Vogel Michael, Glotzer Sharon C
Department of Chemical Engineering, University of Michigan, 2300 Hayward, Ann Arbor, Michigan 48109, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2004 Dec;70(6 Pt 1):061504. doi: 10.1103/PhysRevE.70.061504. Epub 2004 Dec 9.
Molecular dynamics simulations are performed to study spatially heterogeneous dynamics in a model of viscous silica above and below the critical temperature of the mode coupling theory, T(MCT) . Specifically, we follow the evolution of the dynamic heterogeneity as the temperature dependence of the transport coefficients shows a crossover from non-Arrhenius to Arrhenius behavior when the melt is cooled. It is demonstrated that, on intermediate time scales, a small fraction of oxygen and silicon atoms are more mobile than expected from a Gaussian approximation. These highly mobile particles form transient clusters larger than that resulting from random statistics, indicating that the dynamics are spatially heterogeneous. An analysis of the clusters reveals that the mean cluster size is maximum at times intermediate between ballistic and diffusive motion, and the maximum size increases with decreasing temperature. In particular, the growth of the clusters continues into the crossover to Arrhenius behavior for the transport coefficients. These findings imply that the structural relaxation in silica cannot be understood as a statistical bond breaking process. Although the mean cluster sizes for silica are at the lower end of the spectrum of values reported in the literature for other model liquids, we find that spatially heterogeneous dynamics in models of strong and fragile glass formers are similar on a qualitative level. However, unlike the results for fragile liquids, we show that correlated particle motion along quasi-one-dimensional, stringlike paths is of little importance for the structural relaxation in this model of silica, suggesting that stringlike motion is suppressed by the presence of a network structure. To study transient clusters of localized particles, we calculate a generalized susceptibility corresponding to the self-part of a four-point time dependent density correlation function. We find that this generalized susceptibility is maximum on the time scale of the structural relaxation, where a strong increase of the peak height indicates a growing length of spatial correlations between localized particles upon cooling. Characterizing the local structural environments of the most mobile and the most immobile particles, respectively, we show that high particle mobility is facilitated by, but not limited to, the vicinity of defects in the network structure.
进行分子动力学模拟，以研究在模式耦合理论的临界温度T(MCT)之上和之下的粘性二氧化硅模型中的空间非均匀动力学。具体而言，当熔体冷却时，随着输运系数的温度依赖性显示出从非阿累尼乌斯行为到阿累尼乌斯行为的转变，我们追踪动态非均匀性的演变。结果表明，在中间时间尺度上，一小部分氧原子和硅原子比高斯近似预期的更具流动性。这些高流动性粒子形成的瞬态团簇比随机统计产生的团簇更大，表明动力学在空间上是不均匀的。对团簇的分析表明，平均团簇尺寸在弹道运动和扩散运动之间的中间时间达到最大值，并且最大尺寸随温度降低而增加。特别是，团簇的生长一直持续到输运系数转变为阿累尼乌斯行为。这些发现意味着二氧化硅中的结构弛豫不能被理解为统计键断裂过程。尽管二氧化硅的平均团簇尺寸处于文献中报道的其他模型液体值谱的低端，但我们发现强玻璃形成体和弱玻璃形成体模型中的空间非均匀动力学在定性水平上是相似的。然而，与弱液体的结果不同，我们表明在这个二氧化硅模型中，沿着准一维、线状路径的相关粒子运动对结构弛豫不太重要，这表明线状运动被网络结构的存在所抑制。为了研究局部粒子的瞬态团簇，我们计算了与四点时间相关密度关联函数的自部分相对应的广义磁化率。我们发现这种广义磁化率在结构弛豫的时间尺度上最大，在该时间尺度上，峰值高度的强烈增加表明冷却时局部粒子之间空间关联长度的增加。分别表征最具流动性和最不具流动性粒子的局部结构环境，我们表明高粒子流动性受到网络结构中缺陷附近的促进，但不限于该区域。
