Abstract

Physical aging is an inherent property of glassy matter, but understanding its microscopic mechanism remains a challenge particularly at the particle level. In this work, we use a confocal microscope to in-situ trace the particle trajectories in a 3D colloidal glass for 73000 s, aiming at resolving the aging dynamics. By calculating the mean square displacement of particle motions, we find that the glass aging with time can be divided into three stages: β relaxation, α relaxation and free diffusion. The system’s mean square displacement at each aging state is quantitatively resolved into three contributions of particle dynamics modes: vibration within the nearest-neighbor cages, hopping between cages and cooperative rearrangement. We further calculate the particle’s free volume and find that the β-to-α transition is accompanied by the temporary increase of the system-averaged free volume due to pronounced hops of particles. Nevertheless, the temporal autocorrelation of the free volume spatial distribution still obeys a monotonically stretched exponential decay with an exponent of 0.76, which is related to the sub-diffusion dynamics of cooperative rearrangements and hops mixed in α relaxation. According to the resolved vibrational displacements, we calculate the vibrational density of states of this 3D glass, and the characteristic boson peak is reproduced at low frequencies. Our findings shed insight into the particle-level aging dynamics of a real glass under purely thermal activation.

中文翻译：

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解决 3D 胶体玻璃的老化动力学问题

摘要

物理老化是玻璃物质的固有特性，但理解其微观机制仍然是一个挑战，特别是在颗粒水平上。在这项工作中，我们使用共焦显微镜对 3D 胶体玻璃中的粒子轨迹进行了 73000 秒的原位追踪，旨在解决老化动力学问题。通过计算质点运动的均方位移，我们发现玻璃随时间的老化可分为三个阶段：β弛豫、α弛豫和自由扩散。系统在每个老化状态下的均方位移被定量地分解为粒子动力学模式的三个贡献：最近邻笼内的振动、笼之间的跳跃和协作重排。我们进一步计算了粒子的自由体积，发现β到α的转变伴随着由于粒子明显的跳跃而导致的系统平均自由体积的暂时增加。尽管如此，自由体积空间分布的时间自相关仍然服从指数为0.76的单调拉伸指数衰减，这与α松弛中混合的协作重排和跳跃的子扩散动力学有关。根据解析的振动位移，我们计算了该 3D 玻璃的振动态密度，并在低频下再现了特征玻色子峰。我们的研究结果深入了解了纯热激活下真实玻璃的颗粒级老化动力学。