He Xin, Wang Yi-Feng, Zhang Ben-Xi, Wang Shuo-Lin, Yang Yan-Ru, Wang Xiao-Dong, Lee Duu-Jong
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China.
Research Center of Engineering Thermophysics, North China Electric Power University, Beijing 102206, China.
Langmuir. 2021 Dec 21;37(50):14571-14581. doi: 10.1021/acs.langmuir.1c01807. Epub 2021 Dec 13.
In this study, the wetting and dewetting behaviors of water nanodroplets containing various molecule numbers on nanopillar-arrayed surfaces in the presence or absence of an external electric field are investigated via molecular dynamics (MD) simulations, aiming to examine whether there is a scale effect. The results show that, in the absence of an electric field, nanodroplets on coexisting Cassie/Wenzel surfaces may be in the Cassie or the Wenzel state depending on their initial states, and apparent contact angles of the Cassie or Wenzel nanodroplets increase monotonously with increasing the droplet size. Energy analysis shows that on the same coexisting Cassie/Wenzel surface, when an electric field is imposed, a small nanodroplet possesses a lower energy barrier separating the Cassie state from the Wenzel state. Therefore, the small nanodroplet is easier to collapse into the Wenzel state. Moreover, the spontaneous Wenzel-to-Cassie dewetting transition is not observed for the nanodroplets after the removal of the electric field because the Wenzel state is a globally stable energetic state. With the same pillar geometry, both the wetting transition and the dewetting transition are significantly modified for liquids with higher intrinsic contact angles. The energy barrier of the wetting transition increases for both the large and small nanodroplets, meaning that the Cassie state becomes more robust. The energy curve shows that the Wenzel state of the large nanodroplet has higher energy so that the droplet can return to the Cassie state when removing the electric field. Intriguingly, although the small Wenzel nanodroplet has lower energy in the presence of the electric field, the dewetting transition still occurs. The increased solid-liquid interfacial tension when removing the electric field is responsible for this abnormal result. The wetting and dewetting transitions follow different energy pathways, leading to a hysteresis energy loop. There exists a critical water molecule number separating the unstable/stable Wenzel configurations, above which the Cassie state is energetically favorable and the dewetting transition can occur spontaneously after removing the electric field.
在本研究中，通过分子动力学（MD）模拟研究了在有或没有外部电场的情况下，含有不同分子数的水纳米液滴在纳米柱阵列表面上的润湿和去湿行为，旨在研究是否存在尺度效应。结果表明，在没有电场的情况下，共存的Cassie/Wenzel表面上的纳米液滴可能根据其初始状态处于Cassie状态或Wenzel状态，并且Cassie或Wenzel纳米液滴的表观接触角随着液滴尺寸的增加而单调增加。能量分析表明，在同一共存的Cassie/Wenzel表面上，施加电场时，小纳米液滴具有将Cassie状态与Wenzel状态分开的较低能垒。因此，小纳米液滴更容易塌陷到Wenzel状态。此外，去除电场后，纳米液滴未观察到自发的Wenzel到Cassie的去湿转变，因为Wenzel状态是全局稳定的能量状态。对于具有较高固有接触角的液体，在相同的柱几何形状下，润湿转变和去湿转变都有显著改变。大、小纳米液滴的润湿转变能垒均增加，这意味着Cassie状态变得更加稳定。能量曲线表明，大纳米液滴的Wenzel状态具有更高的能量，因此去除电场时液滴可以回到Cassie状态。有趣的是，尽管小的Wenzel纳米液滴在电场存在下具有较低的能量，但去湿转变仍然发生。去除电场时固液界面张力的增加是导致这一异常结果的原因。润湿和去湿转变遵循不同的能量路径，导致能量滞后环。存在一个临界水分子数，将不稳定/稳定的Wenzel构型分开，高于该临界值，Cassie状态在能量上更有利，去除电场后去湿转变可以自发发生。
