Mishra Chandan K, Habdas Piotr, Yodh A G
Department of Physics and Astronomy , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States.
Department of Physics , Saint Joseph's University , Philadelphia , Pennsylvania 19131 , United States.
J Phys Chem B. 2019 Jun 20;123(24):5181-5188. doi: 10.1021/acs.jpcb.9b03419. Epub 2019 Jun 10.
The dynamics in supercooled liquids slow enormously upon approaching the glass transition, albeit without significant change of liquid structure. This empirical observation has stimulated development of many theoretical models which attempt to elucidate microscopic mechanisms in glasses and glass precursors. Here, quasi-two-dimensional colloidal supercooled liquids and glasses are employed to experimentally test predictions of widely used models: mode coupling theory (MCT) and its important extension, inhomogeneous MCT (IMCT). We measure two-point dynamic correlation functions in the glass forming liquids to determine structural relaxation times, τ, and mode coupling exponents, a, b, and γ; these parameters are then used to extract the mode coupling dynamic crossover packing area-fraction, ϕ . This information, along with our measurements of supercooled liquid spatiotemporal dynamics, permits characterization of dynamic heterogeneities in the samples and facilitates direct experimental tests of the scaling predictions of IMCT. The time scales at which dynamic heterogeneities are largest, and their spatial sizes, exhibit power law growth on approaching ϕ . Within experimental error, the exponents of the measured power laws are close to the predictions of IMCT.
过冷液体在接近玻璃化转变时动力学极大地减慢，尽管液体结构没有显著变化。这一经验观察结果刺激了许多理论模型的发展，这些模型试图阐明玻璃及玻璃前驱体中的微观机制。在此，采用准二维胶体过冷液体和玻璃来对广泛使用的模型预测进行实验测试：模式耦合理论（MCT）及其重要扩展——非均匀MCT（IMCT）。我们测量玻璃形成液体中的两点动态关联函数，以确定结构弛豫时间τ以及模式耦合指数a、b和γ；然后利用这些参数提取模式耦合动态交叉堆积面积分数ϕ。这些信息，连同我们对过冷液体时空动力学的测量结果，使得能够对样品中的动态非均匀性进行表征，并有助于对IMCT的标度预测进行直接实验测试。动态非均匀性最大时的时间尺度及其空间尺寸，在接近ϕ时呈现幂律增长。在实验误差范围内，所测幂律的指数接近IMCT的预测值。
