Dynamical heterogeneities below the glass transition

Abstract

We present molecular dynamics simulations of a binary Lennard-Jones mixture at temperatures below the kinetic glass transition. The "mobility" of a particle is characterized by the amplitude of its fluctuation around its average position. The 5% particles with the largest/smallest mean amplitude are thus defined as the relatively most mobile/immobile particles. We investigate for these 5% particles their spatial distribution and find them to be distributed very heterogeneously in that mobile as well as immobile particles form clusters. The reason for this dynamic heterogeneity is traced back to the fact that mobile/immobile particles are surrounded by fewer/more neighbors which form an effectively wider/narrower cage. The dependence of our results on the length of the simulation run indicates that individual particles have a characteristic mobility time scale, which can be approximated via the non-Gaussian parameter.02.70. Ns, 61.20.Lc, 61.43.Fs,

Polymer-specific effects of bulk relaxation and stringlike correlated motion in the dynamics of a supercooled polymer melt

“…27-29 for related work on two-and three-dimensional hard spheres as well as Ref. 30 for dynamical heterogeneities in a LJ-mixture below T g ).…”

Out-of-equilibrium dynamical fluctuations in glassy systems

“…Some recent papers treat the glassy phase of the Lennard-Jones system in a similar way. 19 Following the dynamics of individual particles is very useful to get an intuitive understanding of the evolution of the system, but it is hard to use as a direct input in a theoretical approach. In order to develop such a description, it is much more convenient to use the coarse-grained twotime quantities 20,35 that we shall define in Sec.…”

“…Interest is now shifting to the study of local quantities embodied in intermittent fluctuations and dynamic heterogeneities. The recent development of powerful experimental techniques [2][3][4][5][6][7][8][9][10] and the use of extensive numerical simulations have started to yield valuable information about the dynamics of supercooled liquids [11][12][13][14][15][16][17][18][19] and glassy systems 20,21 at a mesoscopic scale. A detailed analysis of temporal fluctuations in the evolution of global observables allows one to distinguish between intermittent and continuous dynamics, the former being associated with sudden, large rearrangements.…”

Local polymer dynamics under strong connectivity constraints: The dendrimer case

“…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.…”