Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements
Huber, G., Mantz, H., Spolenak, R., Mecke, K., Jacobs, K., Gorb, S., Arzt, E., Proc. Natl Acad. Sci. USA 102, 16293–16296 (2005).
This paper shows that humidity affects gecko adhesion on the spatular level, i.e., the smallest level of the hierarchial gecko foot structure. This finding disagrees with previous studies which rejected the contribution of capillarity and which suggested that van der Waals forces dominate the high adhesion.
Because of the small size of the spatulae (<math>\approx</math> 200 nm wide and long), atomic force microscopy (AFM) was used to determine the pull-off force of a single spatula. A sample of spatula was attached to the end of an AFM cantilever, and the spatula was brought into contact with the surface. Two types of Si wafers were used as surfaces: type N, with a thin Si oxide layer; and type T, with an Si oxide layer <math>\approx</math> 100 times thicker. There were two versions of each of these types of surfaces: a hydrophilic and hydrophobic surface. After contact with the surface, the cantilever was vertically withdrawn from the surface while measuring the pull-off force.
The first part of the experiment showed that the adhesion force of the gecko spatula rises significantly for substrates with increasing hydrophilicity. This suggests that adhesion is determined primarily by short-range forces. The second part showed that adhesion force rises with humidity, which means that water layers between the spatula and surface have a strong influence on adhesion force. The authors suggest that the increased adhesion is due to a change of the effective short-range interaction due to adsorbed monolayers of water.