Chair: Professor Kevin Kendall FRS
van der Waals forces
Professor Kevin Kendall FRS, Adelan Ltd, UK
Adhesion molecules have been thought to control the adhesion of cells . Unfortunately, the ‘lock and key’ model is unacceptable. While there is no doubt that a coating of adhesion molecules such as fibronectin on a surface affects cell adhesion, this is only one factor in the equation. Van der Waals force is the key cause of cell adhesion and is purely electromagnetic. Substrate elasticity and geometry are also important. Originally, the theoretical ideas were defined [2,3] in 1970-1971. By considering the contact of elastic bodies, it became evident that three parameters generally entered the equation for adhesive force F, as indicated below.
F = K [WEd3/(1-2)] 1/ (1)
where K was a constant, W the work of adhesion in Jm
-2, E the elastic modulus in Pa, the Poisson’s ratio and d the dimension in metres. From this model, it is clear that the adhesion molecules have an effect on W, but elasticity E is equally influential and the geometry d is much more important. The most surprising thing about this new theory was that adhesion force was strongest when the surfaces were absolutely smooth and clean, with no projecting ‘lock and key’ and no adhesion molecules present. In other words the effect of adhesion molecules was to reduce the adhesion force, not to cause it.
Umemori, H., The sticky synapse: Cell adhesion molecules, Springer Berlin 2009.
Kendall, K., The adhesion and surface energy of elastic solids, J PhysD: Appl Phys 4(1971) 1186-95.
Johnson, K.L., Kendall, K. and Roberts, A.D., Surface energy and the contact of elastic solids, Proc R Soc Lond A324(1971) 301-313.
Van der Waals adhesion supporting the gecko
Professor Kellar Autumn, Clark University, USA
Geckos climb at speeds of over 1 m/s using adhesive nanostructures on their toes. Gecko toes bear angled arrays of branched, setae formed from stiff, hydrophobic beta-keratin that act as a soft bed of angled springs. Previously, we discovered that setae form a self-cleaning, anisotropic, mechanically switchable adhesive that adheres by van der Waals forces. Subsequently, we showed that humidity softens
and increases viscoelastic losses in setal keratin, increasing van der Waals adhesion. We employed the humidity effect on setal materials properties to test dynamic friction models for multicontact interfaces. Contact forces were materials-dependent domain at low velocity (< 1 mm/s) and materials-independent at higher velocity. This supports the rate-state model of sliding friction, in which shear force is the result of competition between rate-enhancing and contact-area-enhancing mechanisms. Natural and synthetic gecko setae can be employed as a model system in the study of interfacial forces. Smart materials properties of gecko-inspired adhesive nanostructures may enable rigid, inert, recyclable materials to replace glues, screws, and other attachment devices in the future.
Puthoff, J, MJ Holbrook, M Wilkinson, K Jin, N Pesika, and K Autumn*. 2013. Dynamic friction in natural and synthetic gecko setal arrays. Soft Matter 9:4855-4863.
Puthoff J, M Prowse, M Wilkinson, & K Autumn*. 2010 Changes in materials properties explain the effects of humidity on gecko adhesion. J Exp Biol (213):3699-3704.
Wet but not slippery: biomechanics of insect attachment organs
Dr Walter Federle, University of Cambridge, UK
Adhesive pads of climbing animals provide interesting models for synthetic adhesives as they work under the most challenging conditions, including rough, wet or dirty substrates and extremely rapid attachment and detachment during locomotion. Adhesion is controlled dynamically via the directionality and shear force dependence of climbing pads. Insect adhesive organs make contact when pulled towards the body, and their adhesion increases linearly with pulling force, but they detach when pushed. Nevertheless, climbing insects can use their feet for pushing; they have evolved specialised "heel" pads for this purpose.
Although high friction is essential for insect adhesion, insects inject small volumes of fluid into the pad contact zone. In insects with smooth pads, this fluid is a water-in-oil emulsion which helps to reduce slipping. The secretion does not generally increase adhesion, but it helps to maximise contact area on rough substrates and allows insects to maintain strong adhesion during sliding. However, some specialised plant surfaces are lubricated and cause insects to slip.