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arXiv 0811.3811cond-mat.soft
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固液相变自扩散系数的尺寸依赖性研究

A study of the size dependence of self diffusivity across the solid-liquid transition

Manju Sharma, S. Yashonath

摘要:

本研究通过详细的分子动力学模拟研究了由较大溶剂和较小溶质通过Lennard-Jones势相互作用组成的二元混合物的温度诱导相变对溶质自扩散系数的尺寸依赖性的影响。通过两组模拟,一组在固定密度(NVE)和另一个在可变密度(NPT)的固-液转变过程中的密度变化的影响,以及在固体和液体相本身进行了研究。在固相和液相中,除了在低液体密度下,在大多数密度下都可以看到尺寸相关的扩散系数最大值。可以识别两种不同的区域:线性区域和异常区域。可以看出,固-液转变的作用是将扩散系数最大值移向较小尺寸的溶质,并降低最大值的高度。这是由于无序性的影响占主导地位,而密度的影响相对较弱。后者将扩散系数最大值移向较大的溶质尺寸,而前者则正好相反。有趣的是,我们发现,较低的密度导致较弱的扩散率最大值。我们报告首次为系统与大溶质-溶剂相互作用强度,$ε_{uv}$,多重扩散系数最大值被视为溶质大小的函数。其起源尚不清楚。一个显着的差异,在弹道扩散过渡的溶质从线性政权和异常政权。结果发现,这种转变是尖锐的溶质从反常制度。这提供了有趣的洞察异常制度溶质的运动。除此之外,我们报告的线性和异常制度溶质是不同的几个属性。

英文摘要:

The present study investigates the effect of temperature-induced change of phase from face centred cubic solid to liquid on the size dependence of self diffusivity of solutes through detailed molecular dynamics simulations on binary mixtures consisting of a larger solvent and a smaller solute interacting via Lennard-Jones potential. The effect of change in density during the solid-liquid transition as well as in the solid and the liquid phase itself have been studied through two sets of simulations one at fixed density(NVE)and another at variable density(NPT). A size dependent diffusivity maximum is seen in both solid and liquid phases at most densities except at low liquid densities. Two distinct regimes may be identified : linear and anomalous regimes. It is seen that the effect of solid to liquid transition is to shift the diffusivity maximum to smaller sizes of the solutes and decrease the height of the maximum. This is attributed to the predominant influence of disorder and relatively weaker influence of density. The latter shifts the diffusivity maximum towards larger solute size while the former does exactly the opposite. Interestingly, we find that lower densities lead to weaker diffusivity maximum. We report for the first time for systems with large solute-solvent interaction strength, $ε_{uv}$, multiple diffusivity maximum is seen as a function of the solute size. The origin of this is not clear. A remarkable difference in the ballistic-diffusive transition is seen between solutes from the linear regime and anomalous regime. It is found that this transition is sharp for solutes from anomalous regime. This provides interesting insight into motion of anomalous regime solutes. Apart from these, we report several properties which are different for the linear and anomalous regime solutes.

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