Effects of surface morphology, size effect and wettability on interfacial adhesion of carbon nanotube arrays

Vertically-aligned multi-walled carbon nanotube (MWCNT) arrays with various diameters and surface morphologies are fabricated to mimic gecko foot-hairs. The peeling behavior and interfacial adhesion characteristics of the MWCNT arrays are examined by means of depth-sensing nanoindentation technique. The results show that the adhesive force increases with an increasing surface roughness. However, the adhesive force significantly reduces with an increasing cap size due to a reduction in the number of contact sites between the MWCNT array and the indenter. Furthermore, the pull-off force of the MWCNT arrays is also found to increase with an increasing preload due to the corresponding deformation-induced increase in the contact area. Finally, it is shown that the contact angle of a water droplet on the MWCNT surface reduces with an increasing surface roughness. Thus, it is inferred that the enhanced adhesion of the MWCNT arrays with a greater surface roughness is due in part to an increased humidity and wettability of the MWCNT surface.