Molecular dynamics simulations for the motion of evaporative droplets driven by thermal gradients along nanochannels
Molecular dynamics simulations for the motion of evaporative droplets driven by thermal gradients along nanochannels
复制标题
热梯度驱动的蒸发液滴沿纳米通道运动的分子动力学模拟
DOI：
10.1088/0953-8984/25/19/195103
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发表时间：
2013-05-15
影响因子：
2.7
通讯作者：
Qian, Tiezheng
中科院分区：
文献类型：
--
作者：
Wu, Congmin;Xu, Xinpeng;Qian, Tiezheng
对于固体衬底上的单组分流体，在气液界面与固体表面相交的接触线上可能会出现热奇异性。在单组分流体中，气液界面在气液共存温度下几乎等温，而对于导热系数较高的固体，固体表面几乎等温。因此，如果两个等温界面的温度不同，并且在接触线处相交，则形成温度不连续。这导致了所谓的热奇性。接触线附近涉及蒸发/冷凝的局部流体动力学导致接触角取决于衬底温度。这种依赖关系已被证明导致液滴在具有温度梯度的固体衬底上的运动(Xu和Qian 2012 Phys)。修订本E 85 061603)。在目前的工作中，我们进行了分子动力学(MD)模拟作为数值实验，以进一步证实我们之前的连续介质流体动力学建模和模拟所做出的预测，这些预测实际上是半定量的，精确到问题中的小长度尺度。利用分子动力学模拟方法，研究了单组分Lennard-Jones流体中蒸发液滴在具有温度梯度的纳米通道中的运动。液滴沿固壁温度降低的方向迁移，迁移速度与温度梯度成正比。这与我们的连续介质模型的预测是一致的。然后，我们测量液滴大小对液滴运动的影响。研究发现，液滴的迁移率与液滴运动引起的总耗散率的无量纲系数成反比。我们的结果表明，对于本文模拟的小液滴(类似于10 nm)，该系数是有序的，并且随着液滴尺寸的增加而增加。这些发现与我们的连续介质模型的预测是半定量一致的。最后，测量了汽液共存温度对液滴运动的影响。通过对热奇点大小的理论分析，可以看出液滴的迁移率随着共存温度的降低而减小。这是在我们的MD模拟中观察到的。
For a one-component fluid on a solid substrate, a thermal singularity may occur at the contact line where the liquid-vapor interface intersects the solid surface. Physically, the liquid-vapor interface is almost isothermal at the liquid-vapor coexistence temperature in one-component fluids while the solid surface is almost isothermal for solids of high thermal conductivity. Therefore, a temperature discontinuity is formed if the two isothermal interfaces are of different temperatures and intersect at the contact line. This leads to the so-called thermal singularity. The localized hydrodynamics involving evaporation/condensation near the contact line leads to a contact angle depending on the underlying substrate temperature. This dependence has been shown to lead to the motion of liquid droplets on solid substrates with thermal gradients (Xu and Qian 2012 Phys. Rev. E 85 061603). In the present work, we carry out molecular dynamics (MD) simulations as numerical experiments to further confirm the predictions made from our previous continuum hydrodynamic modeling and simulations, which are actually semi-quantitatively accurate down to the small length scales in the problem. Using MD simulations, we investigate the motion of evaporative droplets in one-component Lennard-Jones fluids confined in nanochannels with thermal gradients. The droplet is found to migrate in the direction of decreasing temperature of solid walls, with a migration velocity linearly proportional to the temperature gradient. This agrees with the prediction of our continuum model. We then measure the effect of droplet size on the droplet motion. It is found that the droplet mobility is inversely proportional to a dimensionless coefficient associated with the total rate of dissipation due to droplet movement. Our results show that this coefficient is of order unity and increases with the droplet size for the small droplets (similar to 10 nm) simulated in the present work. These findings are in semi-quantitative agreement with the predictions of our continuum model. Finally, we measure the effect of liquid-vapor coexistence temperature on the droplet motion. Through a theoretical analysis on the size of the thermal singularity, it can be shown that the droplet mobility decreases with decreasing coexistence temperature. This is observed in our MD simulations.