Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension.
Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension.
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DOI：
10.1063/1.4972894
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发表时间：
2016-12
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影响因子：
--
通讯作者：
D. Nicholson;G. Rutledge
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文献类型：
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作者：
D. Nicholson;G. Rutledge
用非平衡分子动力学方法研究了正二十烷在平面剪切和单轴拉伸条件下的结晶成核。采用基于平均首次通过时间的分析方法对模拟结果进行了分析，以确定外加流场类型和应变速率对稳态成核速率和特征成核率的影响，以及对与成核有关的动力学参数：自由能垒、临界核尺寸和单体附着前因子的影响。与剪切相比，拉伸时的临界应变速率较小时，流动增强形核(FEN)的开始发生。当应变率大于临界应变率时，形核率的快速增加伴随着自由能垒和临界核尺寸的减小，以及链伸长的增加。这些观察结果与FEN的机制相一致，在该机制中，FEN是由于流动诱导的熵降低而导致结晶驱动力的增加。在高应变率下，自由能垒、临界核尺寸和拉伸程度达到饱和，而单体附着前因子和取向有序度稳步增加。这一趋势表明，在强流动下，与成核系统中全球取向有序度相关的成核率有显著的扩散贡献。这两个流场都给出了关于折合应变率的所有运动量的相似结果，我们将折合应变率定义为施加的应变率与临界应变率的比率。特征生长速率随应变速率的增加而增大，且与形核速率相对应，与外加的流场类型无关。此外，对结晶团簇的结构分析表明，流场抑制了团簇的致密化和晶化有序化，导致在强流场下形成大的铰接型团簇，在弱流场下形成致密的有序团簇。
Non-equilibrium molecular dynamics is used to study crystal nucleation of n-eicosane under planar shear and, for the first time, uniaxial extension. A method of analysis based on the mean first-passage time is applied to the simulation results in order to determine the effect of the applied flow field type and strain rate on the steady-state nucleation rate and a characteristic growth rate, as well as the effects on kinetic parameters associated with nucleation: the free energy barrier, critical nucleus size, and monomer attachment pre-factor. The onset of flow-enhanced nucleation (FEN) occurs at a smaller critical strain rate in extension as compared to shear. For strain rates larger than the critical rate, a rapid increase in the nucleation rate is accompanied by decreases in the free energy barrier and critical nucleus size, as well as an increase in chain extension. These observations accord with a mechanism in which FEN is caused by an increase in the driving force for crystallization due to flow-induced entropy reduction. At high applied strain rates, the free energy barrier, critical nucleus size, and degree of stretching saturate, while the monomer attachment pre-factor and degree of orientational order increase steadily. This trend is indicative of a significant diffusive contribution to the nucleation rate under intense flows that is correlated with the degree of global orientational order in a nucleating system. Both flow fields give similar results for all kinetic quantities with respect to the reduced strain rate, which we define as the ratio of the applied strain rate to the critical rate. The characteristic growth rate increases with increasing strain rate, and shows a correspondence with the nucleation rate that does not depend on the type of flow field applied. Additionally, a structural analysis of the crystalline clusters indicates that the flow field suppresses the compaction and crystalline ordering of clusters, leading to the formation of large articulated clusters under strong flow fields, and compact well-ordered clusters under weak flow fields.