Anisotropic Wetting Characteristics of Water Droplets on Phosphorene: Roles of Layer and Defect Engineering

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS2, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair oriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplets along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplets on phosphorene.