E22 Micro/nano adhesive structures

Topic

The adhesive micro- and nanostructure of an insect generally exploits a fundamental biomechanical principle: dividing the contact area into multiple smaller contact points increases the insect's total adhesion, following a favorable scaling law relating the number of setae to the total force they generate.

This principle of subdivided contact was quantified by Arzt, Gorb, and Spolenak.

The flexibility of the seta—modeled as an Euler-Bernoulli beam—allows it to conform to rough surfaces, thereby maximizing the actual contact area between the seta and the surface.

However, there is a trade-off between the seta's flexibility and its stability: an overly flexible seta tends to clump together with a neighboring seta—collapsing due to capillary attraction or van der Waals forces—thereby limiting the maximum density of setae the insect can maintain. Greater flexibility in the seta increases the tendency for it to clump with a neighbor.

The hierarchical design—featuring structures repeated across multiple scales—found in the feet of geckos and certain beetles optimizes this trade-off between flexibility and stability. Consequently, in this model—characterized by the division of a contact area into multiple smaller contact points—the Griffith and Hertz-Sneddon equations demonstrate that the adhesion at each of these small contacts depends on both the interfacial fracture energy and the material stiffness associated with that specific contact. If the material of a given small contact is stiffer, the resulting adhesion at that specific contact—as determined by the Griffith and Hertz-Sneddon equations governing the model—is correspondingly lower.

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