Animals that climb up vertical walls and walk up side down on ceilings owe this ability to micron and nano scales attachment elements. There are a variety of mechanisms employed by these animals: tiny claws, adhesive secretions, smooth and hairy adhesive pads, etc.
The highest adhesion forces are encountered in geckos. A gecko walks up walls using physical adsorption forces. The gecko’s ability to walk up walls stems from special structures on their feet called setae ( made of keratin).
Robert Full of the University of California, Berkeley, discovered that the adhesion was due to van der Waals forces ( From Wikipedia: “Van der Waals forces include attractions between atoms, molecules, and surfaces. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations of nearby particlesphysical adsorption forces”). Each seta is 1mm long and 5µm in diameter, terminating in hundreds of fine structures called spatula, which are 200nm long and 10nm in diameter.
In the case of gecko feet, the spatulae are so small and get so close to the surface that an attractive van der Waals force of around 0.4 µN develops between a single spatula and a surface.
Van der Waals force works between the atoms on gecko’s feet and the atoms on the surface it is climbing or hanging on to because there are so many setae and microhairs on gecko feet allows so many atoms to interact with the surface. The force created by one gecko is enough to lift 127 kilograms (the combined force of the millions of spatulae on a single gecko foot produce an adhesion force of around 10 N, or around 2.25 lbs)! For a gecko, each foot is about 100mm2, for a total of 400mm2, and setae have a density of about 10,000 setae/mm2. Each seta can withstand 10-5N shear force, which corresponds to about 0.1N/mm2. Thus, a gecko has an available shear force of 0.1N/mm2* 400mm2=40N. The weight of a gecko, calculated about is 0.245N, and therefore the ratio of shear force to weight is 40N/0.245N=163. We need our animal to obey this relation:
, or rearranging:
. So a gecko has 400mm2/0.025kg=16,000mm2/kg.
Andre Geim discovered that capillary forces (capillary forces are attractive forces created by the surface tension of a molecular layer of absorbed water that forms between two surfaces) also contribute to the adhesion.
Researching how insect and gecko feet have evolved to optimize adhesion strength is leading to bio-inspired development of artificial dry adhesive systems.