Spatial correlations of mobility and immobility in a glass-forming Lennard-Jones liquid

Abstract

Using extensive molecular dynamics simulations of an equilibrium, glass-forming Lennard-Jones mixture, we characterize in detail the local atomic motions. We show that spatial correlations exist among particles undergoing extremely large ("mobile") or extremely small ("immobile") displacements over a suitably chosen time interval. The immobile particles form the cores of relatively compact clusters, while the mobile particles move cooperatively and form quasi-one-dimensional, string-like clusters. The strength and length scale of the correlations between mobile particles are found to grow strongly with decreasing temperature, and the mean cluster size appears to diverge at the mode-coupling critical temperature. We show that these correlations in the particle displacements are related to equilibrium fluctuations in the local potential energy and local composition.

Local polymer dynamics under strong connectivity constraints: The dendrimer case

“…Associations of the temporal scale t * at which the NGP peaks with the time scale for collective motion responsible for the ␣-relaxation process have already been noticed in different glass-forming systems. Studies in supercooled liquids 13,19,22 demonstrated the relation of t * with the lifetime of the dynamic heterogeneities and the cooperative motion necessary for the release of the particles from their cage, which characterizes the early stages of ␣-relaxation. In linear polymer models it was shown 6,9 that NGP peaks in the late-␤/early-␣ regime, while t * was found to follow a similar temperature dependence with that of the ␣-relaxation ͑see Ref.…”

“…Identification and monitoring of regions possessing distinct mobilities rely on a proper selection of subensembles of "fast" and "slow" moving units, for the definition of which various approaches have been followed so far. 6,19,32,33 This introduces a freedom that might be undesirable when it comes to comparison of results obtained from different such selections. In the case of dendrimers, the sets of beads belonging to different generational shells ͑g shells, see next section͒ form ideal subensembles for the elucidation of mobility contrast effects in the realization of local motion, particularly in the proximity of glass transition phenomena.…”

“…The time t * at which NGP exhibits a maximum demarcates a characteristic lifetime of the transient cage 7,9,19,20 and has been correlated with the time scale at which local dynamic heterogeneities approach a maximum. 10,12,20,21 At this transition region connectivity effects become increasingly important.…”

Dynamical Heterogeneity and Jamming in Glass-Forming Liquids

“…[2,27,47,48,49,50,51,52], several approaches -including calculation of a displacement-displacement correlation function, identification of clusters of mobile particles, and calculation of a four-point time-dependent density correlation function -predicted and demonstrated the importance of spatially heterogeneous dynamics in supercooled liquids using a rigorous statistical mechanical analysis. In particular, using the four-point time-dependent density correlation function formalism, we found dynamical correlation lengths of regions of localized and delocalized particles which suggests a picture of fluctuating domains of temporarily localized and delocalized particles, as suggested by Stillinger and Hodgedon [53].…”

Partial jamming and non-locality in dense granular flows

“…More generally, correlated motion of particles [20][21][22], and avalanche-like rearrangements have been observed in various glassy materials like sand [23], granular materials [15,24,25], cohesive grains [26], foams [27], suspensions [8,28], colloids [29,30], and Lennard-Jones glass [31,32]. These observations suggest that clusters of particles may exist in many glassy materials.…”