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.

Figure 1.

(a) A scanning electron microscope image of a mounted setal array. (b) Schematic of the test set-up for determining the adhesion (normal) forces and friction (lateral) forces between an array and a substrate. (Online version in colour.)

Figure 2.

(a) Calculated effective interface potentials for the interaction between a keratin layer and a silicon wafer (type T and type N). The short-range constant was assumed as , resulting in maximal forces of . (b) Relative difference in vdW forces on type T and type N wafers as determined using equation (3.2). (c) Estimated pull-off forces for a β-keratin sphere of radius R = 150 nm on a Si/SiO₂ bilayer material with layer thickness d making use of equation (3.4). (Online version in colour.)

Figure 3.

Results of multiple different experiments on the hydrophobic samples with different setal arrays and substrate pairs: mean adhesion forces are plotted as a function of (a) the drag speed (at 75% RH) and (b) the humidity (with v = 50 mm s⁻¹). By convention adhesion forces are negative. The stars indicate the level of significance. (Online version in colour.)

Figure 4.

Results of multiple experiments (drag speed: 0.5 mms⁻¹; humidities: 10%, 30%, 50%, 70% RH) on the hydrophilic samples with different setal arrays and substrate pairs: (a) mean adhesion forces of the single experiments. (b) Aggregation of the experiments on the hydrophilic samples: mean differences in adhesion forces of ‘concurrent’; single tests on the type N and type T wafers ΔF = F_N − F_T are plotted as a function of the humidity. (Online version in colour.)