Recently, there are hesitations in the application scope of the classical Cassie theory and Wenzel theory. In this paper, Molecular Dynamics (MD) simulations are used to study these two theories used in the nanoscale and find their limitations. The effect of parameters including solid fractions (or roughness factors), arrangement of pillars (with same solid fractions), pillar height, and droplet size on contact angles was investigated. It shows that the Cassie equation is suitable for droplets on uniform pillared surfaces including different solid fractions, arrangement of pillars and pillar height, when there is no meniscus of droplets. The Wenzel equation is also suitable for droplets on uniform pillared surfaces including different roughness factors, arrangement of pillars and pillar heights. Moreover, whether the droplet size has an influence on the contact angle depends on the pinned place of the contact line. In the Wenzel state, the contact line is pinned although increasing the droplet size, resulting in increasing the contact angle, while the contact angle decreases to the initial value again when the droplet size increases enough to allow the contact line moving to the next pillar. The results provide insights toward the wettability of droplets on surfaces in nanoscale.

近来，在经典的卡西理论和温泽尔理论的应用范围上存在犹豫。在本文中，分子动力学（MD）模拟用于研究纳米级中使用的这两种理论并发现它们的局限性。研究了包括固相分数（或粗糙度因子），立柱排列（固含量相同），立柱高度和液滴尺寸等参数对接触角的影响。结果表明，当没有液滴弯月面时，Cassie方程适用于均匀柱表面上的液滴，包括不同的固体分数，柱的排列和柱高。Wenzel方程也适用于均匀的带柱表面上的液滴，包括不同的粗糙度系数，柱的排列和柱的高度。而且，液滴尺寸是否会影响接触角取决于接触线的固定位置。在Wenzel状态下，尽管增加了液滴的大小，接触线仍被钉住，导致接触角增加，而当液滴的大小增加到足以允许接触线移动到下一个支柱时，接触角又再次减小到初始值。结果提供了关于纳米级表面上的液滴的润湿性的见解。