Hard sphere packing

Bridge structures in granular packings
Angularly anisotropic correlation in granular packings
Structures of wet granular packings
Glass order in granular packings


Bridge structures in granular packings

        Static granular packing has an amorphous liquid-like structure while it can hold some stress like a solid. It’s important to understand the origin of the rigidity of amorphous solids. A bridge structure was defined by A. Mehta and coworkers, which was thought to be related with force chains inside the packing. We observed the bridge structures in experimental 3D packings, with all characteristic parameters consistent with numerical results. Studying these bridge structures in packings with different packing fractions helps to understand the jamming phenomenon, and a possible structural origin of amorphous solid rigidity. The work has been published on Europhys. Lett., 102, 24004 (2013).  ( PDF )

Figure. Typical structures of (a) a linear bridge and (c) a complex bridge. (b,d) The backbones of the two bridges.




Angularly anisotropic correlation in granular packings

        This research focuses on the multi-particle correlations in static mono-dispersed sphere packings. It’s demonstrated that the common pair correlation function can be decomposed into translational and orientational parts, while the correlation is locally anisotropic. In addition, this anisotropy majorly originated from some local structures with five-fold symmetry, which are similar to the locally preferred structures in some molecular systems. The work has been published on Phys. Rev. E 90, 062201(2014).  ( PDF )

Figure. The common angularly-averaged pair correlation function (b) and the angular-dependent one (c-e).




Structures of wet granular packings

        Wet granular packings is a model system for attractive glass, since the liquid bridges between particles induce an effective attraction interaction. Lots of locally-prefered-structures with five-fold symmetry are found in short range, which is similar to colloidal system and shows significant difference with dry granular packings. This work suggested that dynamic arrest may be due to geometrical frustrations in both thermal and non-thermal systems. In contrary, pair correlations evolve slightly during compaction, suggesting that higher-order correlations play more important role in glass transition in our systems. The work has been published on Nature Communications 5, 5014 (2014).  ( PDF )

Figure:Fractions of various locally-prefered-structures as a function of tapping number.




Glass order in granular packings

        Spherical granular packing under vertical tapping, as an experimental system of hard-sphere glass, is studied systematically. The microscopic structural evolution of the packing is studied using up to 100 CTs of the packing during tapping processes. We define quasi-regular tetrahedral structures as a structural order parameter, which has five-fold symmetry and a fractal nature, thus cannot tile space. These tetrahedral structures are also correlated with the slow dynamics. In addition, the correlation length of tetrahedral structures grows as packing fraction increases, which follows an entropy-driven nucleation model. This research demonstrated that hard-sphere glass transition may be a special kind of crystallization, which is a structural thermodynamic phase transition. The work has been published on Nature Communications 6, 8409 (2015).  ( PDF )

Figure. Relaxation time (a) and correlation length (b) as a function of effective temperature of the system. (c-f) Clusters of quasi-regular tetrahedra as packing fraction decreases.