CiteULike: weeks's library [274 articles]

Shear-induced particle migration in binary colloidal suspensions

Denis Semwogerere — 2008-04-26T14:00:44-00:00

Physics of Fluids, Vol. 20, No. 4. (2008)

Signatures of Glass Formation in a Fluidized Bed of Hard Spheres

Daniel Goldman — 2008-01-23T18:42:00-00:00

Physical Review Letters, Vol. 96, No. 14. (2006)

We demonstrate that a fluidized bed of hard spheres during defluidization displays properties associated with formation of a glass. The final state is rate dependent, and as this state is approached, the bed exhibits heterogeneity with increasing time and length scales. The formation of a glass results in the arrest of macroscopic particle motion and thus the loss of fluidization. Microscopic motion persists in this state, but the bed can be jammed by application of a small increase in flow rate. Thus a fluidized bed can serve as a test system for studies of glass formation and jamming.

Dynamical Heterogeneity and Jamming in Glass-Forming Liquids

N Lacevic — 2008-03-28T00:05:41-00:00

J. Phys. Chem. B, Vol. 108, No. 51. (23 December 2004), pp. 19623-19633.

Abstract: The relationship between spatially heterogeneous dynamics (SHD) and jamming is studied in a glass-forming binary Lennard-Jones system via molecular dynamics simulations. It has been suggested [Phys. Rev. Lett. 2001, 86, 111]1 that the probability distribution of interparticle forces P(F) develops a peak at the glass transition temperature Tg and that the large force inhomogeneities, responsible for structural arrest in granular materials, are related to dynamical heterogeneities in supercooled liquids that form glasses. It has been further suggested that "force chains" present in granular materials may exist in supercooled liquids and may provide an order parameter for the glass transition. Our goal is to investigate the extent to which the forces experienced by particles in a glass-forming liquid are related to SHD and compare these forces to those observed in granular materials and other glass-forming systems. Our results are summarized as follows. We calculate P(F) for positive (repulsive) instantaneous forces and find no peak in P(F) at any temperature in our system, even below Tg. We also find that particles that have been localized for a long time are less likely to experience high relative force and that mobile particles experience higher relative forces at shorter time scales, indicating a correlation between pairwise forces and particle mobility. We construct force chains based on the magnitude of pairwise positive instantaneous forces. We find that force chains constructed in this manner are composed of both localized and mobile particles; therefore there is no one-to-one correspondence between force chains as defined here and locally mobile or immobile regions of the liquid. We also find that force chains do not play the same role as force chains in granular materials but may indicate a difference in the evolution of the local environment of particles with different mobility. We also discuss a possible relationship between force chains found here and the development of stringlike motion found in this and other glass-forming liquids [Phys. Rev. Lett. 1998, 80, 2338; J. Chem. Phys. 2004, 120, 4415].2,3

Measurement of an unexpectedly large shear-induced self-diffusivity in a dilute suspension of spheres

Isidro Zarraga — 2008-03-19T13:24:13-00:00

Physics of Fluids, Vol. 14, No. 7. (2002), pp. 2194-2201.

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Velocity fluctuations of initially stratified sedimenting spheres

Shang Tee — 2008-03-17T17:28:21-00:00

Physics of Fluids, Vol. 19, No. 11. (2007)

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Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition

HJ Schope — 2008-02-24T19:23:18-00:00

J. Chem. Phys., Vol. 127 (2007), 084505.

We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies.

Application of ferromagnetic nanowires to interfacial microrheology

A Anguelouch — 2008-02-19T17:32:40-00:00

Applied Physics Letters, Vol. 89, No. 11. (2006)

Confocal microscopy of colloidal particles: Towards reliable, optimum coordinates

MC Jenkins — 2008-02-19T17:20:12-00:00

Advances in Colloid and Interface Science, Vol. 136, No. 1-2. (15 January 2008), pp. 65-92.

Over the last decade, the light microscope has become increasingly useful as a quantitative tool for studying colloidal systems. The ability to obtain particle coordinates in bulk samples from micrographs is particularly appealing. In this paper we review and extend methods for optimal image formation of colloidal samples, which is vital for particle coordinates of the highest accuracy, and for extracting the most reliable coordinates from these images. We discuss in depth the accuracy of the coordinates, which is sensitive to the details of the colloidal system and the imaging system. Moreover, this accuracy can vary between particles, particularly in dense systems. We introduce a previously unreported error estimate and use it to develop an iterative method for finding particle coordinates. This individual-particle accuracy assessment also allows comparison between particle locations obtained from different experiments. Though aimed primarily at confocal microscopy studies of colloidal systems, the methods outlined here should transfer readily to many other feature extraction problems, especially where features may overlap one another.

On the relationship between structure and dynamics in a supercooled liquid

Asaph Widmer-Cooper — 2008-02-15T14:48:10-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 49. (2005), pp. S4025-S4034.

We present the dynamic propensity distribution as an explicit measure of the degree to which the dynamics in a liquid over the time scale of structural relaxation is determined by the initial configuration. We then examine, for a binary mixture of soft discs in two dimensions, the correlation between the spatial distribution of propensity and that of two local measures of configuration structure: the local composition and local free volume. While the small particles dominate the high propensity population, we find no strong correlation between either the local composition or the local free volume and the propensity. It is argued that this is a generic failure of purely local structural measures to capture the inherently non-local character of collective behaviour.

Continuous Particle Separation Through Deterministic Lateral Displacement

Lotien Huang — 2007-11-30T17:48:27-00:00

Science, Vol. 304, No. 5673. (14 May 2004), pp. 987-990.

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40seconds with a resolution of [~]10nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of [~]12%. 10.1126/science.1094567

Sheared foam as a supercooled liquid?

SA Langer — 2007-12-31T22:52:55-00:00

EPL (Europhysics Letters), Vol. 49, No. 1. (2000), pp. 68-74.

We conduct numerical simulations on a simple model of a two-dimensional steady-state sheared foam, and define a quantity G that measures stress fluctuations in the constant-area system. This quantity reduces to the temperature in an equilibrium system. We find that the relation between the viscosity and G is the same as that between viscosity and temperature in a very different system, namely a supercooled liquid. This is the first evidence of a common phenomenon linking these two systems.

Magnetic soap films and magnetic soap foams

Florence Elias — 2007-12-21T13:07:36-00:00

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 263, No. 1-3. (1 August 2005), pp. 65-75.

We investigate the physical properties of soap films and soap foams, the liquid phase of which contains a stable colloidal suspension of magnetic particles. The physical properties of such systems result from the equilibrium between the capillary forces, the gravity and the local magnetic forces. Therefore, they depend on the strength of an applied magnetic field or force, which makes it possible to act from outside on the system. We present the effect of the external field on the magnetic soap foams and films through two studies. First, we show that the dynamics of drainage in a freely suspended magnetic soap film is modified by an external magnetic field. Second, we present the control, via an external magnetic force, of the structure adopted by a monodisperse foam in a cylindrical tube. Those studies open possible applications of the ferrofluid foam, in the field of technological applications, but also as a model experimental system to study the drainage, the dynamical rheology, and the stability of soap foams.

Experimental realization of a model glass former in 2D

H König — 2007-12-19T15:56:52-00:00

The European Physical Journal E - Soft Matter, Vol. 18, No. 3. (4 November 2005), pp. 287-293.

We have studied binary two-dimensional (2D) mixtures of superparamagnetic colloidal particles interacting through magnetic dipole moments, which were induced by an external magnetic field B. By tuning B the effective system temperature could be widely adjusted. Time-dependent particle coordinates measured by video-microscopy provide radial pair-distribution functions, mean-square displacements as well as evidence for heterogeneous dynamics. Characteristic features of 3D glass formers are observed experimentally in 2D for the first time.

Evidence for Out-of-Equilibrium Crystal Nucleation in Suspensions of Oppositely Charged Colloids

Eduardo Sanz — 2007-12-19T15:45:09-00:00

Physical Review Letters, Vol. 99, No. 5. (2007)

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.

Crystallization of a Quasi-Two-Dimensional Granular Fluid

PM Reis — 2007-12-10T22:23:17-00:00

Physical Review Letters, Vol. 96, No. 25. (2006), pp. 258001-258001.

We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

Caging Dynamics in a Granular Fluid

PM Reis — 2007-11-30T02:40:21-00:00

Physical Review Letters, Vol. 98, No. 18. (2007)

We report an experimental investigation of the caging motion in a uniformly heated granular fluid for a wide range of filling fractions, . At low the classic diffusive behavior of a fluid is observed. However, as is increased, temporary cages develop and particles become increasingly trapped by their neighbors. We statistically analyze particle trajectories and observe a number of robust features typically associated with dense molecular liquids and colloids. Even though our monodisperse and quasi-2D system is known to not exhibit a glass transition, we still observe many of the precursors usually associated with glassy dynamics. We speculate that this is due to a process of structural arrest provided, in our case, by the presence of crystallization.

Correlation between Dynamic Heterogeneity and Medium-Range Order in Two-Dimensional Glass-Forming Liquids

Takeshi Kawasaki — 2007-12-07T05:03:46-00:00

Physical Review Letters, Vol. 99, No. 21. (2007)

A glassy state of matter results if crystallization is avoided upon cooling or increasing density. However, the physical factors controlling the ease of vitrification and nature of the glass transition remain elusive. Using numerical simulations of polydisperse hard disks, we find a direct relation between medium-range crystalline ordering and the slow dynamics which characterizes the glass transition. This suggests an intriguing scenario that the strength of frustration controls both the ease of vitrification and nature of the glass transition. Vitrification may be a process of hidden crystalline ordering under frustration, at least in our system.

Translation-Rotation Paradox for Diffusion in Glass-Forming Polymers: The Role of the Temperature Dependence of the Relaxation Time Distribution

David Hall — 2007-12-06T15:38:54-00:00

Physical Review Letters, Vol. 79, No. 1. (7 July 1997), 103.

Comparisons are made of the translational and rotational diffusion of small-molecule probes in a polymer near its glass transition temperature; T g . In the rubbery state; 1.1 T g > T > T g ; translational diffusion is much less temperature dependent than rotational reorientation; in a “quenched” glass; translation and rotation have similar temperature dependencies. This is explained to be a consequence of the fact that in the rubbery state near T g the breadth of the polymer relaxation distribution is strongly temperature dependent; while in the quenched glass it is temperature invariant.

Confinement, entropy, and single-particle dynamics of equilibrium hard-sphere mixtures

Jeetain Mittal — 2007-12-01T12:11:26-00:00

The Journal of Chemical Physics, Vol. 127, No. 15. (2007)

High-Throughput Synthesis of Anisotropic Colloids via Holographic Lithography

JH Moon — 2007-11-29T01:55:01-00:00

Advanced Materials, Vol. 19 (2007), pp. 2508-2512.

Monodisperse anisotropic colloids are synthesized (see figure) via holographic lithography and using chemically amplified photoresists. The holographic lithography offers a high-throughput and flexible route to produce particles. To engineer colloidal particles, we apply direct modification of the photoresist surface either uniformly or selectively through chemical coupling, physical grafting, and selective metal deposition.

Control of the Liquid-Liquid Transition in a Molecular Liquid by Spatial Confinement

Rei Kurita — 2007-11-28T18:24:35-00:00

Physical Review Letters, Vol. 98, No. 23. (2007)

Generally, phase transitions are seriously affected by spatial confinement. This effect is important for its own sake, but also for applications to nanotechnology. Here we report the first systematic experimental study on confinement effects on a liquid-liquid transition of a molecular liquid. We found that one liquid can be transformed into another purely by spatial confinement. This indicates that the liquid state cannot be specified by the temperature and pressure alone, but it is also affected by its size in a discontinuous manner: the phase of a liquid in a narrow space can, in principle, be different from that in the bulk. This finding would deepen our basic understanding of the liquid state.

Dynamics of a rigid rod in a glassy medium

AJ Moreno — 2007-11-28T01:19:10-00:00

Europhys. Lett., Vol. 67 (2004), pp. 820-826.

We present simulations of the motion of a single rigid rod in a disordered static 2d array of disk-like obstacles. The rotational, DR, and center-of-mass translational, DCM, diffusion constants are calculated for a wide range of rod length L and density of obstacles ρ. It is found that DCM follows the behavior predicted by kinetic theory for a hard disk with an effective radius R(L). A dynamic crossover is observed in DR for L comparable to the typical distance between neighboring obstacles dnn. Using arguments from kinetic theory and reptation, we rationalize the scaling laws, dynamic exponents, and prefactors observed for DR. In analogy with the enhanced translational diffusion observed in deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for L > 0.6dnn.

Correlated orientational and translational motions in supercooled liquids

Sarika Bhattacharyya — 2007-11-28T00:55:12-00:00

The Journal of Chemical Physics, Vol. 117, No. 6. (2002), pp. 2741-2746.

We have carried out NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture to gain insight into the nature of orientational relaxation (OR) in a viscous liquid. At high pressures when the liquid is highly viscous, the OR is found to occur mainly via correlated hopping, sometimes involving participation of several neighboring atoms, placed in a ring like tunnel. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be gated, restricted primarily by the entropic bottlenecks, with the orientational motion becoming increasingly slower than the translation as the pressure is increased. Orientational relaxation is found to occur with a wide distribution of decay times.

Anisotropic diffusion of nonspherical molecules in dense liquids: A molecular dynamics simulation of isolated ellipsoids in the sea of spheres

S Ravichandran — 2007-11-28T00:50:06-00:00

The Journal of Chemical Physics, Vol. 111, No. 16. (1999), pp. 7505-7511.

Detailed molecular dynamics simulations of the rotational and the translational motions of Gay–Berne ellipsoids in a sea of Lennard-Jones spheres have been carried out. It is found that while the translational motion of an ellipsoid is isotropic at low density, it becomes increasingly anisotropic with density until the ratio of the parallel to the perpendicular diffusion coefficients becomes nearly equal to the value of the aspect ratio at high density. The latter is in agreement with the prediction of Navier–Stokes hydrodynamics with slip boundary condition. The product of the translational diffusion coefficient and the rotational correlation time also attains a hydrodynamic-like density independent behavior only at high density. The reorientational correlation function becomes nonexponential at high density and low temperature where it also develops a slow decay. The perpendicular component of the velocity time correlation function exhibits a clear double minimum, only at high density, which becomes more pronounced as the aspect ratio is increased.

Alternative Explanation of the Difference between Translational Diffusion and Rotational Diffusion in Supercooled Liquids

KL Ngai — 2007-11-28T00:42:04-00:00

J. Phys. Chem. B, Vol. 103, No. 48. (2 December 1999), pp. 10684-10694.

Abstract: We offer an alternative explanation of the breakdown of Debye-Stokes-Einstein and Stokes-Einstein relations observed as enhanced translational diffusion compared with rotational diffusion of probe molecules in supercooled liquids and polymers close to the glass transition temperature. By showing that the breakdowns of Debye-Stokes-Einstein and Stokes-Einstein relations in glass-forming liquids are special cases of a more general phenomenon, it becomes clear that a more general explanation than spatially heterogeneous dynamics is needed. In the framework of the coupling model, the explanation is based on the fact that different dynamic variables weigh the intermolecular cooperativity differently and have different coupling parameters (i.e., degrees of intermolecular cooperativity), n, which enter into the stretch exponents of their correlation functions, <(0)(t)> = exp[-(t/)1-n], represented in the Kohlrausch form. In some of the applications we made, the values of n's are known from experiment and the difference between them is absolutely clear. The explanation of the difference between the 's and their temperature dependencies then becomes quantitative, requiring no adjustable parameter (Ngai, K. L.; Mashimo, S.; Fytas, G. Macromolecules 1988, 21, 3030). An exception is the present explanation of the breakdown of Stokes-Einstein relation where experimental technique has not yet been developed to determine the microscopic correlation function and, hence, the coupling parameter of translational diffusion. The explanation from the coupling model is based on the assumption that the known coupling parameter of rotational diffusion is larger than that of translational diffusion, which has been justified on theoretical grounds and can be falsified experimentally when the latter becomes known in the future. The effect depends on the degree of probe participation in the cooperative dynamics with the host molecules characterized by the ratio, c/, of the probe rotational relaxation time c to the -relaxation time of the neat host . The observed variation in the magnitude of the enhanced translation with the size of the probes in supercooled liquids and polymers is explained. In the process, we have established a correlation between the enhanced translation and c/ as well as a correlation between the Kohlrausch exponent of the rotation correlation function and c/. These two correlations when combined give rise to the correlation between enhanced translational diffusion and (1-) established earlier by Ediger and co-workers. The present explanation of enhanced translational diffusion can also explain the recent findings of (a) a parallel enhancement of dielectric relaxation in supercooled liquids by Chang and Sillescu and (b) a breakdown of the Debye-Stokes-Einstein relation of deploarized light scattering data of diglycidyl ether of bisphenol A by Comez et al. On the other hand, the spatially heterogeneous dynamics explanation of enhanced translation no longer can explain these results.

Reorientation and translation of individual dye molecules in a polymer matrix

A Schob — 2007-11-28T00:34:21-00:00

European Polymer Journal, Vol. 40, No. 5. (May 2004), pp. 1019-1026.

The orientational and translational motion of individual dye molecules embedded in a polymer matrix is studied in the temperature regime above the glass transition. The rotational diffusion close to the glass transition is heterogeneous on the single molecule level and few sudden changes in the reorientational speed of single molecules are found. The exchange between these reorientational speeds is found to be one order of magnitude slower than the reorientational time constant of the molecules. Translational motion can be clearly identified at about 1.2 Tg. However, the translational diffusion shows no signs of heterogeneity on the timescale of our experiments, from which we conclude, that the timescale of the exchange process between microenvironments has become too fast or that no heterogeneity exists at the temperatures above 1.2 Tg.

Heterogeneity at the glass transition: what do we know?

Hans Sillescu — 2007-11-28T00:27:28-00:00

Journal of Non-Crystalline Solids, Vol. 307-310 (September 2002), pp. 16-23.

We critically discuss the information that can be obtained from experiments with respect to the existence, the life time, and the length scale of dynamical heterogeneity in glass-forming liquids. The ability to select a dynamically distinguishable subensemble and observe its return to the full equilibrium ensemble is illustrated by examples from multi-dimensional NMR. We also discuss non-resonant hole burning spectroscopy as an example for which two separate time scales are involved.

Direct observation of molecular cooperativity near the glass transition

E Vidalrussell — 2007-11-28T00:22:47-00:00

Nature, Vol. 408, No. 6813. (7 December 2000), pp. 695-698.

The increasingly sluggish response of a supercooled liquid as it nears its glass transition1 (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated2 to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses3 and in simulations of binary liquids well above the glass transition4. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking5. Important issues to be resolved include the precise form of the cooperativity and its length scale6, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity7, 8, 9 in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.

Spatially heterogeneous dynamics in liquids: insights from simulation

Sharon Glotzer — 2007-11-28T00:15:59-00:00

Journal of Non-Crystalline Solids, Vol. 274, No. 1-3. (September 2000), pp. 342-355.

In this paper, review of recent theoretical and simulation work on dynamical heterogeneity and correlated particle motion in glass-forming liquids and polymers is presented. Evidence for increasing mobility fluctuations in these systems with decreasing temperature, and the relationship between dynamical heterogeneity and decoupling of diffusion and structural relaxation is described. The relationship between dynamical heterogeneity, string-like collective motion, the clustering of strings, and mode-coupling theory is briefly discussed.

Heterogeneity at the glass transition: a review

Hans Sillescu — 2007-11-28T00:10:01-00:00

Journal of Non-Crystalline Solids, Vol. 243, No. 2-3. (February 1999), pp. 81-108.

Theoretical concepts and experimental evidence of heterogeneity in glass-forming liquids and polymers are reviewed. The main purpose is to provide an introduction to theoretical developments and recent experiments which have led to rapidly increasing knowledge. Realizing that there is no consensus in regard to the various scenarios of the glass transition starting from rather different assumptions we try to give a balanced overview although we also compare and interrelate some of the approaches. The experimental part describes recent nuclear magnetic resonance, dielectric, and optical experiments from which dynamically distinguishable subensembles can be selected thus proving the existence of a well defined dynamical heterogeneity.

Dynamics of a glass-forming triepoxide studied by dielectric spectroscopy

S Corezzi — 2007-11-27T18:54:18-00:00

Journal of Physics: Condensed Matter, Vol. 11, No. 50. (1999), pp. 10297-10314.

Dielectric measurements of an epoxy resin, N, N-diglycidyl-4-glycidyloxyaniline, have been carried out in the supercooled and glassy phase over a broad frequency range (102-6 ×109 Hz). The measurements reveal electrical transport due to ionic impurities as well as three different dipolar relaxations - in addition to the alpha- and beta-relaxation, a slower alpha´-relaxation is recognized, whose loss peak is disclosed after subtraction of the dc conductivity contribution. The glass transition is found to affect markedly the secondary relaxation, whose strength and shape parameters change across Tg. The major inference from the results concerns the existence of a transition in the dynamics, occurring some tens of degrees above Tg, in the vicinity of the temperature TS where the peaks of the alpha- and beta-relaxations merge. Evidence in favour of such a transition is given by: (i) the change in the temperature dependence of the alpha-relaxation time; (ii) independently, the change in the temperature dependence of the dc conductivity; (iii) the breakdown of the Debye-Stokes-Einstein model, replaced at lower temperatures by a fractional regime. Concerning the alpha´-process, it shows a Vogel-Fulcher behaviour with the same temperature T0 as the alpha-relaxation but, unlike this last, it is not involved in a splitting phenomenon with the beta-relaxation. Several hypotheses concerning the nature of the alpha´-process are explored.

Heterogeneity at the Glass Transition: Translational and Rotational Self-Diffusion

I Chang — 2007-11-27T18:50:35-00:00

J. Phys. Chem. B, Vol. 101, No. 43. (23 October 1997), pp. 8794-8801.

Abstract: Self-diffusion coefficients, D, have been measured in the glass forming liquids salol, glycerol, phenolphthaleine dimethyl ether (PDE), cresolphthaleine dimethyl ether (CDE), and -trinaphthylbenzene (TNB) in the supercooled regime. The NMR static magnetic field gradient technique was applied where D >10-14 m2 s-1 can be attained. The results are similar to previous diffusion experiments where an enhancement of translational diffusion was found in comparison with rotational diffusion and shear viscosity. Various models of spatial heterogeneity are related to a phenomenological environmental fluctuation model in view of recent diffusion and relaxation data close to the glass transition.

Test of the fractional Debye-Stokes-Einstein equation in low-molecular-weight glass-forming liquids under condition of high compression

Stella Bielowka — 2007-11-27T18:46:25-00:00

Physical Review E, Vol. 63, No. 6. (22 May 2001), 062301.

From temperature studies at ambient pressure; it was pointed out for several glass-forming liquids that the α-relaxation time (τ) can be related to the dc-ionic conductivity (σ) through the phenomenological fractional Debye-Stokes-Einstein (DSE) equation. In the present paper we test the validity of fractional DSE equation for relaxation data obtained from pressure variable experiments. To this end we carried out broadband dielectric measurements (10 mHz–10 MHz) in a wide range of pressures (0.1–300 MPa). The material under study were N;N -diglycidyl-4-glycidyloxyaniline and N;N -diglycidylaniline. As a result we found that the fractional DSE equation is also obeyed for pressure pathways.

On the scaling in the rotational dynamics of molecular probes in salol and ortho-terphenyl: a possible role of the energy landscape basins

L Andreozzi — 2007-11-27T18:44:13-00:00

Journal of Physics: Condensed Matter, Vol. 18, No. 3. (2006), pp. 931-940.

The reorientational dynamics of a stiff paramagnetic tracer dissolved in the glass former salol is investigated by means of electron spin resonance over a broad temperature range. The Debye-Stokes-Einstein law describing rotational diffusion in simple liquids is found to break down in the supercooled region where the diffusion is less temperature dependent than the viscosity. Over a large temperature interval a simple power law relates diffusion and viscosity, whereas at lower temperatures the decoupling is stronger and an activated dynamics is observed. These experiments are discussed together with previous data concerning other tracer/glass former couples. Starting from some observed common features, an attempt is made to obtain a unifying interpretation of the data in the framework of the energy landscape picture.

A study of the Debye - Stokes - Einstein law in supercooled fluids

L Andreozzi — 2007-11-27T18:42:45-00:00

Journal of Physics: Condensed Matter, Vol. 8, No. 47. (1996), pp. 9605-9608.

The reorientation of a nearly spherical paramagnetic tracer dissolved in supercooled o-terphenyl is studied by electron spin resonance in the range 200 K < T < 370 K. For T > 300 K the rotational correlation time follows the Debye - Stokes - Einstein law. On decreasing the temperature, the rotational motion of the probe and the viscosity decouple. In particular, in the region around T = 290 K it is found that with .

Evidence of a fractional Debye-Stokes-Einstein law in supercooled o-terphenyl

L Andreozzi — 2007-11-27T18:40:06-00:00

Europhys. Lett., Vol. 38 (1997), pp. 669-674.

The rotational dynamics of nearly spherical and cylindrical tracers dissolved in supercooled o-terphenyl is studied via the Electron Spin Resonance spectroscopy. On decreasing the temperature, a sharp crossover is observed from the regime where the Debye-Stokes-Einstein law (DSE) holds to a new one which is accounted for by a fractional DSE (FDSE) Dr propto (η/T)-ξ with ξ ≤ 1. Dr is the rotational diffusion constant, η the shear viscosity and T the temperature. Tumbling and spinning of the cylindrical tracer exhibit distinct crossovers. A simple model of FDSE based on the presence of dynamic heterogeneities is presented.

Molecular Reorientation in Liquids and Gases

BJ Berne — 2007-11-27T17:34:51-00:00

The Journal of Chemical Physics, Vol. 49, No. 7. (1968), pp. 3125-3129.

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Test of the Einstein-Debye Relation in Supercooled Dibutylphthalate at Pressures up to 1.4 GPa

M Paluch — 2007-11-27T16:27:10-00:00

Physical Review Letters, Vol. 90, No. 17. (2 May 2003), 175702.

Broadband dielectric measurements were carried out on di- n -butyl phthalate (DBP) under isothermal conditions at hydrostatic pressures up to 1.6 GPa. A comparison of the dielectric relaxation times with the viscosity revealed that no breakdown of the Einstein-Debye relation is induced by high compression. This absence of any decoupling is attributed to the weak intermolecular cooperativity of DBP and its negligible change with pressure. Because of the latter; the dielectric spectra conform to time-pressure superpositioning.

Enhanced diffusion upon approaching the kinetic glass transition

Cliff Liu — 2007-11-27T16:18:47-00:00

Physical Review E, Vol. 53, No. 1. (January 1996), pp. 799-802.

The apparent failure of the Stokes-Einstein law in strongly supercooled liquids has provoked recent experimental and theoretical studies. In an attempt to explain this phenomenon; we study the diffusion in a dynamically disordered continuum in which small; compact regions of greater diffusivity appear and disappear in time. Within the confines of the model; we show that a systematic increase in the ratio of the diffusivity of fluidized domains to the background diffusivity appears to be the single most important factor in explaining the deviation from the Stokes relation. © 1996 The American Physical Society.

Reply to “Comment on `Translation-rotation paradox for diffusion in fragile glass-forming liquids' ”

Frank Stillinger — 2007-05-16T17:50:04-00:00

Physical Review E, Vol. 53, No. 3. (March 1996), pp. 2995-2997.

We discuss several aspects of the fluidized domain model advanced earlier to explain rotational and translational diffusion rates for fragile glass formers and; in particular; we point out weaknesses in the preceding Comment [Sillescu; Phys. Rev. E 53 ; 2992 (1996)]. Recent experimental results are cited to refine estimates of domain characteristics for orthoterphenyl at T g and to suggest that domain heterogeneity in space and time underlies stretched exponential relaxation behavior for rotational motion. © 1996 The American Physical Society.

Comment on `Translation-rotation paradox for diffusion in fragile glass-forming liquids'

Hans Sillescu — 2007-05-16T17:46:28-00:00

Physical Review E, Vol. 53, No. 3. (March 1996), pp. 2992-2994.

F. H. Stillinger and J. A. Hodgdon [Phys. Rev. E 50 ; 2064 (1994)] have proposed a ‘‘fluidized domain’’ model to explain that the diffusion coefficient ratio D trans / D rot increases by a factor of ∼10 2 on approaching the glass transition from above in fragile liquids. In their numerical estimates they make ad hoc assumptions that lead to a very small volume fraction φ 0 ∼10 -5 of the fluidized domains. It is shown how these assumptions can be avoided and φ 0 ∼10 -1 results from experimental values of D trans ; D rot ; and the width of the D rot distribution. A modification of the Stillinger-Hodgdon model can be derived from the two-state model of Beckert and Pfeifer [Ann. Phys. (Leipzig) 16 ; 262 (1965)] and yields experimental estimates of a few nanometers for the fluidized domains. © 1996 The American Physical Society.

Breakdown of the Stokes--Einstein relation in supercooled liquids

Gilles Tarjus — 2007-11-27T15:56:06-00:00

The Journal of Chemical Physics, Vol. 103, No. 8. (1995), pp. 3071-3073.

The Stokes–Einstein and Stokes–Einstein–Debye relations hold well in nonsupercooled liquids; however, sizeable deviations from the former appear in supercooled liquids, leading to a ‘‘decoupling’’ of translational diffusion from viscosity and reorientational relaxation. We attribute this breakdown and this ‘‘decoupling’’ to the existence of structured domains in the supercooled liquid.

Fractional Stokes-Einstein and Debye-Stokes-Einstein Relations in a Network-Forming Liquid

Stephen Becker — 2007-06-30T17:34:58-00:00

Physical Review Letters, Vol. 97, No. 5. (2006), pp. 055901-055901.

We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations for translational and rotational motion in a prototypical model of a network-forming liquid, the ST2 model of water. We find that the emergence of fractional SE and DSE relations at low temperature is ubiquitous in this system, with exponents that vary little over a range of distinct physical regimes. We also show that the same fractional SE relation is obeyed by both mobile and immobile dynamical heterogeneities of the liquid.

How do molecules move near $T_g$? Molecular rotation of six probes in o-terphenyl across 14 decades in time

Marcus Cicerone — 2006-12-07T16:37:01-00:00

The Journal of Chemical Physics, Vol. 102, No. 1. (1995), pp. 471-479.

Time resolved optical spectroscopy was used to observe molecular rotation over more than 14 decades in time for six probes in o-terphenyl (OTP). In contrast to previous studies, probe rotation times are found to depend significantly upon probe size in the deeply supercooled regime. Systematic deviations from the temperature dependence of the Debye–Stokes–Einstein equation are observed, however, these deviations are relatively small. These observations are inconsistent with some models of cooperative molecular motion near Tg which invoke rigid aggregates or locally liquidlike regions. The width of the relaxation spectrum (characterized by the KWW parameter) systematically decreases with increasing probe size. Near Tg, the largest probe (rubrene) rotates with nearly a single exponential correlation function. Based on the observed trend in , it is estimated that OTP is homogeneous on length scales greater than 2.5 nm at Tg. ©1995 American Institute of Physics.

Photobleaching technique for measuring ultraslow reorientation near and below the glass transition: tetracene in o-terphenyl

Marcus Cicerone — 2007-06-28T20:19:04-00:00

Journal of Physical Chemistry, Vol. 97, No. 40. (1993), pp. 10489-10497.

no abstract

Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering

I Chang — 2007-06-26T23:53:25-00:00

Journal of Non-Crystalline Solids, Vol. 172-174 (1994), pp. 248-255.

It has been shown that translational diffusion coefficients, D[t], in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T≤1.2 T[g] and can be described by D[t]∼η[-ξ] with ξ<1, WHEREAS D[r]∼η[-1] applies for the mean rotational diffusion coefficients, D[r], down to the glass transition temperature, T[g]. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to T[g]

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

F Fujara — 2007-05-16T18:11:08-00:00

Zeitschrift für Physik B Condensed Matter, Vol. 88, No. 2. (21 June 1992), pp. 195-204.

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toDt=3·10-14 m2s-1 using a1H-NMR technique applying static field gradients up to 53T m-1 In a range of more than two decades theDt values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureTg. A change of mechanism is found for translational diffusion atTc˜1.2Tg whereDt is proportional to the inverse shear viscosity?-1 atT>Tc butDt ~?? with ?=0.75 atTTc. Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal to?-1 down toTg. Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.

Time-correlation functions and molecular motion

G Williams — 2007-11-26T19:49:07-00:00

Chem. Soc. Rev., Vol. 7 (1978), pp. 89-131.

The time-independent thermodynamic properties of molecular liquids and solids and their rationalization in terms of time-independent statistical mechanics receive considerable attention in undergraduate courses in chemistry and physics. The dynamical properties of such systems involve translational, vibrational, and reorientational modes of molecular motion and whereas the vibrational motions, which give rise to infrared, Raman, and neutron-scattering spectra, are well covered in undergraduate courses, considerably less attention is given to translational and reorientational modes of motion. Many of the important physical properties of liquids and solids relate to the latter modes of motion, and in recent years there has been a considerable interest in their study using many experimental techniques. Table 1 lists a selection of the techniques, and will be discussed below. In parallel with experimental studies, a sound theoretical framework has emerged, based on time-correlation functions, which allows translational and reorientational motions to be described formally and in physical terms.

Translational and reorientational heterogeneity in the glass-forming liquid $Ca_0.4K_0.6(NO_3)_1.4$

Mauro Ribeiro — 2007-11-26T19:14:32-00:00

Phys. Chem. Chem. Phys., Vol. 6 (2004), pp. 771-774.

Molecular dynamics (MD) simulations were performed in order to reveal dynamical heterogeneity in the glass-forming liquid Ca0.4K0.6(NO3)1.4(CKN). A polarizable model for the nitrate anion has been used in a 10.0 ns simulation of 996 ions at 450 K, which is close to the previously estimated glass transition temperature for this model (Tg380 K). Mobile and immobile ions within 500.0 ps time windows were identified, and short-range spatial correlations between them were analysed. Mobile ions are strongly correlated, but they are much less correlated with immobile ones. In the case of the NO3– species, the sub-ensemble of translationally mobile ions also results in a rapidly decaying reorientational correlation function. Thus, if one probes spatial heterogeneity of rotational degrees of freedom in CKN, one will be also probing spatial heterogeneity of translational degrees of freedom, and vice-versa. By using a reorientation analogous of the van Hove correlation function, small angle rotational diffusion in the dynamics of mobile anions, and large angular jumps in the dynamics of immobile ones were observed.

On the mechanism of reorientational and structural relaxation in supercooled liquids: The role of border dynamics and cooperativity

Joohyun Kim — 2007-06-26T21:45:18-00:00

The Journal of Chemical Physics, Vol. 121, No. 9. (2004), pp. 4237-4245.

Molecular dynamics simulation and analysis based upon the many-body potential energy landscape (PEL) are employed to characterize single molecule reorientation and structural relaxation, and their interrelation, in deeply supercooled liquid CS2. The rotational mechanism changes from small-step Debye diffusion to sudden large angle reorientation (SLAR) as the temperature falls below the mode-coupling temperature Tc. The onset of SLAR is explained in terms of the PEL; it is an essential feature of low-T rotational dynamics, along with the related phenomena of dynamic heterogeneity and the bifurcation of slow and fast relaxation processes. A long trajectory in which the system is initially trapped in a low energy local minimum, and eventually escapes, is followed in detail, both on the PEL and in real space. During the trapped period, "return" dynamics occurs, always leading back to the trap. Structural relaxation is identified with irreversible escape to a new trap. These processes lead to weak and strong SLAR, respectively; strong SLAR is a clear signal of structural relaxation. Return dynamics involves small groups of two to four molecules, while a string-like structure composed of all the active groups participates in the escape. It is proposed that, rather than simple, nearly instantaneous, one-dimensional barrier crossings, relaxation involves activation of the system to the complex, multidimensional region on the borders of the basins of attraction of the minima for an extended period. ©2004 American Institute of Physics.