Contact Line Dynamics of Water Droplets Spreading on Nano-Structured Teflon Surfaces in Dropwise Condensation

In dropwise condensation process, superhydrophobicity is usually achieved by introducing micro/nano-roughness to hydrophobic materials. The analysis of droplets growing and moving and the optimization of the surface structures entails a comprehensive knowledge of the contact line dissipation. However, it in many cases is neglected due to the insufficient understanding, particularly regarding its magnitude and characteristics. In this study, we report a study on the contact line dynamics of water droplets spreading on nano-structured Teflon surfaces. The Teflon surfaces are modeled on Gromacs 5.1.2 and based on the OPLSAA force field. The Teflon model is then validated by examining the glass transition temperature and thermal expansion coefficient. Patterned pillars are created by a confined layer method. The contact line dynamics of water on as-formed surfaces with different solid fraction is then analyzed using the molecular kinetic theory modified by incorporating both viscous damping and solid-liquid retarding. The unit displacement length of contact line is demonstrated to be a constant value of 0.605 nm on both flat and pillar-arrayed surfaces. The contact line friction coefficient is calculated to be on the same order of magnitude with the dynamic viscosity of water, and can be significantly decreased on superhydrophobic surfaces as a result of reduced liquid-solid contact, although contact line experiences stronger resistance on a single pillar.