Tung Chi-Huan, Chang Shou-Yi, Yip Sidney, Wang Yangyang, Carrillo Jan-Michael Y, Sumpter Bobby G, Shinohara Yuya, Do Changwoo, Chen Wei-Ren
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300044, Taiwan.
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2024 Mar 7;160(9). doi: 10.1063/5.0189938.
A method for characterizing the topological fluctuations in liquids is proposed. This approach exploits the concept of the weighted gyration tensor of a collection of particles and permits the definition of a local configurational unit (LCU). The first principal axis of the gyration tensor serves as the director of the LCU, which can be tracked and analyzed by molecular dynamics simulations. Analysis of moderately supercooled Kob-Andersen mixtures suggests that orientational relaxation of the LCU closely follows viscoelastic relaxation and exhibits a two-stage behavior. The slow relaxing component of the LCU corresponds to the structural, Maxwellian mechanical relaxation. Additionally, it is found that the mean curvature of the LCUs is approximately zero at the Maxwell relaxation time with the Gaussian curvature being negative. This observation implies that structural relaxation occurs when the configurationally stable and destabilized regions interpenetrate each other in a bicontinuous manner. Finally, the mean and Gaussian curvatures of the LCUs can serve as reduced variables for the shear stress correlation, providing a compelling proof of the close connection between viscoelastic relaxation and topological fluctuations in glass-forming liquids.
提出了一种表征液体中拓扑涨落的方法。该方法利用了一组粒子的加权回转张量的概念，并允许定义一个局部构型单元（LCU）。回转张量的第一主轴用作LCU的指向矢，可通过分子动力学模拟对其进行跟踪和分析。对中度过冷的Kob-Andersen混合物的分析表明，LCU的取向弛豫紧密跟随粘弹性弛豫，并表现出两阶段行为。LCU的缓慢弛豫成分对应于结构上的麦克斯韦力学弛豫。此外，发现在麦克斯韦弛豫时间，LCU的平均曲率近似为零，而高斯曲率为负。这一观察结果意味着，当构型稳定和不稳定区域以双连续方式相互渗透时，结构弛豫就会发生。最后，LCU的平均曲率和高斯曲率可作为剪切应力相关性的约化变量，为玻璃形成液体中粘弹性弛豫与拓扑涨落之间的紧密联系提供了有力证据。
