Microscopic origin of the fragile to strong crossover in supercooled water: The role of activated processes
Microscopic origin of the fragile to strong crossover in supercooled water: The role of activated processes
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DOI：
10.1063/1.4975387
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发表时间：
2017-02-28
影响因子：
4.4
通讯作者：
Gallo, P.
中科院分区：
文献类型：
--
作者：
De Marzio, M.;Camisasca, G.;Gallo, P.
我们对TIP4P/2005过冷水在液液临界点附近的密度自关联函数进行了精确的分析。在对该模型的前期工作中，我们提供了深过冷区动力学行为从脆弱到强烈交叉的证据。结构松弛在脆性区遵循模式耦合理论，然后偏离模式耦合区域，表现出较强的Arrhenius行为。这种交叉在水中特别重要，因为它与过冷区的热力学有关。为了更好地理解这种交叉的起源，我们现在计算Van Hove自相关函数。特别是，我们的目标是研究简单液体中的跳跃现象的存在和作用，跳跃现象是导致简单液体中脆弱到强烈交叉的原因。在TIP4P/2005水中，我们也发现了跳跃过程，并分析了它们在接近脆弱到强交叉和模耦合理想交叉温度时对温度和密度的依赖关系。我们的结果表明，水的行为就像一个简单的玻璃形体。在初始弹道状态之后，笼子效应在轻度过冷区占主导地位，并在很长一段时间内发生扩散。在脆弱到强大的交叉中，我们发现跳跃(激活的)过程开始发挥作用。范霍夫关联函数中出现的峰值证明了这一点。在深过冷区，我们的分析清楚地表明，激活过程主导着动力学。通过对Van Hove函数和径向分布函数的比较，可以更好地理解过冷水中跳跃现象的机制，并将跳跃现象的发生与Widom线的交叉直接联系起来。由AIP出版公司出版。
We perform an accurate analysis of the density self-correlation functions of TIP4P/2005 supercooled water on approaching the region of the liquid-liquid critical point. In a previouswork on this model, we provided evidence of a fragile to strong crossover of the dynamical behavior in the deep supercooled region. The structural relaxation follows the Mode Coupling theory in the fragile region and then deviates from Mode Coupling regime to a strong Arrhenius behavior. This crossover is particularly important in water because it is connected to the thermodynamics of the supercooled region. To better understand the origin of this crossover, we compute now the Van Hove self-correlation functions. In particular we aim at investigating the presence and the role of the hopping phenomena that are the cause of the fragile to strong crossover in simple liquids. In TIP4P/2005 water, we find hopping processes too and we analyze how they depend on temperature and density upon approaching the fragile to strong crossover and the Mode Coupling ideal crossover temperature. Our results show that water behaves like a simple glass former. After an initial ballistic regime, the cage effect dominates the mild supercooled region, with diffusion taking place at long time. At the fragile to strong crossover, we find that hopping (activated) processes start to play a role. This is evidenced by the appearance of peaks in the Van Hove correlation functions. In the deep supercooled regime, our analysis clearly indicates that activated processes dominate the dynamics. The comparison between the Van Hove functions and the radial distribution functions allows one to better understand the mechanism of hopping phenomena in supercooled water and to connect their onset directly with the crossing of the Widom Line. Published by AIP Publishing.