Nanotribological behavior of bioinspired textured surfaces with directional characteristics

Friction and adhesion becomes extremely important at nano and micro length scales, modulating the durability of several nano/micro electromechanical systems (NEMS/MEMS). Bio-mimicking the surface texture of different living organisms has helped improve the tribological performance of many such systems. In this study, we examined the friction and wetting behaviour of textured surfaces derived by mimicking the surface morphology of butterfly wing. Three different derivatives of mimicked structure with similar solid/air fraction but different contact aspect ratios were fabricated on Si wafer using photolithography and deep reactive ion-etching techniques. The textured surfaces patterns were subsequently coated with polytetrafluoroethylene (PTFE), diamond-like carbon (DLC), and fluorine incorporated diamond-like carbon (F-DLC) using plasma-enhanced chemical vapor deposition (PECVD) technique. Atomic force microscope was used to investigate the friction behaviour of the coated and un-coated samples at different applied normal load. Wetting behaviour of the textured and control surfaces was measured using sessile-drop method. Results showed that both wettability and friction were significantly influenced by the shape, orientation and surface chemistry of the textured structure. The PTFE and F-DLC coatings helped reduce the friction compared to Si control surface. The developed patterned displayed dual character with wetting and friction being function of the texture shape. The increase in aspect ratio of textured geometry enhanced directional wettability and friction. The wetting was controlled by the contact-line pinning phenomenon modulated by the texture geometry. The friction behaviour of the textured geometry varied in direct correlation with the contact area. Further, the edge-effect showed prominent influence leading to an increase in friction force in lateral direction. The effect of surface chemistry and texture geometry is explained on basis of intermolecular forces and contact mechanics. The directional friction and wetting characteristics of the developed surface would be of potential use for transport applications in different systems.