Correlation of Local Order with Particle Mobility in Supercooled Liquids Is Highly System Dependent

Abstract

We investigate the connection between local structure and dynamical heterogeneity in supercooled liquids. Through the study of four different models we show that the correlation between a particle's mobility and the degree of local order in nearby regions is highly system dependent. Our results suggest that the correlation between local structure and dynamics is weak or absent in systems that conform well to the mean-field picture of glassy dynamics and strong in those that deviate from this paradigm. Finally, we investigate the role of order-agnostic point-to-set correlations and reveal that they provide similar information content to local structure measures, at least in the system where local order is most pronounced.Supercooled liquids display markedly heterogeneous dynamics, despite possessing structural properties that appear nearly unchanged from those of normal liquids from which they are prepared [1]. While there has been intense focus on understanding dynamical heterogeneity in a wide variety of systems, the structural origin of this phenomenon is not well understood [2, 3]. Simulations of model supercooled liquids are useful for understanding the connections between structure and dynamics because particle locations may be followed precisely for all times. Nonetheless, new theoretical tools are needed to filter out extraneous detail from the key structural and dynamical fluctuations in glassy systems.One particularly useful simulation-based tool for quantifying the influence of structure on dynamics is the isoconfigurational ensemble, where a large number of molecular dynamics (MD) simulations are initiated from the same starting configuration with momenta sampled randomly from a Boltzmann distribution [4, 5]. Under glassy conditions, spatial heterogeneities are immediately evident in the isoconfigurational displacement (or propensity) field. A reasonable hypothesis is that particles with low propensity have a larger measure of local structural stability. Surprisingly, however, simple structural quantities, such as free volume and local potential energy, show little correlation with the heterogeneity of the propensity field [6]. In some models, localized soft modes [7][8][9] or unstable modes [10] appear to correlate strongly with propensity, but the degree of universality of this connection has not been thoroughly investigated.Recently, focus has turned to the study of specific structural motifs and their putative connection with the dynamics of supercooled liquids. The notion that the frustration of local order incommensurate with bulk crystalline periodicity may be related to glass formation is an old one [11][12][13][14]. New evidence for the growth of domains associated with local packing motifs has been presented for several simple [15] and realistic model systems [16], where particles tend to be found in certain "locally preferred structures" (LPS) with increased supercooling. As a general rule, more fragile systems display a more rapid increase in LPS concentration and domain extent [15, 16]....

Recasting a model atomistic glassformer as a system of icosahedra

“…8,13,14 In particular, the occurrence of superArrhenius dynamics coincides with the emergence of a population of icosahedra ( Figs. 1 and 2).…”

“…1,2 It has been proposed that upon cooling, icosahedral arrangements of atoms might form in supercooled liquids 3 and that dynamical arrest may be related to a (geometrically frustrated) transition to a phase of such icosahedra. 4 It is now possible to identify geometric motifs such as icosahedra and related locally favored structures (LFSs) using computer simulation [5][6][7][8][9][10][11][12][13][14][15] and particle-resolved studies in colloid experiments. [16][17][18][19] Further evidence of increasing numbers of LFSs upon cooling is also found in metallic glassformers.…”

“…20,21 The discovery of dynamic heterogeneity, i.e., locally fast and slow regions 22,23 has spurred attempts to correlate LFS such as icosahedra with the dynamically slow regions. This has met with some success; 7,8,[11][12][13][14]24 however, such correlation does not by itself demonstrate a mechanism for arrest 24,25 and in any case is dependent on the model system under consideration.…”

Structure in sheared supercooled liquids: Dynamical rearrangements of an effective system of icosahedra

“…Geometric motifs such as icosahedra and other locally favoured structures (LFS) can be identified in particleresolved colloidal experiments [6][7][8][9][10][11][12] and computer simulations [13][14][15][16][17][18][19][20]. In particular, it has been shown that the onset of slow dynamics in simulated Lennard-Jones systems is closely coupled to the local structure, characterized by the LFS [18,21,22].…”

Assessing the role of static length scales behind glassy dynamics in polydisperse hard disks

“…It is intuitively defined as the average distance between pinned particles that forces the system to stay in an amorphous configuration with a vanishing configurational entropy. The reasons behind the popularity of PTS correlation lengths in the study of the glass transition are at least twofold: (i) they are expected to provide an "order-agnostic" method to measure static correlations (23,24); (ii) it is theoretically established that no divergence of the relaxation time of a glass at finite temperature can occur without the concomitant divergence of the static correlation length (25).…”