Characterization of Intermediate Wetting States and Anisotropic Sliding on Micro-directional Grooved Surfaces

The wetting state of a water droplet remarkably affecting its sliding behavior is characterized by the droplet boundary contact line. This chapter presents experimental studies of the apparent contact angle against droplet deposition time, as well as contact angle hysteresis, and compares the results with the Wenzel and Cassie–Baxter models. Observations indicate that different intermediate wetting phenomena exist. The sliding performance of a droplet under intermediate wetting states is also investigated. It is found that the droplet does not slide under partial wetting but slides when the side walls of the grooves have been wetted by part of the droplet. Furthermore, the droplets under different wetting states on surfaces with varied groove spacing and widths, under the same groove spacing to width ratio, present altered sliding performance before rolling off from the micro-grooves in a parallel direction. This study broadens the characterization method of intermediate wetting states, which determines the condition of anisotropic sliding on micro-grooves. The findings have great potential for application to artificial self-cleaning surfaces.