Tip geometry controls adhesive states of superhydrophobic surfaces.

Inspired by biological attachment systems, aligned polystyrene (PS) nanopillars terminating in flat, concave tips and nanotubes were fabricated by a simple and reproducible method. All the obtained surfaces show both the contact angles larger than 150 degrees (superhydrophobicity) and high adhesion of water to it. The tip geometry plays an important role in determining the adhesive property. Surface with the concave tips has the highest adhesion, and then the surface with flat tips, whereas aligned nanotube surface has a relatively lower adhesion. Besides different van der Waals forces between the PS surfaces and water, another important factor, i.e., different negative pressures produced by the different volumes of sealed air, may be the crucial factor for their different adhesions. These findings provide the experimental evidence of the influence of the tip geometry on the adhesion of structured superhydrophobic surfaces, which is helpful for us to further understand the biological attachment systems and to optimum design of artificial analogues.