Geckos are endowed with specialized toe pads that enable them to climb vertical surfaces and ceilings. Researchers are keen to imitate the extraordinary sticky features of these toes to synthesize materials for fabricating artificial adhesives.

Biomimetics research – mimicking nature – indicates that carbon nanotubes (CNTs) are excellent candidates for producing artificial adhesive structures on par with geckos. However, devising experiments to measure the adhesive properties of individual CNTs is challenging.

Japanese researchers at the Department of Mechanical Engineering of Osaka University, in collaboration with Nitto Denko Corporation in Osaka, investigated the adhesive properties of a single CNT using a unique two-stage transmission electron microscope (TEM). The adhesion of a CNT orientated perpendicularly to a gold surface was a remarkable ~7 nN, and the degree of adhesion had a linear relationship to the cross-section area of CNTs.

In the experiments, the CNT was fixed to one of the two stages, and a silicon (Si) tip coated with gold (Au) or silicon dioxide was fixed to the other. The position of the Si tip was adjusted with a high-precision piezoelectric actuator.

High-resolution TEM images show that the tip of the CNT became flattened against the much larger Si tip covered with Au, and thus the CNT-Au contact is effectively between a layer of graphene and the Au tip.

Furthermore, estimates show that the adhesion of an array of multiwall CNTs with ~5.0 × 1010 fibres/cm2 would be ~900 N/cm2 – which is 100 times larger than reports to date, underscoring the importance of flat contact between the surfaces.

The work is reported in Applied Physics Express.