J Bionic Eng 18 (2021) 398–408 DOI: https://doi.org/10.1007/s42235-021-0029-8
Study of the Classical Cassie Theory and Wenzel Theory Used in Nanoscale
Hao Li1,3, Xiaolei Feng1 , Kai Zhang2*
1. School of Material Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
2. College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao 266580, China
3. National Engineering Laboratory of Offshore Geophysical and Exploration Equipment, China University of Petroleum, Shandong 266580, China
Abstract 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.
Keywords: wettability, Cassie theory, Wenzel theory, nanoscale, molecular dynamics simulations