以平行化分子動力學模擬探討奈米液滴的碰撞及汽液間的表面張力

Parallel molecular dynamics Simulation of binary head-on Collision and liquid-vapor interfacial tension

莊瑞榮(RUEI-RONG JHUANG)

指導教授 ： 趙修武

中原大學/工學院/機械工程學系/博士(2008年)

https://doi.org/10.6840/cycu200900062

摘要

摘 要
本文主要分為兩大部分，第一部分是利用平行化分子動力學模擬程式(Parallelized cellular molecular dynamics simulation code, PCMD)來模擬探討兩個在奈米尺度下由氦(Argon)原子所構成的兩個相同液滴在真空環境下以及含背壓環境狀態下的碰撞動力學行為。兩個相同大小的奈米氬液滴以低於10 m/s的相對速度正向碰撞。以前的文獻研究顯示彈性碰撞(bouncing)只在一狹窄的範圍內才會發生，這主要歸因於液滴之間有背景氣體的存在。然而，經由徹底改變正向碰撞條件的模擬，我們發現只要相對速度小於一臨界值，彈性碰撞就能很容易地發生，此一臨界值的大小強烈決定於背景氣壓的大小。一般說來這相對速度的臨界值隨背景氣壓的增加而減少。這種奈米液滴之間的回彈現象主要歸因於從兩液滴的表面所發射出的蒸發原子，在真空狀態下這更是一個明顯且重要的因素。
第二部份是以分子動力學模擬方法從局部內能函數來計算表面張力。本文提出一種修正的熱力學的方法藉由分子動力學模擬來從超額內能獲得表面張力，並與Baidakov等人的方法[J. Chem. Phys. 126, 214505 (2007)]相比較。改進的地方包括︰ 1) 超額內能密度從原來的界面模擬裡以局部內能函數的計算來獲得，不必進行另外的分子動力學模擬； 2)用一個從熱力學關係式直接導出的函數來擬合超額內能密度，如此可使表面張力的計算更簡單且直接。將本方法所得出的結果與使用力學方法所得出的結果相互比較驗証。一般而言，本文提出的修正方法所得出的結果平均上比Baidakov等人的方法更接近於實驗數據。

關鍵字

正向碰撞 ； 分子動力學模擬 ； 氬液滴 ； 超額內能 ； 表面張力

並列摘要

Abstract
[PART I]: Head-on collision of two equal-size nanoscale argon droplets with relative speed less than 10 m/s is investigated using a parallel cellular molecular dynamics code (PCMD). Previous studies showed that bouncing only occurred within a narrow range of head-on collision conditions, which was mainly attributed to the existence of background gas between the droplets. However, through simulations by thoroughly varying the head-on collision conditions, we have found that bouncing can easily occur as long as the relative speed is less than a critical value, which the magnitude strongly depends on the background gas pressure. This critical value of relative speed generally decreases with increasing background gas pressure. We attribute the bouncing between nanoscale droplets to the vaporizing atoms emitting from the head-on surfaces of the two droplets, which becomes the dominated factor under vacuum condition.
[PART II]: A revised thermodynamic approach based on the excess internal energy obtained from molecular dynamics simulation, as compared to Baidakov et al. [J. Chem. Phys. 126, 214505 (2007)], for determining the surface tension is proposed. Improvement includes: 1) The density of excess internal energy is obtained in the same interface simulation from the calculation of the local internal energy profile without performing additional molecular dynamics simulation; 2) The density of excess internal energy is fitted by a function which is directly derived from the thermodynamic relation, thus make the calculation of the surface tension more simple and straightforward. Results of the present approach are validated against those obtained using mechanical approach. In general, the present results are closer to the experimental data on the average than those by Baidakov et al.

並列關鍵字

MD Simulation ； head-on collision ； argon droplets ； surface tension ； excess internal energy

參考文獻

1. K. Gunn, Science 150, 888 (1965).

3. G. B. Foote, J. Atmos. Sci. 32, 390 (1975).

8. J. S. Rowlinson and B. Widom, Molecular Theory of Capillarity (Clarendon, Oxford,1982).

9. D. Henderson, Fundamentals of Inhomogeneous Fluids (Dekker, New York, 1992).

10. H. T. Davis, Statistical Mechanics of Phases, Interfaces, and Thin Films (VCH, Weinheim, 1996).