Recently the research group led by Prof. Yujie Wang, School of Physics and Astronomy, SJTU, has published a paper entitled “Connecting packing efficiency of binary hard sphere systems to their intermediate range” in Physical Review Letters.
Hard sphere (HS) systems play a paramount role in statistical mechanics and material science since they are important models to study the behavior of many-particle systems like liquids, colloids, and metallic glasses. In the context of the glass transition, one also often uses weakly polydisperse samples or a slightly asymmetric binary mixture, since this suppresses the crystallization of the liquid while keeping its structure close to the well understood one-component case. These studies have shown that, depending on the composition and the packing fraction, the local structure shows a surprisingly rich variety. Understanding this structure is important since such highly asymmetric systems are relevant for describing the behavior of real granular materials, which are usually highly polydisperse, as well for the glass-forming ability of multicomponent systems like metallic glasses. The theoretical description is, however, significantly complex with the difficulty to characterize their structure on intermediate length scales. And experiments on colloidal systems are hampered by the precise control of size ratio, while granular systems are prone to the phenomenon of phase separation. Because of these difficulties, there is at present little insight on how the size ratio or the composition affects the packing density or the structure of asymmetric HS systems.
Prof. Yujie Wang’s group, in cooperation with Prof. Walter Kob from University of Montpellier and Prof. Zhang Zhen from Xi'an Jiaotong University, has employed computational X-ray tomography, to determine the three dimensional structure of binary hard sphere packings. By employing a novel method to analyze this structure, Prof. Yujie Wang’s group has been able to decide the static correlation length for structure. The experiments show that the arrangement of the particles is in fact much less disordered than expected in that one finds spatial correlations that extend to surprisingly large distances. Furthermore the experiments reveal the presence of symmetries in the structure, a type of order that has not been detected in any previous experiments or theoretical calculations. The results illustrate that a high packing fraction is intimately related to the presence of a short structural correlation length, demonstrating that probing the structure in 3D is a powerful approach which allows to characterize the structure of many-body disordered systems. Exploiting this approach to multicomponent systems will thus allow not only to advance our understanding of disordered systems but also facilitate the creation of novel self-assembled materials and glass-forming systems
This work is supported by National Natural Science Foundation of China (No. 11974240) and Shanghai Science and Technology Commission (No. 19XD1402100).