Purely repulsive systems such as colloids, granular materials, and foams undergo the jamming transition from unjammed state without rigidity to disordered solid state with rigidity when the packing density increases. The jamming transition at zero temperature and shear stress is denoted as point J, which exhibits interesting but elusive phase transition features. This transition has mixed first and second order phase transition properties and cannot be simply classified into well-known types of transitions. In the past decade, the criticality of point J has been widely studied in various perspectives. Unlike conventional critical phenomena, there has been no report of the direct observation of a static diverging length scale associated with point J. As the simplest model to study noncrystalline liquid-solid transitions, the jamming transition at point J has become a hot topic in soft condensed matter physics. The concept of jamming has been adopted as well by many other fields to understand the formation of noncrystalline solids. 

When subject to the shear stress larger than the yield stress, jammed solids undergo the unjamming transition as well into shear flows. How to determine the yield stress is also a research focus of noncrystalline materials. In the perspective of the potential energy landscape, the yield stress should be the maximum stress under which mechanically stable solids can survive. It can be measured numerically via the fast search of stable solids in the potential energy landscape under the constraint of constant shear stress. However, such a numerical method is still lacking.

The team led by Prof. Ning Xu in the CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Physics has made an important progress on the calculation of the yield stress of jammed solids and the study of the criticality of point J by performing intensive simulations. The paper has been published in Phys. Rev. Lett. on April 11 of 2014 as an Editor’s Suggestion [Phys. Rev. Lett. 112, 145502 (2014)].

In this work, the PhD candidate LIU Hao, undergraduate student XIE Xiaoyi, and Prof. XU Ning propose to minimize a thermodynamic-like potential to obtain mechanically stable solids under desired shear stress. Through intensive samplings, they obtain the probability of finding jammed states under constant shear stress, from which they determine the yield stress and its uncertainty of jammed solids. The yield stress as a function of the packing density satisfies the finite size scaling, which implies a diverging length scale at point J and provides another evidence of the criticality of point J. Interestingly, the critical exponent is not limited to the yield stress. Multiple quantities exhibiting critical scaling near point J satisfy finite size scaling as well with the same critical exponent as the yield stress. Therefore, the length scale observed in this work is robust. Furthermore, by comparing properties of jammed solids obtained from the new method and quasistatic shear, they find that quasistatic shear tends to explore low-energy states, which suggests a possible and simple way to search for ultrastable glasses. 

This work was supported by the National Natural Science Foundation, Ministry of Science and Technology, Chinese Academy of Sciences, and Ministry of Education. 

Link to the paper：http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.145502

(HFNL)