Xueyun Zhang

Dynamics of a Stick-Jump Contact Line of Water Drops on a Strip Surface

In this study, we prepared microscale periodic rough structures consisting of parallel strips on a silicon surface. The width of each strip was equal to the gap between the strips, and the silicon surface was silanized with perfluorooctyltrichlorosilane. We studied the wetting characteristics of water drops as they advanced and receded on patterned surfaces in a direction perpendicular to the strip. Water drops were observed to advance or recede in a smooth manner when the strip width was smaller than 32 microm but in a stick-jump manner when the strip width was larger than 50 microm. The regular strip-patterned substrates enabled us to deduce the relationship between the stick-jump behavior and the feature size of the substrate. For surfaces on which water drops showed stick-jump behavior, the oscillation amplitude of the contact angle decreased with decreasing strip width. In addition, the jumping distances of the contact lines, for both advancing and receding water drops, were nearly equal to the strip period. A 2D model was applied to analyze the contact line motion on the patterned surfaces, which showed reasonable agreement with the experimental results.

Anisotropic Wetting on Checkerboard-Patterned Surfaces

A series of surfaces with microscale checkerboard patterns consisting of continuous central lines and discontinuous lateral lines were fabricated. The surface wetting properties of these checkerboard patterns were found to be anisotropic. The central continuous lines were found to have a strong influence on the dynamic wetting properties and moving trajectories of the water droplets. The droplets move more easily in the direction parallel to the central continuous lines and less easily in the direction perpendicular to the central continuous lines. Meanwhile, the droplets moving path tends to incline toward the central continuous lines from a tilting direction. When the microsurface was modified with a layer of nanowire, the surface wettability was found to be isotropic and superhydrophobic.

Effect of pattern topology on the self-cleaning properties of textured surfaces

The water contact angle and self-cleaning property of microfabricated surface textures possessing different topologies are compared. In one kind of surface textures, the protruded regions form a connected square network. In the other kind of surface textures, the protruded regions form a regular array of square posts. We find that the water apparent contact angle of the connected textures agrees with the Cassie equation [Discuss. Faraday Soc. 3, 11 (1948)], but that of the disconnected textures is much larger. Nevertheless, the disconnected textures exhibit inferior self-cleaning property, contrary to conventional conception. We discuss the possible reasons for these observations.