Influences of micro-groove size on surface anisotropic wetting behaviors

Abstract

Biomimetic microstructure has been used widely in the fields of microfluidics, micro-mixers, flow drag reduction, tribology, etc. When solid surface is modified with microstructure, it will inevitably influence the solid-liquid interfacial behaviors, such as adhesion, surface wetting, shear viscous resistance, and interfacial slip. Surface anisotropic wetting can be achieved by using either of anisotropic surface microstructure and chemically heterogeneous patterned surface, or both of them. And anisotropic wetting properties can be used to control the micro-flowing behaviors, like mixing, flowing direction and speed. The effect of microstructure on the surface wetting behavior is closely related to the size, shape and arrangement of microstructure. In the paper, the influence of micro-groove size on liquid anisotropic wetting behavior is studied. The results indicate that the droplet wetting state of the patterned surface used is Cassie state. According to the experimental results, we can see that the liquid flows easily along the groove direction with small motion resistance, thus resulting in a small contact angle. While the water droplet has a higher flowing resistance in the direction perpendicular to the groove direction due to the energy barrier caused by micro-groove, thus showing a larger contact angle. Meanwhile, the water droplet shows pinning and jump behavior during the spreading in the direction perpendicular to the micro-groove direction. The contact angle along the micro-groove direction <i>θ</i>_// increases with groove width <i>G</i> increasing, and decreases with ridge width <i>R</i> increasing, which means that the parallel direction contact angle <i>θ</i>_// is inversely proportional to the solid fraction <i>R</i>/(<i>R</i> + <i>G</i>). And the experimental contact angle <i>θ</i>_// shows good consistence with that obtained from theoretical Cassie model. While the contact angle of water droplet perpendicular to groove direction <i>θ</i>_⊥ almost keeps no change with groove width <i>G</i> nor ridge width <i>R</i>. Both the droplet deformation ratio <i>L</i>/<i>W</i> and contact angle ratio of the two featured direction <i>θ</i>_⊥/<i>θ</i>_// are proportional to the solid fraction <i>R</i>/(<i>R</i> + <i>G</i>). The water droplet shows anisotropic wetting behaviors, which means that the liquid motion resistances are different in these directions. The high droplet deformation ratio <i>L</i>/<i>W</i> and the high contact angle ratio <i>θ</i>_⊥/<i>θ</i>_// correspond to the large difference in motion resistance. And surface wetting behavior has a great influence on the micro-flowing behavior. Thus, the micro-flowing behavior can be regulated by changing the microgroove size. The present research can conduce to the understanding the wetting mechanism and flowing behaviors of liquid droplet on patterned surface.