Surface energy tunable nanohairy dry adhesive by broad ion beam irradiation

We present a simple and shape-controllable method to tune the surface energy and tilting angle of nanohairy structures using broad ion beam irradiation. First, the vertical nanohair arrays made of ultraviolet (UV)-curable polyurethane acrylate (PUA) material were prepared by replica molding. Subsequently, the nanohairs were bent to a wide range of angles from 0 (vertical) to 75° (stooped hairs) by the oblique broad Ar ion beam. The surface energy was tuned with normal oxygen (O ₂) ion beam irradiation together with aging effect (47.1 to 71.9 mJ m ^-2). It was observed that the shear adhesion strength was significantly increased from 25.2 to 58.1 N cm ^-2 on the hydrophilic test surface and from 22.8 to 45.7 N cm ^-2 on the hydrophobic test surface, respectively, with the variation of surface energy from 47.1 to 71.9 mJ m ^-2. In addition, the shear adhesion strength was increased from 58.1 to 124.1 N cm ^-2 and from 45.7 to 110.6 N cm ^-2 on hydrophilic and hydrophobic test surfaces, respectively, as the tilting angle was varied from 0 to 75° at the highest surface energy of 71.9 mJ m ^-2. The measured adhesion data were compared with the theoretical adhesion forces based on Johnson-Kendall-Roberts (JKR), modified-JKR, peel zone (PZ), and Kendall peeling models, suggesting that the contact state of nanohairs is a mixture of tip and side contacts.