Anisotropic wetting properties of trapezoidal profile surfaces with hierarchical stripes

Anisotropic wetting properties of patterned surfaces are critical for fundamental research and potential applications. In spite of significant development in investigation of such surfaces, the importance of geometries parameter of the micro/nanostructure on anisotropic wettability is not understood completely, particularly in thermodynamic analysis aspect. In this work, a trapezoidal profile surfaces with hierarchical stripes were analyzed using a two-dimensional model by cutting a three-dimensional structure along a given orientation. According to the calculations of free energy and free energy barrier, the effects of cutting orientation and all the geometrical parameters for the surfaces on equilibrium contact angle (ECA) and contact angle hysteresis (CAH) were investigated systematically. It is demonstrated that the ECA and CAH are closely related to the cutting orientation and the trapezoidal base angle of hierarchical striped surfaces. In addition, the wetting state at the nanoscale facilitates the transition from isotropic to anisotropic ECA and CAH of the striped surfaces with Cassie-type state at the microscale. As for micro/nanostructured surfaces at Wenzel-type wetting state, the anisotropic behavior of ECA and CAH is further enhanced by the existence of nanostructures. These calculated results are well agreement with the related experimental results, which will provide fundamental principles for the design of demanded geometry of micro/nanostructured surfaces with controllable anisotropic wetting behavior.