An investigation of the dynamics of phase transitions in Lennard-Jones fluids
An investigation of the dynamics of phase transitions in Lennard-Jones fluids
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发表时间：
1998
影响因子：
4
通讯作者：
N. Hadjiconstantinou
中科院分区：
文献类型：
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作者：
N. Hadjiconstantinou
本文报道了一种模拟外加热量的方法在Lennard-Jones流体的分子动力学模拟中的发展、验证和应用。这种仿真能力对于纯理论和实际应用都是非常重要的。这里我们考察了一个理论应用，即恒压下液态Ar团簇的蒸发。该算法基于由牛顿方程推导出的修正运动方程，并利用文献中所知的高斯最小约束原理。修正后的运动方程满足所有体系分子的线性(时间)能量相加的约束。论文的第一部分介绍了热添加算法的有效性：该方法只有在不对模拟材料的性质产生不利影响的情况下才是有用的。验证包括对Lennard-Jones流体在两个平行(分子)壁之间的二维通道中的一系列模拟。壁面保持恒温，而流体则使用新的模拟方法进行外部加热。根据连续介质理论，这个问题的温度分布解是抛物线的。在给定加热率的情况下，可以从温度分布的曲率获得导热率的估计值。将导热系数的估计值与流体的实验数据和基于牛顿(精确)运动方程的模拟数据进行了比较。我们发现，由我们的模拟得到的导热系数估计值与使用牛顿运动方程得到的基线结果是一致的。论文的第二部分利用加热法实时研究了恒压下流体团簇的相变过程。这之前从未尝试过；文献中只存在准静态模拟的结果，其中通过在不同温度下执行一系列平衡模拟来恢复Lennard-Jones流体的相变行为。通过新提出、发展和验证的时间相关方法得到的结果与线性响应理论预测的准静态模拟结果一致。最后，我们用均匀成核理论解释了我们的结果。我们发现我们的结果与均匀成核是一致的，它预测相分离开始于纳米水平，蒸发和冷凝的临界半径都在几个纳米量级。蒸发的临界原子核是气态的，可以预见的是，它们比处于液态的凝聚原子核要大。我们的结果与实验数据吻合得很好。这项工作可以为研究与成核理论和成核动力学有关的公开问题，如亚稳态团簇寿命和成核频率奠定基础。还可以研究其他的相变机制，例如调幅节点分解。论文导师：托马斯·阿里亚斯职称：助理教授
This thesis reports the development, validation and application of a method to simulate external heat addition in molecular dynamics simulations of Lennard-Jones fluids. This simulation capability is very important for both purely theoretical and practical applications. Here we examine one theoretical application, namely the evaporation of clusters of liquid Argon under constant pressure. The algorithm is based on modified equations of motion derived from Newton's equations with the use of what is known in the literature as Gauss' least constraint principle. The modified equations of motion satisfy the constraint of linear (in time) energy addition to all the system molecules. The first part of the thesis presents the validation of the heat addition algorithm: the method is useful only if it does not adversely affect the properties of the simulated material. The validation consists of a series of simulations of a Lennard-Jones fluid in a two-dimensional channel bounded between two parallel (molecular) walls. The walls are kept at constant temperature, while the fluid is externally heated using the new simulation method. The temperature profile solution for this problem is, according to (the exact) continuum theory, parabolic. Given the heat addition rate, estimates for the value of the thermal conductivity can be obtained from the curvature of the temperature profile. The estimates for the thermal conductivity are compared to experimental data for the fluid, and simulation data based on the Newtonian (exact) equations of motion for the same fluid. We find that the thermal conductivity estimates obtained from our simulations are in agreement with the baseline results utilizing the Newtonian equations of motion. The second part of the thesis reports on the investigation of the phase change of fluid clusters at constant pressure in real time using the heat addition algorithm. This has not been attempted before; results exist in the literature only for quasistatic simulations whereby the phase change behavior of a Lennard-Jones fluid is recovered by performing a series of equilibrium simulations at varying temperatures. The results obtained through the newly proposed, developed, and validated time dependent method are in agreeement with the results of the quasistatic simulations as linear response theory predicts. We conclude with the interpretation of our results using homogeneous nucleation theory. We find that our results are consistent with homogeneous nucleation which predicts that phase separation starts at the nanoscopic level with critical radii of the order of a few nanometers for both evaporation and condensation. The critical nuclei for evaporation, which are gaseous, are predictably larger than the nuclei for condensation, which are in the liquid state. Our results are in good agreement with experimental data. This work can form the basis for the investigation of open problems related to nucleation theory and nucleation kinetics, such as metastable cluster lifetimes, and nucleation frequencies. Alternative phase change mechanisms, such as spinodal decomposition, can also be investigated. Thesis Supervisor: Tomas A. Arias Title: Assistant Professor