is investigated by depolarized light scattering, neutron scattering, and dielectric loss. Fast beta relaxation evolves as in calcium potassium nitrate. The dynamic susceptibilities can be described by the asymptotic scaling law of mode-coupling theory with a shape parameter lambda=0.79; the temperature dependence of the amplitudes extrapolates to T(c) approximately equal 378 K. However, the frequencies of the minima of the three different spectroscopies never coincide, in conflict with the factorization prediction, indicating that the true asymptotic regime is unreachable.

Light-scattering spectra of supercooled molecular liquids

The light-scattering spectra of molecular liquids are derived within a generalized hydrodynamics. The wave-vector and scattering-angle dependencies are given in the most general case and the change of the spectral features from liquid to solidlike is discussed without phenomenological model assumptions for (general) dielectric systems without long-ranged order. Exact microscopic expressions are derived for the frequency dependent transport kernels, generalized thermodynamic derivatives, and the background spectra.

A Multipurpose Instrument To Measure the Vitreous Properties of Charged Colloidal Solids

We present a new high-precision light-scattering setup to study the properties of colloidal solids. It combines static light scattering, dynamic light scattering, and torsional resonance spectroscopy with a flexible and reliable preparational procedure. All three experiments can be performed quasi-simultaneously on the same mechanically undisturbed sample. Thus, unequivocal identification of glassy behavior in a comprehensive characterization dependent on interaction parameters becomes possible. A detailed description of the mechanics is given. We thoroughly tested the new apparatus on dilute colloidal samples and against commercial reference instruments. We performed a first systematic series of experiments on near index matched charged particles, varying the packing volume fraction between 0.006 and 0.11. The results indicate a new route into the glassy state different from that observed in hard-sphere systems. The range of applications, accuracy, limitations, and possible extensions of the technique are discussed. Copyright 2001 Academic Press.

Dielectric and transport properties of a supercooled symmetrical molten salt

The liquid-glass transition of the restricted primitive model for a symmetrical molten salt is studied using mode-coupling theory. The transition at high densities is predicted to obey the Lindemann criterion for melting, and the charge-density peak found in neutron-scattering experiments on ionic glass formers is qualitatively reproduced. Frequency-dependent dielectric functions, shear viscosities, and dynamical conductivities of the supercooled liquid are presented. Comparing the latter to the diffusion constant, we find that mode-coupling theory reproduces the Nernst-Einstein relation. The Stokes-Einstein radius is found to be approximately equal to the particle radius only near the high-density glass transition.

Broadband dielectric spectroscopy on glass-forming propylene carbonate

Dielectric spectroscopy covering more than 18 decades of frequency has been performed on propylene carbonate in its liquid and supercooled-liquid state. Using quasioptic submillimeter and far-infrared spectroscopy, the dielectric response was investigated up to frequencies well into the microscopic regime. We discuss the alpha process whose characteristic time scale is observed over 14 decades of frequency and the excess wing showing up at frequencies some three decades above the peak frequency. Special attention is given to the high-frequency response of the dielectric loss in the crossover regime between alpha peak and boson peak. Similar to our previous results in other glass-forming materials, we find evidence for additional processes in the crossover regime. However, significant differences concerning the spectral form at high frequencies are found. We compare our results to the susceptibilities obtained from light scattering and to the predictions of various models of the glass transition.