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28 Mar 2013

Volume 138, Issue 12, Articles (12xxxx)

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J. Chem. Phys. 138, 124701 (2013); http://dx.doi.org/10.1063/1.4794685 (9 pages)

Luying Wang, Randall S. Dumont, and James M. Dickson
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Preface: Special Topic on the Glass Transition

Marsha I. Lester, Editor

J. Chem. Phys. 138, 12A101 (2013); http://dx.doi.org/10.1063/1.4796105 (1 page)

Online Publication Date: 26 March 2013

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This Special Topic on the Glass Transition contains a perspective article and a collection of original research articles that showcase recent experimental and theoretical advances in the field. This special issue provides a timely discussion of modern developments in our understanding of the behavior of supercooled liquids and amorphous materials, which have implications in diverse fields ranging from biology to materials science.
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64.70.P- Glass transitions of specific systems
61.43.-j Disordered solids
back to top Perspective
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Perspective: The glass transition

Giulio Biroli and Juan P. Garrahan

J. Chem. Phys. 138, 12A301 (2013); http://dx.doi.org/10.1063/1.4795539 (13 pages) | Cited 2 times

Online Publication Date: 26 March 2013

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We provide here a brief perspective on the glass transition field. It is an assessment, written from the point of view of theory, of where the field is and where it seems to be heading. We first give an overview of the main phenomenological characteristics, or “stylised facts,” of the glass transition problem, i.e., the central observations that a theory of the physics of glass formation should aim to explain in a unified manner. We describe recent developments, with a particular focus on real space properties, including dynamical heterogeneity and facilitation, the search for underlying spatial or structural correlations, and the relation between the thermal glass transition and athermal jamming. We then discuss briefly how competing theories of the glass transition have adapted and evolved to account for such real space issues. We consider in detail two conceptual and methodological approaches put forward recently, that aim to access the fundamental critical phenomenon underlying the glass transition, be it thermodynamic or dynamic in origin, by means of biasing of ensembles, of configurations in the thermodynamic case, or of trajectories in the dynamic case. We end with a short outlook.
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64.70.qj Dynamics and criticality
64.70.qd Thermodynamics and statistical mechanics
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Are the dynamics of a glass embedded in its elastic properties?

Marcel Potuzak, Xiaoju Guo, Morten M. Smedskjaer, and John C. Mauro

J. Chem. Phys. 138, 12A501 (2013); http://dx.doi.org/10.1063/1.4730525 (5 pages) | Cited 1 time

Online Publication Date: 2 January 2013

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The low temperature dynamics of glass are critically important for many high-tech applications. According to the elastic theory of the glass transition, the dynamics of glass are controlled by the evolution of shear modulus. In particular, the elastic shoving model expresses dynamics in terms of an activation energy required to shove aside the surrounding atoms. Here, we present a thorough test of the shoving model for predicting the low temperature dynamics of an oxide glass system. We show that the nonequilibrium viscosity of glass is governed by additional factors beyond changes in shear modulus.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
61.43.Fs Glasses

Are polar liquids less simple?

D. Fragiadakis and C. M. Roland

J. Chem. Phys. 138, 12A502 (2013); http://dx.doi.org/10.1063/1.4769262 (6 pages)

Online Publication Date: 2 January 2013

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Strong correlation between equilibrium fluctuations under isochoric conditions of the potential energy, U, and the virial, W, is a characteristic of liquids that implies the presence of certain dynamic properties, such as density scaling of the relaxation times and isochronal superpositioning of the relaxation function. In this work we employ molecular dynamics simulations on methanol and two variations, lacking hydrogen bonds and a dipole moment, to assess the connection between the correlation of U and W and these dynamic properties. We show, in accord with prior results of others [T. S. Ingebrigtsen, T. B. Schrøder, and J. C. Dyre, Phys. Rev. X 2, 011011 (2012)10.1103/PhysRevX.2.011011], that simple van der Waals liquids exhibit both strong correlations and the expected dynamic behavior. However, for polar liquids this correspondence breaks down—weaker correlation between U and W is not associated with worse conformance to density scaling or isochronal superpositioning. The reason for this is that strong correlation between U and W only requires their proportionality, whereas the expected dynamic behavior depends primarily on constancy of the proportionality constant for all state points. For hydrogen-bonded liquids, neither strong correlation nor adherence to the dynamic properties is observed; however, this nonconformance is not directly related to the concentration of hydrogen bonds, but rather to the greater deviation of the intermolecular potential from an inverse power law (IPL). Only (hypothetical) liquids having interactions governed strictly by an IPL are perfectly correlating and exhibit the consequent dynamic properties over all thermodynamic conditions.
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64.30.Jk Equations of state of nonmetals
65.20.Jk Studies of thermodynamic properties of specific liquids
61.50.Lt Crystal binding; cohesive energy

Glass transition of poly(ethylmethacrylate) admixed and bound to nanoparticles

Cornelius Friedrichs, Sebastian Emmerling, Gunnar Kircher, Robert Graf, and Hans Wolfgang Spiess

J. Chem. Phys. 138, 12A503 (2013); http://dx.doi.org/10.1063/1.4769252 (6 pages)

Online Publication Date: 2 January 2013

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The chain dynamics at the glass transition of poly(ethylmethacrylate) in the bulk is compared to that of mixtures of the polymer with nanoparticles by advanced NMR methods. In order to make the two components compatible, the particles were functionalized with the polymer itself. Particular emphasis is placed on the extended local chain conformations of this polymer accessible by 13C NMR spectroscopy. The isotropization dynamics of these extended conformations is only slightly changed in the mixtures, but is significantly slowed down by attachment of the chains to the nanoparticles themselves. The slowing down is studied at various distances from the nanoparticle and is observed for most of the attached chains segments except for the chain ends. The results are put into perspective to the glass transition in polymers attached to surfaces, thin polymer layers, and the chain dynamics of star polymers.
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64.70.pj Polymers
76.60.Cq Chemical and Knight shifts

Modeling the relaxation of polymer glasses under shear and elongational loads

S. M. Fielding, R. L. Moorcroft, R. G. Larson, and M. E. Cates

J. Chem. Phys. 138, 12A504 (2013); http://dx.doi.org/10.1063/1.4769253 (11 pages)

Online Publication Date: 2 January 2013

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Glassy polymers show “strain hardening”: at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes then to re-vitrify. Here, we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a “crinkle factor” describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally, we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
64.70.pj Polymers
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Higher-order correlation functions and nonlinear response functions in a Gaussian trap model

Gregor Diezemann

J. Chem. Phys. 138, 12A505 (2013); http://dx.doi.org/10.1063/1.4769254 (7 pages)

Online Publication Date: 2 January 2013

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The four-time correlation function of a general dynamical variable obeying Gaussian statistics is calculated for the trap model with a Gaussian density of states. It is argued that for energy-independent variables this function is reminiscent of the four-time functions that have been discussed earlier in the interpretation of the results of four-dimensional NMR experiments on supercooled liquids. Using an approximative relation between the four-time correlation function and the cubic response function the nonlinear susceptibility is calculated and the results are compared with the corresponding ones resulting from an exact calculation. It is found that the results of the approximation change the qualitative behavior of the modulus of the susceptibility. Whereas in the exact calculation a peak is found in the modulus in most cases, depending on temperature and the additional model parameters no such peak occurs in the approximation. This difference has its origin mainly in an incorrect estimate of the static response. The results are discussed in relation to recent experimental findings.
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02.50.Ng Distribution theory and Monte Carlo studies
76.60.-k Nuclear magnetic resonance and relaxation
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Multiple length and time scales of dynamic heterogeneities in model glass-forming liquids: A systematic analysis of multi-point and multi-time correlations

Kang Kim and Shinji Saito

J. Chem. Phys. 138, 12A506 (2013); http://dx.doi.org/10.1063/1.4769256 (12 pages) | Cited 1 time

Online Publication Date: 2 January 2013

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We report an extensive and systematic investigation of the multi-point and multi-time correlation functions to reveal the spatio-temporal structures of dynamic heterogeneities in glass-forming liquids. Molecular dynamics simulations are carried out for the supercooled states of various prototype models of glass-forming liquids such as binary Kob–Andersen, Wahnström, soft-sphere, and network-forming liquids. While the first three models act as fragile liquids exhibiting super-Arrhenius temperature dependence in their relaxation times, the last is a strong glass-former exhibiting Arrhenius behavior. First, we quantify the length scale of the dynamic heterogeneities utilizing the four-point correlation function. The growth of the dynamic length scale with decreasing temperature is characterized by various scaling relations that are analogous to the critical phenomena. We also examine how the growth of the length scale depends upon the model employed. Second, the four-point correlation function is extended to a three-time correlation function to characterize the temporal structures of the dynamic heterogeneities based on our previous studies [K. Kim and S. Saito, Phys. Rev. E 79, 060501(R) (2009)10.1103/PhysRevE.79.060501; K. Kim and S. Saito, J. Chem. Phys. 133, 044511 (2010)10.1063/1.3464331] . We provide comprehensive numerical results obtained from the three-time correlation function for the above models. From these calculations, we examine the time scale of the dynamic heterogeneities and determine the associated lifetime in a consistent and systematic way. Our results indicate that the lifetime of the dynamical heterogeneities becomes much longer than the α-relaxation time determined from a two-point correlation function in fragile liquids. The decoupling between the two time scales is remarkable, particularly in supercooled states, and the time scales differ by more than an order of magnitude in a more fragile liquid. In contrast, the lifetime is shorter than the α-relaxation time in tetrahedral network-forming strong liquid, even at lower temperatures.
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64.70.pm Liquids
61.20.Ja Computer simulation of liquid structure
61.20.Lc Time-dependent properties; relaxation
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Dynamic criticality at the jamming transition

Atsushi Ikeda, Ludovic Berthier, and Giulio Biroli

J. Chem. Phys. 138, 12A507 (2013); http://dx.doi.org/10.1063/1.4769251 (17 pages) | Cited 2 times

Online Publication Date: 2 January 2013

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We characterize vibrational motion occurring at low temperatures in dense suspensions of soft repulsive spheres over a broad range of volume fractions encompassing the jamming transition at (T = 0, φ = φJ). We find that characteristic time and length scales of thermal vibrations obey critical scaling in the vicinity of the jamming transition. We show in particular that the amplitude and the time scale of dynamic fluctuations diverge symmetrically on both sides of the transition, and directly reveal a diverging correlation length. The critical region near φJ is divided in three different regimes separated by a characteristic temperature scale T⋆(φ) that vanishes quadratically with the distance to φJ. While two of them, (T < T⋆(φ), φ > φJ) and (T < T⋆(φ), φ < φJ), are described by harmonic theories developed in the zero temperature limit, the third one for T > T⋆(φ) is inherently anharmonic and displays new critical properties. We find that the quadratic scaling of T⋆(φ) is due to nonperturbative anharmonic contributions, its amplitude being orders of magnitude smaller than the perturbative prediction based on the expansion to quartic order in the interactions. Our results show that thermal vibrations in colloidal assemblies directly reveal the critical nature of the jamming transition. The critical region, however, is very narrow and has not yet been attained experimentally, even in recent specifically-dedicated experiments.
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82.70.Kj Emulsions and suspensions
82.70.Dd Colloids

Nonequilibrium static growing length scales in supercooled liquids on approaching the glass transition

Étienne Marcotte, Frank H. Stillinger, and Salvatore Torquato

J. Chem. Phys. 138, 12A508 (2013); http://dx.doi.org/10.1063/1.4769422 (10 pages) | Cited 1 time

Online Publication Date: 2 January 2013

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The small wavenumber k behavior of the structure factor S(k) of overcompressed amorphous hard-sphere configurations was previously studied for a wide range of densities up to the maximally random jammed state, which can be viewed as a prototypical glassy state [A. Hopkins, F. H. Stillinger, and S. Torquato, Phys. Rev. E 86, 021505 (2012)]10.1103/PhysRevE.86.021505. It was found that a precursor to the glassy jammed state was evident long before the jamming density was reached as measured by a growing nonequilibrium length scale extracted from the volume integral of the direct correlation function c(r), which becomes long-ranged as the critical jammed state is reached. The present study extends that work by investigating via computer simulations two different atomic models: the single-component Z2 Dzugutov potential in three dimensions and the binary-mixture Kob-Andersen potential in two dimensions. Consistent with the aforementioned hard-sphere study, we demonstrate that for both models a signature of the glass transition is apparent well before the transition temperature is reached as measured by the length scale determined from the volume integral of the direct correlation function in the single-component case and a generalized direct correlation function in the binary-mixture case. The latter quantity is obtained from a generalized Ornstein-Zernike integral equation for a certain decoration of the atomic point configuration. We also show that these growing length scales, which are a consequence of the long-range nature of the direct correlation functions, are intrinsically nonequilibrium in nature as determined by an index X that is a measure of the deviation from thermal equilibrium. It is also demonstrated that this nonequilibrium index, which increases upon supercooling, is correlated with a characteristic relaxation time scale.
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64.70.pm Liquids
64.70.qj Dynamics and criticality

Static correlations functions and domain walls in glass-forming liquids: The case of a sandwich geometry

Giacomo Gradenigo, Roberto Trozzo, Andrea Cavagna, Tomás S. Grigera, and Paolo Verrocchio

J. Chem. Phys. 138, 12A509 (2013); http://dx.doi.org/10.1063/1.4771973 (9 pages) | Cited 2 times

Online Publication Date: 2 January 2013

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The problem of measuring nontrivial static correlations in deeply supercooled liquids made recently some progress thanks to the introduction of amorphous boundary conditions, in which a set of free particles is subject to the effect of a different set of particles frozen into their (low temperature) equilibrium positions. In this way, one can study the crossover from nonergodic to ergodic phase, as the size of the free region grows and the effect of the confinement fades. Such crossover defines the so-called point-to-set correlation length, which has been measured in a spherical geometry, or cavity. Here, we make further progress in the study of correlations under amorphous boundary conditions by analyzing the equilibrium properties of a glass-forming liquid, confined in a planar (“sandwich”) geometry. The mobile particles are subject to amorphous boundary conditions with the particles in the surrounding walls frozen into their low temperature equilibrium configurations. Compared to the cavity, the sandwich geometry has three main advantages: (i) the width of the sandwich is decoupled from its longitudinal size, making the thermodynamic limit possible; (ii) for very large width, the behaviour off a single wall can be studied; (iii) we can use “anti-parallel” boundary conditions to force a domain wall and measure its excess energy. Our results confirm that amorphous boundary conditions are indeed a very useful new tool in the study of static properties of glass-forming liquids, but also raise some warning about the fact that not all correlation functions that can be calculated in this framework give the same qualitative results.
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64.70.pm Liquids
61.43.Fs Glasses

Evolution of the dynamic susceptibility in molecular glass formers: Results from light scattering, dielectric spectroscopy, and NMR

N. Petzold, B. Schmidtke, R. Kahlau, D. Bock, R. Meier, B. Micko, D. Kruk, and E. A. Rössler

J. Chem. Phys. 138, 12A510 (2013); http://dx.doi.org/10.1063/1.4770055 (15 pages) | Cited 3 times

Online Publication Date: 3 January 2013

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Although broadly studied, molecular glass formers are not well investigated above their melting point. Correlation times down to 10−12 s are easily accessible when studying low-Tg systems by depolarized light scattering, employing a tandem-Fabry-Perot interferometer and a double monochromator. When combining these techniques with state-of-the-art photon correlation spectroscopy (PCS), broad band susceptibility spectra become accessible which can compete with those of dielectric spectroscopy (DS). Comparing the results with those from DS, optical Kerr effect, and NMR, we describe the evolution of the susceptibilities starting from the boiling point Tb down to Tg, i.e., from simple liquid to glassy dynamics. Special attention is given to the emergence of the excess wing contribution which is also probed by PCS and which signals a crossover of the spectral evolution. The process is attributed to a small-angle precursor process of the α-relaxation, and the apparent probe dependent stretching of the α-process is explained by a probe dependent contribution of the excess wing. Upon cooling, its emergence is linked to a strong decrease of the strength of the fast dynamics which is taken as reorientational analog of the anomaly of the Debye-Waller factor. Many glass formers show in addition a slow β-process which manifests itself rather universally in NMR, in DS, however, with different amplitudes, but not at all in PCS experiments. Finally, a three-parameter function is discussed interpolating τα(T) from Tb to Tg by connecting high- and low-temperature dynamics.
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64.70.P- Glass transitions of specific systems
78.35.+c Brillouin and Rayleigh scattering; other light scattering
78.20.-e Optical properties of bulk materials and thin films
64.70.Q- Theory and modeling of the glass transition

Non-exponential nature of calorimetric and other relaxations: Effects of 2 nm-size solutes, loss of translational diffusion, isomer specificity, and sample size

G. P. Johari and J. Khouri

J. Chem. Phys. 138, 12A511 (2013); http://dx.doi.org/10.1063/1.4770056 (18 pages) | Cited 1 time

Online Publication Date: 3 January 2013

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Certain distributions of relaxation times can be described in terms of a non-exponential response parameter, β, of value between 0 and 1. Both β and the relaxation time, τ0, of a material depend upon the probe used for studying its dynamics and the value of β is qualitatively related to the non-Arrhenius variation of viscosity and τ0. A solute adds to the diversity of an intermolecular environment and is therefore expected to reduce β, i.e., to increase the distribution and to change τ0. We argue that the calorimetric value βcal determined from the specific heat [Cp = T(dS/dT)p] data is a more appropriate measure of the distribution of relaxation times arising from configurational fluctuations than β determined from other properties, and report a study of βcal of two sets of binary mixtures, each containing a different molecule of ∼2 nm size. We find that βcal changes monotonically with the composition, i.e., solute molecules modify the nano-scale composition and may increase or decrease τ0, but do not always decrease βcal. (Plots of βcal against the composition do not show a minimum.) We also analyze the data from the literature, and find that (i) βcal of an orientationally disordered crystal is less than that of its liquid, (ii) βcal varies with the isomer's nature, and chiral centers in a molecule decrease βcal, and (iii) βcal decreases when a sample's thickness is decreased to the nm-scale. After examining the difference between βcal and β determined from other properties we discuss the consequences of our findings for theories of non-exponential response, and suggest that studies of βcal may be more revealing of structure-freezing than studies of the non-Arrhenius behavior. On the basis of previous reports that β → 1 for dielectric relaxation of liquids of centiPoise viscosity observed at GHz frequencies, we argue that its molecular mechanism is the same as that of the Johari-Goldstein (JG) relaxation. Its spectrum becomes broader on cooling and its unimodal distribution reversibly changes to a bimodal distribution, each of β < 1. Kinetic freezing of the slower modes of the bimodal distribution produces a glass. After this bifurcation, the faster, original relaxation persists as a weak JG relaxation at T → Tg, and in the glassy state.
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64.70.P- Glass transitions of specific systems
61.43.-j Disordered solids
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
77.22.Gm Dielectric loss and relaxation

Nonlinear active micro-rheology in a glass-forming soft-sphere mixture

D. Winter and J. Horbach

J. Chem. Phys. 138, 12A512 (2013); http://dx.doi.org/10.1063/1.4770335 (10 pages) | Cited 2 times

Online Publication Date: 3 January 2013

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We present extensive molecular dynamics computer simulations of a glass-forming Yukawa mixture, investigating the nonlinear response of a single particle that is pulled through the system by a constant force. Structural changes around the pulled particle are analyzed by pair correlation functions, measured in the deeply supercooled state of the system. A regime of intermediate force strengths is found where the structural changes around the pulled particle are small, although its steady-state velocity shows a strong nonlinear response. This nonlinear response regime is characterized by a force-temperature superposition principle of a Peclet number and anisotropic diffusive behavior. In the direction parallel to the force, mean-square displacements show anomalous superdiffusion in the long time limit. We analyze this superdiffusive behavior by means of the van Hove correlation function of the pulled particle. Perpendicular to the force, the driven particle shows diffusive behavior for all considered force strengths and temperatures. We discuss the dynamics perpendicular and parallel to the force in terms of effective temperatures.
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83.10.Tv Structural and phase changes
05.45.-a Nonlinear dynamics and chaos
02.70.Ns Molecular dynamics and particle methods
05.60.-k Transport processes

On the Bauschinger effect in supercooled melts under shear: Results from mode coupling theory and molecular dynamics simulations

Fabian Frahsa, Amit Kumar Bhattacharjee, Jürgen Horbach, Matthias Fuchs, and Thomas Voigtmann

J. Chem. Phys. 138, 12A513 (2013); http://dx.doi.org/10.1063/1.4770336 (13 pages)

Online Publication Date: 3 January 2013

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We study the nonlinear rheology of a glass-forming binary mixture under the reversal of shear flow using molecular dynamics simulations and a schematic model of the mode-coupling theory of the glass transition (MCT). Memory effects lead to a history-dependent response, as exemplified by the vanishing of a stress-overshoot phenomenon in the stress–strain curves of the sheared liquid, and a change in the apparent elastic coefficients around states with zero stress. We investigate the various retarded contributions to the stress response at a given time schematically within MCT. The connection of this macroscopic response to single-particle motion is demonstrated using molecular-dynamics simulation.
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62.10.+s Mechanical properties of liquids
61.20.Ja Computer simulation of liquid structure
64.70.pm Liquids
61.25.hk Polymer melts and blends
64.70.pj Polymers

Dynamics of thermal vibrational motions and stringlike jump motions in three-dimensional glass-forming liquids

Takeshi Kawasaki and Akira Onuki

J. Chem. Phys. 138, 12A514 (2013); http://dx.doi.org/10.1063/1.4770337 (9 pages) | Cited 1 time

Online Publication Date: 3 January 2013

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Using molecular dynamics simulation on a glass-forming liquid in three dimensions, we investigate the thermal vibrational motions, the configuration changes caused by stringlike jump motions, and their close correlations. The heterogeneous vibrational motions are visualized in terms of a vibration length Si(t) defined for each particle i. The structure factor for the inhomogeneity of Si(t)2 is also calculated, which exhibits considerable long wavelength enhancement. By examining the birth times of strings, they are shown to appear collectively and intermittently. We show that particles with larger Si(t) tend to trigger jump motions more frequently at later times than those with smaller Si(t). We also show that the particles with fewer bonds tend to have larger Si(t) and participate more frequently in the stringlike motions.
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61.43.Fs Glasses
64.70.pm Liquids
63.50.-x Vibrational states in disordered systems
61.20.Ja Computer simulation of liquid structure
61.66.Fn Inorganic compounds

Geometrical frustration and static correlations in hard-sphere glass formers

Benoit Charbonneau, Patrick Charbonneau, and Gilles Tarjus

J. Chem. Phys. 138, 12A515 (2013); http://dx.doi.org/10.1063/1.4770498 (20 pages) | Cited 2 times

Online Publication Date: 3 January 2013

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We analytically and numerically characterize the structure of hard-sphere fluids in order to review various geometrical frustration scenarios of the glass transition. We find generalized polytetrahedral order to be correlated with increasing fluid packing fraction, but to become increasingly irrelevant with increasing dimension. We also find the growth in structural correlations to be modest in the dynamical regime accessible to computer simulations.
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61.20.Ja Computer simulation of liquid structure
64.70.pm Liquids
61.20.Gy Theory and models of liquid structure

Can a stable glass be superheated? Modelling the kinetic stability of coated glassy films

Ian Douglass and Peter Harrowell

J. Chem. Phys. 138, 12A516 (2013); http://dx.doi.org/10.1063/1.4772480 (6 pages)

Online Publication Date: 3 January 2013

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The fabrication of ultra-stable glass films by vapour deposition and their subsequent front-like response to annealing are both manifestations of the enhancement of dynamics at the amorphous surface. We use the facilitated kinetic Ising model to model the behaviour of ultra-stable amorphous films when a coating is applied that suppresses the dynamics at the film surface. The consequences of this manipulation of the film include glass films that can be heated to temperatures in excess of the glass transition without transforming into the liquid, the possibility of direct visualization of the spatial distribution of intrinsic dynamic heterogeneities, and the possibility of using surface treatment to engineer relaxation of these glass films.
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81.05.Kf Glasses (including metallic glasses)
81.40.Gh Other heat and thermomechanical treatments
81.65.-b Surface treatments
64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
68.55.aj Insulators
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Manipulating the properties of stable organic glasses using kinetic facilitation

A. Sepúlveda, Stephen F. Swallen, and M. D. Ediger

J. Chem. Phys. 138, 12A517 (2013); http://dx.doi.org/10.1063/1.4772594 (6 pages) | Cited 3 times

Online Publication Date: 3 January 2013

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In contrast to ordinary glasses, when highly stable organic glasses are annealed at temperatures above Tg, they transform heterogeneously into the liquid state by a constant velocity propagating front that initiates at the free surface. The evolution of this growth front has been interpreted as kinetic facilitation, i.e., efficiently packed molecules become un-jammed only when mobility is available in adjacent regions. Here we use physical vapor deposition to prepare highly stable glasses of indomethacin in which mobile regions are either added to or eliminated from the samples in an attempt to use the kinetic facilitation concept to manipulate the properties of these materials. The addition of higher mobility layers in the interior of a thin stable glass film or at the substrate surface is shown to initiate new growth fronts, thus demonstrating that kinetic facilitation occurs independently of free surface mobility. Conversely, capping the free surface with a higher Tg stable glass stops the growth front, apparently by eliminating surface mobility, thus increasing sample stability by slowing the transformation to the supercooled liquid.
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68.55.A- Nucleation and growth
61.43.Fs Glasses
81.40.Gh Other heat and thermomechanical treatments
64.70.D- Solid-liquid transitions
64.70.P- Glass transitions of specific systems

Microrheology of supercooled liquids in terms of a continuous time random walk

Carsten F. E. Schroer and Andreas Heuer

J. Chem. Phys. 138, 12A518 (2013); http://dx.doi.org/10.1063/1.4772627 (6 pages) | Cited 2 times

Online Publication Date: 3 January 2013

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Molecular dynamics simulations of a glass-forming model system are performed under application of a microrheological perturbation on a tagged particle. The trajectory of that particle is studied in its underlying potential energy landscape. Discretization of the configuration space is achieved via a metabasin analysis. The linear and nonlinear responses of drift and diffusive behavior can be interpreted and analyzed in terms of a continuous time random walk. In this way, the physical origin of linear and nonlinear response can be identified. Critical forces are determined and compared with predictions from literature.
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66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation
46.65.+g Random phenomena and media
83.10.Mj Molecular dynamics, Brownian dynamics

Dynamics of glass-forming liquids. XVI. Observation of ultrastable glass transformation via dielectric spectroscopy

Z. Chen, A. Sepúlveda, M. D. Ediger, and R. Richert

J. Chem. Phys. 138, 12A519 (2013); http://dx.doi.org/10.1063/1.4771695 (8 pages)

Online Publication Date: 3 January 2013

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The transformation of vapor-deposited ultrastable glasses of indomethacin (IMC) into the supercooled liquid state near Tg is monitored by means of dielectric spectroscopy. Films with thickness between 400 and 800 nm are deposited on differential interdigitated electrode cells and their loss profiles are measured during isothermal annealing using a dual-channel impedance technique for frequencies between 0.03 and 100 Hz. All dielectric loss spectra observed during the transformation process can be explained by a volume fraction of the supercooled liquid that increases linearly with time. From the early stages of the transformation to the liquid that is formed via complete annealing of the ultrastable glass, the average dielectric relaxation time as well as the distribution of relaxation times of the liquid component are identical to those of the conventional liquid obtained by cooling the melt. The dependence of the transformation rate on the film thickness is consistent with a growth front mechanism for the direct conversion from the ultrastable glass to the equilibrium supercooled liquid. We conclude that the IMC liquid recovered from the ultrastable glass is structurally and dynamically identical to the conventional supercooled state.
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64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
68.55.aj Insulators
77.55.-g Dielectric thin films
77.22.Gm Dielectric loss and relaxation
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.40.Gh Other heat and thermomechanical treatments

Local elastic response measured near the colloidal glass transition

D. Anderson, D. Schaar, H. G. E. Hentschel, J. Hay, Piotr Habdas, and Eric R. Weeks

J. Chem. Phys. 138, 12A520 (2013); http://dx.doi.org/10.1063/1.4773220 (9 pages) | Cited 1 time

Online Publication Date: 7 January 2013

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We examine the response of a dense colloidal suspension to a local force applied by a small magnetic bead. For small forces, we find a linear relationship between the force and the displacement, suggesting the medium is elastic, even though our colloidal samples macroscopically behave as fluids. We interpret this as a measure of the strength of colloidal caging, reflecting the proximity of the samples' volume fractions to the colloidal glass transition. The strain field of the colloidal particles surrounding the magnetic probe appears similar to that of an isotropic homogeneous elastic medium. When the applied force is removed, the strain relaxes as a stretched exponential in time. We introduce a model that suggests this behavior is due to the diffusive relaxation of strain in the colloidal sample.
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62.10.+s Mechanical properties of liquids
82.70.Dd Colloids
64.70.pv Colloids
64.70.Q- Theory and modeling of the glass transition

Microscopic theory of the glassy dynamics of passive and active network materials

Shenshen Wang and Peter G. Wolynes

J. Chem. Phys. 138, 12A521 (2013); http://dx.doi.org/10.1063/1.4773349 (10 pages)

Online Publication Date: 7 January 2013

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Signatures of glassy dynamics have been identified experimentally for a rich variety of materials in which molecular networks provide rigidity. Here we present a theoretical framework to study the glassy behavior of both passive and active network materials. We construct a general microscopic network model that incorporates nonlinear elasticity of individual filaments and steric constraints due to crowding. Based on constructive analogies between structural glass forming liquids and random field Ising magnets implemented using a heterogeneous self-consistent phonon method, our scheme provides a microscopic approach to determine the mismatch surface tension and the configurational entropy, which compete in determining the barrier for structural rearrangements within the random first order transition theory of escape from a local energy minimum. The influence of crosslinking on the fragility of inorganic network glass formers is recapitulated by the model. For active network materials, the mapping, which correlates the glassy characteristics to the network architecture and properties of nonequilibrium motor processes, is shown to capture several key experimental observations on the cytoskeleton of living cells: Highly connected tense networks behave as strong glass formers; intense motor action promotes reconfiguration. The fact that our model assuming a negative motor susceptibility predicts the latter suggests that on average the motorized processes in living cells do resist the imposed mechanical load. Our calculations also identify a spinodal point where simultaneously the mismatch penalty vanishes and the mechanical stability of amorphous packing disappears.
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68.03.Cd Surface tension and related phenomena
75.10.Hk Classical spin models
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
65.20.De General theory of thermodynamic properties of liquids, including computer simulation
61.25.Em Molecular liquids
62.10.+s Mechanical properties of liquids

Statistics of modifier distributions in mixed network glasses

John C. Mauro

J. Chem. Phys. 138, 12A522 (2013); http://dx.doi.org/10.1063/1.4773356 (8 pages) | Cited 1 time

Online Publication Date: 7 January 2013

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The constituents of any network glass can be broadly classified as either network formers or network modifiers. Network formers, such as SiO2, Al2O3, B2O3, P2O5, etc., provide the backbone of the glass network and are the primary source of its rigid constraints. Network modifiers play a supporting role, such as charge stabilization of the network formers or alteration of the network topology through rupture of bridging bonds and introduction of floppy modes. The specific role of the modifiers depends on which network formers are present in the glass and the relative free energies of modifier interactions with each type of network former site. This variation of free energy with modifier speciation is responsible for the so-called mixed network former effect, i.e., the nonlinear scaling of property values in glasses having fixed modifier concentration but a varying ratio of network formers. In this paper, a general theoretical framework is presented describing the statistical mechanics of modifier speciation in mixed network glasses. The model provides a natural explanation for the mixed network former effect and also accounts for the impact of thermal history and relaxation on glass network topology.
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64.70.qd Thermodynamics and statistical mechanics
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Dynamic heterogeneities above and below the mode-coupling temperature: Evidence of a dynamic crossover

Elijah Flenner and Grzegorz Szamel

J. Chem. Phys. 138, 12A523 (2013); http://dx.doi.org/10.1063/1.4773321 (13 pages)

Online Publication Date: 11 January 2013

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We examine dynamic heterogeneities in a model glass-forming fluid, a binary harmonic sphere mixture, above and below the mode-coupling temperature Tc. We calculate the ensemble independent susceptibility χ4(τα) and the dynamic correlation length ξ4(τα) at the α-relaxation time τα. We also examine in detail the temperature dependence of τα and the diffusion coefficient D. For higher temperatures, we find that the standard Stokes-Einstein relationship, D ∼ τα−1, holds, but at lower temperatures a fractional Stokes-Einstein relationship, D ∼ τα−σ with σ = 0.65, emerges. By examining the relationships between τα, D, χ4(τα), and ξ4(τα) we determine that the emergence of the fractional Stokes-Einstein relationship is accompanied by a dynamic crossover from τα ∼ ek2ξ4 at higher temperatures to τα ∼ ek1ξ43/2 at lower temperatures.
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66.10.C- Diffusion and thermal diffusion
61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids
64.75.Ef Mixing
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