Furukawa Akira, Tanaka Hajime
Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan.
Phys Rev E. 2016 Nov;94(5-1):052607. doi: 10.1103/PhysRevE.94.052607. Epub 2016 Nov 17.
Glass-forming liquids are often classified into strong glass formers with nearly Arrhenius behavior and fragile ones with super-Arrhenius behavior. We reveal a significant difference in the dynamics between these two types of glass formers through molecular dynamics simulations: In strong glass formers, the relaxation dynamics of density fluctuations is nondiffusive, whereas in fragile glass formers it exhibits diffusive behavior. We demonstrate that this distinction is a direct consequence of the fundamental difference in the underlying elementary relaxation process between these two dynamical classes of glass formers. For fragile glass formers, a density-exchange process proceeds the density relaxation, which takes place locally at the particle level in normal states but is increasingly cooperative and nonlocal as the temperature is lowered in supercooled states. On the other hand, in strong glass formers, such an exchange process is not necessary for density relaxation due to the presence of other local relaxation channels. Our finding provides a novel insight into Angell's classification scheme from a hydrodynamic perspective.
玻璃形成液体通常分为具有近似阿仑尼乌斯行为的强玻璃形成体和具有超阿仑尼乌斯行为的易碎玻璃形成体。我们通过分子动力学模拟揭示了这两种类型的玻璃形成体在动力学方面的显著差异：在强玻璃形成体中，密度涨落的弛豫动力学是非扩散性的，而在易碎玻璃形成体中，它表现出扩散行为。我们证明，这种区别是这两类动力学玻璃形成体在潜在基本弛豫过程中根本差异的直接结果。对于易碎玻璃形成体，密度交换过程先于密度弛豫，密度弛豫在正常状态下于粒子层面局部发生，但在过冷状态下随着温度降低，其协同性和非局部性越来越强。另一方面，在强玻璃形成体中，由于存在其他局部弛豫通道，这种交换过程对于密度弛豫并非必要。我们的发现从流体动力学角度为安吉尔的分类方案提供了全新见解。
