JKR Theory for the Stick Slip Peeling and Adhesion Hysteresis of Gecko Mimetic Patterned Surfaces with a Smooth Glass Surface

Geckos are highly efficient climbers and can run over any kind of surface with impeccable dexterity due to the typical design of their hierarchical foot structure. We have fabricated tilted, i.e., asymmetric, poly(dimethylsiloxane) (PDMS) microflaps of two different densities that mimic the function of the micrometer sized setae on the gecko foot pad. The adhesive properties of these microflaps were investigated in a modified surface forces apparatus; both for normal pure loading and unloading (detachment), as well as unloading after the surfaces were sheared, both along and against the tilt direction. The tilted microflaps showed directional, i.e., anisotropic adhesive behavior when sheared against an optically smooth (RMS roughness approximate to 10 +/- 8 nm) SiO2 surface. Enhanced adhesion was measured after shearing the flaps along the tilted (gripping) direction and low adhesion when sheared against the tilted (releasing) direction. A Johnson-Kendall-Roberts (JKR) theory using an effective surface energy and modulus of rigidity (stiffness) quantitatively described the contact mechanics of the tilted microflaps against the SiO2 surface. We also find an increasing adhesion and stick-slip of the surfaces during detachment which we explain qualitatively in terms of the density of flaps, considering it to increase from 0% (no flaps, smooth surface) to 100% (close-packed flaps, effectively smooth surface). Large energy dissipation at the PDMS-silica interface caused by the viscoelastic behavior of the polymer results in stick-slip peeling and hence an enhanced adhesion energy is observed during the separation of the microflaps surface from the smooth SiO2 surface after shearing of the surfaces. For structured multiple contact surfaces, hysteresis as manifested by different loading and unloading paths can be due entirely to the elastic JKR micro-contacts. These results have important implications in the design of biomimetic adhesives.