Receding dynamics of contact lines and size-dependent adhesion on microstructured hydrophobic surfaces.

The microstructure size on textured surfaces of a given solid fraction exhibits an important effect on their properties. To understand the size effect on surface adhesion, we study the receding dynamics of the microscopic three-phase contact lines, the adhesive properties, and the relation between them on microstructured surfaces. Two types of surfaces are used, which are micropillar and micropore, respectively. First, the receding process of the contact line is directly observed by laser scanning confocal microscopy (LSCM), which shows distinct characteristics on the two types of surfaces. The micro contact line experiences pinnning, sliding, and rupture on micropillar-patterned surfaces while no rupture occurs on micropore-patterned surfaces. The three-dimensional morphology of the micromeniscus on the micropillared surfaces and the two-dimensional scanning of the cross-sections of the micromeniscus along the diagonal direction are imaged. Based on the images, the local contact angles around the micropillar at the receding front, and the curvatures of the micro-meniscus are obtained. Then, the adhesive force on these surfaces is measured, which surprisingly shows an increasing trend with the size of the microstructure for micropillared surfaces but no obvious size dependence for micropored surfaces. Wetting hysteresis is also measured to testify the similar trend with the size for the two types of surfaces. Further investigation shows that the monotonic increase of the adhesive force with the increasing size of micropillars is due to the growing difficulty of the detachment of the contact lines. The underlying mechanism responsible for the size dependence of the adhesive force is the enhancement of the local reduced pressure exerted on the top of the micropillar with increasing size, resulting from the concave profile of the outer micromeniscus.