Gecko and fly "stick" with nano-contacts

Liability is based on size reduction and special form of the contact points

Gecko, Gecko, Fly, Spider and Beetle adhesive hair © MPI for Metals Research
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The adhesive structures on the feet of geckos and many insects consist of only a few hundred nanometers fine hairs. These nanostructures probably evolved in the course of evolution to optimize the adhesion of insects to substrates. This is shown by recent research at the Max Planck Institute for Metals Research in Stuttgart: Optimal adhesion depends on the fact that these hairs are optimally shaped at their contact surfaces.

But this strong shape dependence can be compensated by minimizing the adhesive contacts. Because below 100 nanometers, the contacts adhere optimally - regardless of changes in the shape of the contact surfaces. An optimal, fault-tolerant adhesion can thus be achieved by a combination of size reduction and shape optimization. The following applies: the smaller the characteristic size of the individual adhesive contact, the less important is its shape. This also makes it plausible why hair contacts of biological adhesion systems are only between a few hundred nanometers and a few micrometers in size. These findings are important for the design of adhesive systems in the art. (PNAS, Early Edition, May 17, 2004).

Puzzle of optimal adhesion

When two objects are joined by adhesion or adhesion, and then subjected to external stresses, stress concentrations near the bond may occur. As the load continues to increase, the stress intensity eventually reaches a critical level and a small crack occurs. This gets bigger and bigger until the connection finally breaks. The reason is that not all the material is included in the liability, but only a small part in the vicinity of the stress concentration is intensively loaded and spread cracks. How to achieve a robust and reliable adhesion between structurally different components is a little understood problem for engineers so far.

Therefore, liability mechanisms that have been "tested" and improved over millions of years in biological evolution are of interest not only to biologists but also to engineers. For example, geckos and many insects have hairy structures (called spatulae) on their feet that act as adhesive devices. It turned out that the density of the surface hair increases with the body weight of the animals. Among all the species studied to date, geckos have the highest number of spatulae per unit area. They are - compared to flies and other insects - also relatively heavy animals.

Van der Waals forces crucial

In the meantime, it has been proven that the Geckos' detention mechanism is dominated by van der Waals forces. These forces arise through short-term asymmetric charge distributions around the atoms. The fact that Van der Waals forces should play a dominant role at first seems surprising, because we need a much greater force to pull a gecko from the ceiling than to take our hand off the table and that, though in both cases the same van der Waals force works. This raises the question of what exactly determines the strength of the adhesion (adhesion). In any case, the chemical structure of the materials can not explain why the same van der Waals force gives such a strong adhesion to gecko but not to humans. It appears that nature has developed other sophisticated mechanisms to allow certain species of animals, for which adhesion allows survival, to use the weak van der Waals forces. display

H. Gao and H. Yao from the Max Planck Institute for Metals Research in Stuttgart have now developed a model for the adhesion between a single spatula and a substrate, which is based on van der Waals interactions. Thereafter, the shape of the surface of a spatula has great influence on the strength of the adhesion. The scientists show that there is a special form of spatulae, in which - regardless of their size - in any case, the maximum theoretical adhesion is achieved. If the spatula has this optimal shape, the adhesive force distributes itself evenly over the entire contact area. This corresponds to optimal material usage.

Technology lags behind nature

But why is such an optimal form so far not used in technology? One reason is certainly that the maximum achievable adhesive force is very sensitive to small variations in the geometry of the adhesive contacts. Thus, for a fiber of one millimeter radius, the bond strength decreases by more than two orders of magnitude, if the shape of the fiber deviates by only one to two percent from its optimum shape. However, the researchers found that this sensitivity to form can, interestingly, be eliminated by reducing the size, that is, the diameter of the fiber. If the fiber diameter reduces to a critical size, the adhesive force reaches the maximum theoretical value, regardless of small changes in their shape. The scientists estimate the critical length scale to be around 100 nanometers.

Consequently, one can achieve optimum adhesion in nature and potentially also in technology through a combination of size reduction and shape optimization. The smaller the fiber, the less important is its shape. Nevertheless, if large areas of contact are necessary, an optimal adhesion can be achieved if it is possible to produce the shape of the adhesive contacts in sufficient precision. From a practical point of view, however, it is necessary to reduce the contact size as much as possible in order to achieve a robust and, at the same time, optimal adhesion. This relationship between size reduction and shape optimization could also find important applications in technology.

(MPG, 26.05.2004 - NPO)