Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition

Abstract

Surface energies are commonly used to determine the adhesion forces between materials. However, the component of surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers. Previous theoretical results and indirect experimental evidence suggest that the van der Waals energies of subsurface layers will influence interfacial adhesion forces. We discovered that nanometre-scale differences in the oxide layer thickness of silicon wafers result in significant macroscale differences in the adhesion of isolated gecko setal arrays. Si/SiO 2 bilayer materials exhibited stronger adhesion when the SiO 2 layer is thin (approx. 2 nm). To further explore how layered materials influence adhesion, we functionalized similar substrates with an octadecyltrichlorosilane monolayer and again identified a significant influence of the SiO 2 layer thickness on adhesion. Our theoretical calculations describe how variation in the SiO 2 layer thickness produces differences in the van der Waals interaction potential, and these differences are reflected in the adhesion mechanics. Setal arrays used as tribological probes provide the first empirical evidence that the 'subsurface energy' of inhomogeneous materials influences the macroscopic surface forces.

In vivoadhesion force measurements ofChlamydomonason model substrates

“…These model substrates have shown to be well suited to probe the influence of vdW interactions in biological systems, since their surface characteristics are identical within the experimental error while their subsurface contribution is different. 33,[37][38][39] As shown in Fig. 4, adhesion force measurements with the same cells (N = 25 cells) on both types of substrates are in excellent agreement.…”

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

“…45 Moreover, adhesion experiments with setal arrays of live geckos revealed that the adhesion force was significantly varied by a change in the substrates subsurface composition. 46 …”

Self‐assembled silane monolayers: an efficient step‐by‐step recipe for high‐quality, low energy surfaces

“…Silane layers in particular are mechanically robust, thermally stable up to at least 250 ∘ C and are not subject to swelling in the presence of solvents. These properties render silane‐coated substrates ideal model surfaces to study a wide range of physical, chemical and biological phenomena such as adhesion, adsorption, friction or nanofluidics of thin liquid films . Technical applications also benefit from the unique properties of SAMs: They act, for example as lubrication layers in micro‐electro‐mechanical systems or as coatings in microfluidic devices .…”