Abstract:

In order to reveal the influence of van der Waals' force on Lennard-Jones fluid viscoelasticity from the micro scale, equilibrium molecular simulation method was used to research the liquid-solid coexistence Lennard-Jones fluid of 30 conditions in the range of ρ^*=0.85-1.0 and T^*=0.6-1.5 in this study. First, the viscosity of argon is simulated by this model and the results are consistent with the experimental value of the error within 6.69%, which verifies the expansibility of the model to the real substance. Then the liquid-solid coexistence Lennard-Jones fluids simulated, and the accuracy of the simulation in this range is verified by comparing the simulation viscosities with the literature values within the error of 5.16%. Finally, the variation of both static viscoelasticity (viscosity η^*, high-frequency shear modulus G_∞^*) and dynamic viscoelasticity (storage modulus G'^*, loss modulus G"^*) were observed from the external factors (temperature and density) and internal factors (Lennard-Jones potential parameters, ε and σ), in addition, the microscopic mechanism of van der Waals' force effect on the viscoelasticity was elaborated as well. The results show that both density increment and temperature decrement lead to the increase of the static viscoelasticity (η^*, G_∞^*), meanwhile, G'^* and G"^* in the high frequency region were also increased by the increment of temperature and density, which suggests the enhancement of the viscosity and elasticity. Furthermore, when Lennard-Jones potential parameter, ε or σ increases, the Lennard-Jones fluid tends to be more solidified, this enhances both static and dynamic viscoelasticity and provides a guide to improve the viscoelastic properties of monatomic material in the engineering applications.

Key words: molecular simulation, viscoelasticity, viscosity, elasticity, van der Waals' force

摘要：

采用平衡态分子模拟方法对ρ^*=0.85~1.0，T^*=0.6~1.5范围内共30组的液固共存态Lennard-Jones体的黏弹性进行研究。模拟所得真实物质Ar的约化黏度与实验值误差为6.69%，验证了本模型对真实物质的可拓展性；同时，液固共存态下Lennard-Jones体模型的黏度模拟值与文献值吻合较好，误差小于5.16%，模拟精度较高。从环境参数（T^*、ρ^*）和分子参数（L-J势参数ε、σ）两方面，观测了Lennard-Jones体的静态（黏度η^*、无限大频率的剪切模量G_∞^*）及动态（储能模量G'^*、损耗模量G"^*）黏弹性的变化规律，并在此基础上解释范德华力对黏弹性的影响机理。结果表明，ρ^*升高或T^*降低将导致η^*、G_∞^*的升高，而T^*、ρ^*的升高则会使得中高频区的G'^*及G"^*增大；L-J势参数ε、σ的增大均能促进体系固态化，增强其静态及动态黏弹性，可为工程上高效利用单原子物质的黏弹性提供理论指导。

关键词: 分子模拟, 黏弹性, 黏度, 弹性, 范德华力