Structural correlations and cooperative dynamics in supercooled liquids (original) (raw)
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Physical Review Letters, 2010
We identify the pattern of microscopic dynamical relaxation for a two dimensional glass forming liquid. On short timescales, bursts of irreversible particle motion, called cage jumps, aggregate into clusters. On larger time scales, clusters aggregate both spatially and temporally into avalanches. This propagation of mobility, or dynamic facilitation, takes place along the soft regions of the systems, which have been identified by computing isoconfigurational Debye-Waller maps. Our results characterize the way in which dynamical heterogeneity evolves in moderately supercooled liquids and reveal that it is astonishingly similar to the one found for dense glassy granular media.
Avalanches and Dynamical Correlations in supercooled liquids
2009
We identify the pattern of microscopic dynamical relaxation for a two dimensional glass forming liquid. On short timescales, bursts of irreversible particle motion, called cage jumps, aggregate into clusters. On larger time scales, clusters aggregate both spatially and temporally into avalanches. This propagation of mobility, or dynamic facilitation, takes place along the soft regions of the systems, which have been identified by computing isoconfigurational Debye-Waller maps. Our results characterize the way in which dynamical heterogeneity evolves in moderately supercooled liquids and reveal that it is astonishingly similar to the one found for dense glassy granular media.
Dynamic entropy as a measure of caging and persistent particle motion in supercooled liquids
Physical Review E, 1999
The length-scale dependence of the dynamic entropy is studied in a molecular dynamics simulation of a binary Lennard-Jones liquid above the mode-coupling critical temperature Tc. A number of methods exist for estimating the entropy of dynamical systems and we utilize an approximation based on calculating the mean first-passage time (MFPT) for particle displacement because of its tractability and its accessibility in real and simulation measurements. The MFPT dynamic entropy S(ǫ) is defined to equal the inverse of the average first-passage time for a particle to exit a sphere of radius ǫ. This measure of the degree of chaotic motion allows us to identify characteristic time and space scales and to quantify the increasingly correlated particle motion and intermittency occurring in supercooled liquids. In particular, we identify a "cage" size defining the scale at which the particles are transiently localized, and we observe persistent particle motion at intermediate length scales beyond the scale where caging occurs. Furthermore, we find that the dynamic entropy at the scale of one interparticle spacing extrapolates to zero as the mode-coupling temperature Tc is approached.
Physical Review Letters, 2008
We unveil the existence of non-affinely rearranging regions in the inherent structures (IS) of supercooled liquids by numerical simulations of two-and three-dimensional model glass formers subject to static shear deformations combined with local energy minimizations. In the liquid state IS, we find a broad distribution of rather large rearrangements which are correlated only over small distances. At low temperatures, the onset of the cooperative dynamics corresponds to much smaller displacements correlated over larger distances. This finding indicates the presence of nonaffinely rearranging domains of relevant size in the IS deformation, which can be seen as the static counterpart of the cooperatively rearranging regions in the dynamics. This idea provides new insight into possible structural signatures of slow cooperative dynamics of supercooled liquids and supports the connections with elastic heterogeneities found in amorphous solids. PACS numbers: 61.43.Fs,64.70.Q-,05.20.Jj
Configurational entropy and collective modes in normal and supercooled liquids
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 1999
Soft vibrational modes have been used to explain anomalous thermal properties of glasses above 1 K. The soft-potential model consists of a collection of double-well potentials that are distorted by a linear term representing local stress in the liquid. Double-well modes contribute to the configurational entropy of the system. Based on the Adam-Gibbs theory of entropically driven relaxation in liquids, we show that the presence of stress drives the transition from Arrhenius to Zwanzig-Bässler temperature dependence of relaxation times. At some temperature below the glass transition, the energy scale is dominated by local stress, and soft modes are described by single wells only. It follows that the configurational entropy vanishes, in agreement with the "Kauzmann paradox." We discuss a possible connection between soft vibrational modes and ultrafast processes that dominate liquid dynamics near the glass transition.
Evidence for compact cooperatively rearranging regions in a supercooled liquid
Journal of Physics: Condensed Matter, 2009
We examine structural relaxation in a supercooled glass-forming liquid simulated by NVE molecular dynamics. Time correlations of the total kinetic energy fluctuations are used as a comprehensive measure of the system's approach to the ergodic equilibrium. We find that, under cooling, the total structural relaxation becomes delayed as compared with the decay of the component of the intermediate scattering function corresponding to the main peak of the structure factor. This observation can be explained by collective movements of particles preserving many-body structural correlations within compact 3D cooperatively rearranging regions.
How do packing defects modify the cooperative motions in supercooled liquids?
Chemical Physics, 2017
We use molecular dynamic simulations to investigate the relation between the presence of packing defects in a glass-former and the spontaneous cooperative motions called dynamic heterogeneity. For that purpose we use a simple diatomic glass-former and add a small number of larger or smaller diatomic probes. The diluted probes modify locally the packing, inducing structural defects in the liquid, while we find that the number of defects is small enough not to disturb the average structure. We find that a small packing modification around a few molecules can deeply influence the dynamics of the whole liquid, when supercooled. When we use small probe molecules, the dynamics accelerates and the dynamic heterogeneity decreases. In contrast, for large probes the dynamics slows down and the dynamic heterogeneity increases. The induced heterogeneities and transport coefficient modification increase when the temperature decreases and disappear around the onset temperature of the cage dynamics.
Inherent Structure Entropy of Supercooled Liquids
Physical Review Letters, 1999
We present a quantitative description of the thermodynamics in a supercooled binary Lennard-Jones liquid via the evaluation of the degeneracy of the inherent structures, i.e., of the number of potential energy basins in configuration space. We find that the contribution of the inherent structures to the free energy of the liquid almost completely decouples from the vibrational contribution. An important
Dynamical Heterogeneities in a Supercooled Lennard-Jones Liquid
Physical Review Letters, 1997
We present the results of a large scale molecular dynamics computer simulation study in which we investigate whether a supercooled Lennard-Jones liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian parameter for the self part of the van Hove correlation function and use it to identify "mobile" particles. We find that these particles form clusters whose size grows with decreasing temperature. We also find that the relaxation time of the mobile particles is significantly shorter than that of the bulk, and that this difference increases with decreasing temperature.
Physical Review Applied
Probing dynamic and static correlations in glass-forming supercooled liquids has been a challenge for decades despite extensive research. Dynamic correlation, which manifests itself as dynamic heterogeneity, is ubiquitous in various systems starting from molecular glass-forming liquids, dense colloidal systems to collections of cells. On the other hand, the mere concept of growing many-body static correlations in these dense disordered systems in the supercooled regime remains somewhat elusive. Its existence is still actively debated. We propose a method to extract dynamic and static correlations using rodlike particles as a probe. This method can be implemented in experiments to study the growth of static and dynamic correlations in molecular glass-forming liquids and other soft-matter systems, including biological systems that show glassy dynamics. Finally, we analytically derive the exact form of the distribution of rotational decorrelation time of the probe rod molecules and rationalize the observed log-normal-like distribution reported in previous experimental studies on the dynamics of elongated probe molecules in supercooled glycerol.