Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, USA.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2018 Oct 14;149(14):144504. doi: 10.1063/1.5037282.
We investigate local structural fluctuations in a model equilibrium fluid with the aim of better understanding the structural basis of locally heterogeneous dynamics identified in recent simulations and experimental studies of glass-forming liquids and other strongly interacting particle systems, such as lipid membranes, dusty plasmas, interfacial dynamics of crystals, the internal dynamics of proteins, etc. In particular, we utilize molecular dynamics simulation methods to study a single component Lennard-Jones condensed material at constant temperature in two dimensions over a range of densities covering both liquid and crystalline phase regimes. We identify three distinct structural classes of particles by examining the immediate neighborhood of individual particles relying on a solid-angle based tessellation technique. The area distribution of the neighborhoods reveals cages having hexagonal, pentagonal, and square symmetries. Pentagonal cells appear to be the predominant motif in the liquid phase, while the solid phase is dominated by hexagonal cells, as in the case of a perfect crystal. An examination of the spatial organization of particles belonging to each structural class further indicates that finite-size clusters of the hexagonal and pentagonal particle populations arise within both liquids and solids, and the size of these clusters grows in a complementary way as a function of density. Both particle populations form percolation clusters in the liquid-crystal coexistence regime. Interestingly, the populations of particles with different local structures, defined by the arrangement of neighboring particles, are found to maintain different diffusivities, as computed from the velocity autocorrelation function for each type of particle for all densities studied. Our analysis provides a new conceptual framework for understanding the structural origin of dynamical heterogeneity in soft materials.
我们研究了模型平衡流体中的局部结构波动，旨在更好地理解最近在玻璃形成液体和其他强相互作用粒子系统（如脂质膜、尘埃等离子体、晶体界面动力学、蛋白质内部动力学等）的模拟和实验研究中发现的局部不均匀动力学的结构基础。特别是，我们利用分子动力学模拟方法研究了二维单组分 Lennard-Jones 凝聚物质在恒定温度下的密度范围，涵盖了液体和晶体相区。我们通过检查单个粒子的紧邻区域，利用基于立体角的细分技术，确定了三种不同的粒子结构类别。邻居的面积分布揭示了具有六边形、五边形和正方形对称性的笼子。在液体相中，五边形单元似乎是主要的图案，而在固体相中，六边形单元占主导地位，就像完美晶体的情况一样。对属于每个结构类别的粒子的空间组织的检查进一步表明，在液体和固体中都出现了六边形和五边形粒子群的有限大小簇，并且这些簇的大小作为密度的函数以互补的方式增长。在液体-晶体共存区，两种粒子群都形成了渗流簇。有趣的是，不同局部结构的粒子（由相邻粒子的排列定义）的种群被发现具有不同的扩散率，这是通过对所有研究密度的每种类型的粒子的速度自相关函数计算得出的。我们的分析为理解软物质中动力学不均匀性的结构起源提供了一个新的概念框架。
