Jammed Particle Configurations and Dynamics in High-Density Lennard-Jones Binary Mixtures in Two Dimensions

Abstract

We examine the changeover in the particle configurations and the dynamics in dense Lennard-Jones binary mixtures composed of small and large particles. By varying the composition at a low temperature, we realize crystal with defects, polycrystal with small grains, and glass with various degrees of disorder. In particular, we show configurations where small crystalline regions composed of the majority species are enclosed by percolated amorphous layers composed of the two species. We visualize the dynamics of configuration changes using the method of bond breakage and following the particle displacements. In quiescent jammed states, the dynamics is severely slowed down and is highly heterogeneous at any compositions. We apply shear flow by relative motions of boundary layers. Then plastic deformations multiply occur in relatively fragile regions, growing into large-scale shear bands where the strain is highly localized. Such bands appear on short time scales and evolve on long time scales with finite lifetimes.

Homogeneity and packing structure of a 2D sheared granular system

“…The large and small particles have radii 8.0 mm and 6.4 mm respectively, and number ratio 1 : 3.3. This configuration is carefully selected to meet three criteria: 1) the particle sizes should be comparable to the slat width, 2) the size ratio should considerably deviate from unity to maximally ensure disordered packing [4], and 3) the system should have relatively more small particles than big ones, in order to increase the total number of particles in the apparatus and enhance statistics. The density of the packing is chosen within a range φ S ≤ φ ≤ φ J , where φ J 0.84 is the isotropic jamming point in 2D and φ S 0.75 is the minimum density for shear jamming [1,5].…”

Construction of a disorder variable from Steinhardt order parameters in binary mixtures at high densities in three dimensions

“…1). In our previous work, 7,8 we introduced the two-dimensional counterparts, D j andD, to analyze the structural disorder in two dimensions.…”

“…where the averaged bond-orientational parametersq j 6m in Eq. (8) are used in the right hand side. This quantity has been used to detect crystalline regions in nucleation.…”

“…The particle configurations in binary particle systems are much more complicated than those of one component systems, [1][2][3][4][5][6][7][8] which strongly depend on the temperature T, the average number density n, and the composition c. At high densities, it is known to be profoundly influenced also by the size ratio σ 1 /σ 2 between the diameters of the two components, σ 1 and σ 2 . If σ 1 /σ 2 is close to unity at large n, the system becomes a crystal at low T or a liquid at high T. If σ 1 /σ 2 considerably deviates from unity, glass states are realized at large n and at low T. In crystal and polycrystal, there emerge many kinds of defects such as point defects, dislocations, grain boundaries, and stacking faults in the background of various crystal structures.…”

“…Its amplitude |ψ j 6 | was also used to detect mesoscopic order in glass. 18 Furthermore, a disorder variable D j was constructed from ψ j 6 , in terms of which the degree of disorder has been visualized in simulations 7,8 and in an experiment. 19 In three dimensions, on the other hand, disordered crystal states are much more complex under the strong influence of the underlying crystal structure.…”