Department of Chemical, Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80, 80125 Napoli, Italy.
Int J Mol Sci. 2022 Mar 24;23(7):3556. doi: 10.3390/ijms23073556.
Glass transition is a most intriguing and long-standing open issue in the field of molecular liquids. From a macroscopic perspective, glass-forming systems display a dramatic slowing-down of the dynamics, with the inverse diffusion coefficient and the structural relaxation times increasing by orders of magnitude upon even modest supercooling. At the microscopic level, single-molecule motion becomes strongly intermittent, and can be conveniently described in terms of "cage-jump" events. In this work, we investigate a paradigmatic glass-forming liquid, the Kob-Andersen Lennard-Jones model, by means of Molecular Dynamics simulations, and compare the macroscopic and microscopic descriptions of its dynamics on approaching the glass-transition. We find that clear changes in the relations between macroscopic timescales and cage-jump quantities occur at the crossover temperature where Mode Coupling-like description starts failing. In fact, Continuous Time Random Walk and lattice model predictions based on cage-jump statistics are also violated below the crossover temperature, suggesting the onset of a qualitative change in cage-jump motion. Interestingly, we show that a fully microscopic relation linking cage-jump time- and length-scales instead holds throughout the investigated temperature range.
玻璃化转变是分子液体领域中一个最引人入胜和长期存在的开放性问题。从宏观角度来看，玻璃形成系统的动力学明显减慢，即使是适度的过冷，逆扩散系数和结构弛豫时间也会增加几个数量级。在微观水平上，单分子运动变得强烈的间歇性，可以方便地用“笼跳跃”事件来描述。在这项工作中，我们通过分子动力学模拟研究了一种典型的玻璃形成液体，即 Kob-Andersen Lennard-Jones 模型，并比较了在接近玻璃转变时其动力学的宏观和微观描述。我们发现，在模式耦合描述开始失效的交叉温度处，宏观时间尺度和笼跳跃量之间的关系发生了明显的变化。事实上，基于笼跳跃统计的连续时间随机行走和格子模型预测在交叉温度以下也被违反，这表明笼跳跃运动发生了定性变化。有趣的是，我们表明，一个完全微观的关系，将笼跳跃时间和长度尺度联系起来，在整个研究的温度范围内都成立。
