Dynamic heterogeneity, believed to be one of the hallmarks of the dynamics of supercooled liquids, is expected to affect the diffusion of the particles comprising the liquid. We have carried out extensive molecular dynamics simulations of two model glass forming liquids in three dimensions to study the time scales at which Fick's law of diffusion starts to set in. We have identified two different Fickian time scales: one at which the mean squared displacement begins to show a linear dependence on time, and another one at which the distribution of particle displacements becomes Gaussian. These two times scales are found to be very different from each other and from the α relaxation time in both systems. An interesting connection is found between one of these Fickian time scales and the time scale obtained from the bond-breakage correlation function. We discuss the relation among these different time scales and their connection to dynamic heterogeneity in the system.

动态异质性被认为是过冷液体动力学的标志之一，预计会影响包含液体的颗粒的扩散。我们在三个维度上对两种模型玻璃成型液体进行了广泛的分子动力学模拟，以研究菲克扩散定律开始建立的时间尺度。我们确定了两种不同的菲克时间尺度：一种是均方位移开始的时间尺度。表现出对时间的线性依赖性，另一种情况下，粒子位移的分布变为高斯分布。发现这两个尺度在两个系统中彼此之间和与α弛豫时间上都非常不同。在这些Fickian时间尺度之一与从键断裂相关函数获得的时间尺度之间发现了一个有趣的联系。我们讨论了这些不同时标之间的关系以及它们与系统中动态异质性的联系。