Geckos, adhesive forces

Autumn, K., Liang, Y. A., Hsieh, S. T., et al. (2000). Adhesive force of a single gecko foot-hair. Nature 405 681-685. 

Geckos, and maybe ladybugs, crawl on ceilings by van der Waals attraction. The adhesive force of a gecko foot hair or "seta" to a silicon surface is of the right magnitude to... 

Geckos use a sophisticated assembly of submicroscopic foot hairs to walk along a ceiling upside down [1], Such a dendritic network of hairs takes advantage of multiple adhesive forces, resulting in an efficient multivalency effect. 

Directionality is an important feature of gecko adhesion tilted fiber orientation results in strong shear adhesion forces in one direction and easy surface lift-off in the other direction. Theoretical analyses comparing vertical versus tilted pillars in model materials indicate that adhesion is optimized in the latter condition. " While the preparation of tilted or curved pillars requires more complex fabrication techniques, efforts to generate dry adhesive arrays of pillars with such geometries have confirmed model predictions of improved adhesion (Figure... 

Sun W, Neuzil P, Kustandi TS, Oh S, Samper VD. (2005) The Nature of the Gecko Lizard Adhesive Force. Biophys J Biophys Lett%9 L14-L17. 

Liu, K., Du,J.,Wu,J.,Jiang, L., 2012. Superhydrophobic gecko feet with high adhesive forces towards water and their bio-inspired materials. Nanoscale 4,768. 

The pads on the feet and/or legs of insects or animals such as beetles, spiders and geckoes have nanoscale hairs that enable them to attach and detach from different kinds of surfaces, allowing them to walk on them. These provide natural examples of reversible adhesion or smart adhesion. It has been suggested that the adhesion arises from a combination of van der Waals attractive forces and capillary forces (depending on the humidity) [103]. Synthetic surfaces comprising arrays of 400 X 600 nm nanopillars have been produced that mimic the gecko-foot effect with reversible adhesion for at least a few contact cycles [103]. 

As it turns out, geckos do not use any chemical adhesives, nor do they use suction. Instead, their abilities arise from the intermolecular forces of attraction between the molecules in their feet and the molecules in the surface on which they are walking. When you place your hand on a surface, there are certainly intermolecular forces of attraction between the molecules of your hand and the surface, but the microscopic topography of your hand is quite bumpy. As a result, your hand only makes contact with the surface at perhaps a few thousand points. In contrast, the foot of a gecko has approximately half a million microscopic flexible hairs, called setae, each of which has even smaller hairs. 

A subsequent set of experiments expanding upon Hiller s findings, were conducted in order to ascertain the type of force primarily responsible for adhesion in gekkotans. The inability of geckos to stick to hydrophobic, weakly polarizable surfaces (those with a high water contact angle) could have been due to either reduced capillary adhesion or reduced van der Waals forces. In order to resolve this problem, polarizability was separated from hydrophilicity/hydrophobicity. This is important because van der Waals adhesion depends on the creation of instantaneous dipoles, and... 

---