Wetting and adhesion energy of droplets on wettability gradient surfaces

Wettability gradient surfaces can be used for heat and mass transfer in many fields, which has aroused extensive interest of research. However, since numerous studies only focused on the surfaces with identical microstructures, the wetting and adhesion energy of droplets on wettability gradient surfaces are still unclear. In this work, three types of microstructured surfaces with gradient wettability were fabricated via photolithography and etching methods. We found that the contact angle, contact angle hysteresis and critical sliding angle will increase with area fraction. Meanwhile, the gradient microstructures will affect the triple line and wetted area of droplets. In addition, it is found that the critical sliding angle depends on not only the droplet volume but the size and types of microstructures, which is consistent with our force analysis. Based on the morphological characteristics and actual wetted area of droplets, an adhesion energy model for microstructured surfaces with gradient wettability is proposed. The model is also verified by other types of wettability gradient surfaces, which may be helpful to design wettability gradient surfaces with low adhesion energy for practical applications.