Bernini S, Puosi F, Leporini D
Dipartimento di Fisica "Enrico Fermi," Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy.
Laboratoire de Physique de l'École Normale Supérieure de Lyon, UMR CNRS 5672, 46 allée d'Italie, 69007 Lyon, France.
J Chem Phys. 2015 Mar 28;142(12):124504. doi: 10.1063/1.4916047.
We investigate by molecular-dynamics simulations, the fast mobility-the rattling amplitude of the particles temporarily trapped by the cage of the neighbors-in mildly supercooled states of dense molecular (linear trimers) and atomic (binary mixtures) liquids. The mixture particles interact by the Lennard-Jones potential. The non-bonded particles of the molecular system are coupled by the more general Mie potential with variable repulsive and attractive exponents in a range which is a characteristic of small n-alkanes and n-alcohols. Possible links between the fast mobility and the geometry of the cage (size and shape) are searched. The correlations on a per-particle basis are rather weak. Instead, if one groups either the particles in fast-mobility subsets or the cages in geometric subsets, the increase of the fast mobility with both the size and the asphericity of the cage is revealed. The observed correlations are weak and differ in states with equal relaxation time. Local forces between a tagged particle and the first-neighbour shell do not correlate with the fast mobility in the molecular liquid. It is concluded that the cage geometry alone is unable to provide a microscopic interpretation of the known, universal link between the fast mobility and the slow structural relaxation. We suggest that the particle fast dynamics is affected by regions beyond the first neighbours, thus supporting the presence of collective, extended fast modes.
我们通过分子动力学模拟研究了处于轻度过冷状态的致密分子（线性三聚体）液体和原子（二元混合物）液体中，粒子被相邻粒子构成的笼暂时困住时的快速迁移率——即粒子的晃动幅度。混合物粒子通过 Lennard-Jones 势相互作用。分子系统中的非键合粒子通过更通用的 Mie 势耦合，其排斥和吸引指数可变，范围是小链烷烃和链醇的特征范围。我们探寻了快速迁移率与笼的几何结构（尺寸和形状）之间可能存在的联系。基于单个粒子的相关性相当弱。相反，如果将粒子按快速迁移率子集分组，或者将笼按几何子集分组，就会发现快速迁移率随笼的尺寸和非球度的增加而增大。观察到的相关性很弱，并且在具有相同弛豫时间的状态下有所不同。在分子液体中，标记粒子与第一近邻壳层之间的局部力与快速迁移率不相关。得出的结论是，仅笼的几何结构无法对快速迁移率与缓慢结构弛豫之间已知的普遍联系提供微观解释。我们认为粒子的快速动力学受到第一近邻之外区域的影响，从而支持了集体、扩展快速模式的存在。
