Slow relaxations and stringlike jump motions in fragile glass-forming liquids: Breakdown of the Stokes-Einstein relation

Kawasaki, Takeshi; Ōnuki, Akira

doi:10.1103/physreve.87.012312

Cited by 54 publications

(60 citation statements)

References 40 publications

(85 reference statements)

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“…We also find that the generalized SE ratio for bond relaxation is preserved, i.e., D ∝ τ −1 B , indicating that the bond relaxation function is a descriptor for the 2D diffusive motion in a similar manner to other 3D supercooled liquids [32,39].…”

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confidence: 53%

Local Density Fluctuation Governs the Divergence of Viscosity Underlying Elastic and Hydrodynamic Anomalies in a 2D Glass-Forming Liquid

2019

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If a liquid is cooled rapidly to form a glass, its structural relaxation becomes retarded, producing a drastic increase in viscosity. In two dimensions, strong long-wavelength fluctuations persist, even at low temperature, making it difficult to evaluate the microscopic structural relaxation time. This Letter shows that, in a 2D glass-forming liquid, relative displacement between neighbor particles yields a relaxation time that grows in proportion to the viscosity. In addition to thermal elastic vibrations, hydrodynamic fluctuations are found to affect the long-wavelength dynamics, yielding a logarithmically diverging diffusivity in the long-time limit.In many two-dimensional ordering phenomena, fluctuations at long wavelengths are so strong that perfect order is destroyed. For example, the transition between a liquid and a crystalline solid is continuous or nearly continuous [1][2][3][4][5][6]. Recently, large-scale molecular dynamics (MD) simulations [7,8] and colloidal experiments [9, 10] have revealed that such long-wavelength fluctuations also exist in two-dimensional (2D) liquids that are rapidly cooled toward the glass transition. Although retaining a random amorphous structure, elastic vibrations appear as the rigidity emerges with the decrease in temperature. The excess of low-frequency phonons in two dimensions [7,11] leads to enhanced thermal elastic vibrations at long wavelengths. Even in the presence of these long-wavelength fluctuations, the structural relaxation in 2D and 3D supercooled liquids appears to be similar, once the effect of these fluctuations has been eliminated by introducing quantities that characterize the local switching between neighbor particles [7,9,10].The glass transition is marked by a drastic increase in macroscopic viscosity, which is intimately related to the divergence of the microscopic structural relaxation time. As such, theoretical and computational studies have focused on the dynamical mechanism of growth in the microscopic structural relaxation time, most typically the α-relaxation time [12][13][14][15], which is defined as the decay time of the autocorrelation function for the local density i.e., the intermediate scattering function. However, in two dimensions, the decay time is strongly suppressed by the long-wavelength thermal elastic vibrations [7-10], preventing the α-relaxation time from reflecting the microscopic structural relaxation time. Therefore, to enhance our understanding of 2D glass formation, it is essential to identify the relaxation time related to the viscosity divergence by taking the mechanical properties into account.When a liquid approaches the glass transition via rapid cooling, a transient elastic response emerges, giving rise to an intermediate plateau in the shear stress relaxation function and an increase in the overall viscosity. In a recent study, while the viscosity enhancement is similar between 2D and 3D glassforming liquids, the plateau in the stress relaxation function turns out to be less clear in two dimensions under light super-...

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confidence: 53%

Local Density Fluctuation Governs the Divergence of Viscosity Underlying Elastic and Hydrodynamic Anomalies in a 2D Glass-Forming Liquid

2019

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“…Surface effects penetrate up to z 5.5σ for τ = 20 and 40, but reach deeper to z 4.5σ for τ = τ max . To establish this more clearly, we numerically evaluate an integrated hopping event correlation defined by Particle back-and-forth motions have long been studied in glassy systems [38,[45][46][47][48][49][50][51][52] and cause the main peak at r = 0 in Fig. 11(b)-(c).…”

Section: Hopping Event Correlationsmentioning

confidence: 99%

Deeper penetration of surface effects on particle mobility than on hopping rate in glassy polymer films

Lam

2018

The Journal of Chemical Physics

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Free surfaces in glassy polymer films are known to induce surface mobile layers with enhanced dynamics. Using molecular dynamics simulations of a bead-spring model, we study a wide variety of layer-resolved structural and dynamical properties of polymer films equilibrated at a low temperature. Surface enhancement on thermally induced particle hopping rate is found to terminate abruptly only about 5 particle diameters from the free surface. In contrast, enhancement on the net motions of particles measured at longer time scales penetrates at least 2 particle diameters deeper. The diverse penetration depths show the existence of a peculiar sublayer, referred to as the innersurface layer, in which surface enhanced mobility is not caused by more frequent particle hops but instead by a reduced dynamic heterogeneity associated with diminished hopping anti-correlations. Confinement effects of the free surface thus provide a unique mechanism for varying the dynamic heterogeneity and hopping correlations while keeping the hopping rate constant. Our results highlight the importance of correlations among elementary motions to glassy slowdown and suggest that dynamic facilitation is mediated via perturbations to the correlations rather than the rate of elementary motions. arXiv:1810.00192v1 [cond-mat.soft]

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“…The cage-jump scenario also suggests intermittent molecular motions, which can be modeled by the continuous-time random walk using a random waiting time between cage-jumps. 3 The cage-jump motion in glassy dynamics has been extensively addressed with molecular dynamics (MD) simulations [4][5][6][7][8][9][10][11][12][13][14][15][16][17] and experiments using colloidal glasses. [18][19][20][21][22][23][24][25] As the temperature is decreased, the mean square displacement (MSD) exhibits a plateau in the intermediate time scales between ballistic and diffusive regimes, reflecting the localized motion inside the cage.…”

Section: Introductionmentioning

confidence: 99%

Diffusion dynamics of supercooled water modeled with the cage-jump motion and hydrogen-bond rearrangement

Kikutsuji

Kim

Matubayasi

2019

The Journal of Chemical Physics

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The slow dynamics of glass-forming liquids is generally ascribed to the cage-jump motion. In the cage-jump picture, a molecule remains in a cage formed by neighboring molecules, and after a sufficiently long time, it jumps to escape from the original position by cage-breaking. The clarification of the cage-jump motion is therefore linked to unraveling the fundamental element of the slow dynamics. Here, we develop a cagejump model for the dynamics of supercooled water. The caged and jumping states of a water molecule are introduced with respect to the hydrogen-bond (H-bond) rearrangement process, and describe the motion in supercooled states. It is then demonstrated from the molecular dynamics simulation of the TIP4P/2005 model that the characteristic length and time scales of cage-jump motions provide a good description of the selfdiffusion constant that is determined in turn from the long-time behavior of the mean square displacement. Our cage-jump model thus enables to connect between H-bond dynamics and molecular diffusivity. arXiv:1903.06060v2 [cond-mat.soft]

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Slow relaxations and stringlike jump motions in fragile glass-forming liquids: Breakdown of the Stokes-Einstein relation

Cited by 54 publications

References 40 publications

Local Density Fluctuation Governs the Divergence of Viscosity Underlying Elastic and Hydrodynamic Anomalies in a 2D Glass-Forming Liquid

Local Density Fluctuation Governs the Divergence of Viscosity Underlying Elastic and Hydrodynamic Anomalies in a 2D Glass-Forming Liquid

Deeper penetration of surface effects on particle mobility than on hopping rate in glassy polymer films

Diffusion dynamics of supercooled water modeled with the cage-jump motion and hydrogen-bond rearrangement

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