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

Abstract

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 ≈ 10 ± 8 nm) SiO 2 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 SiO 2 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 SiO 2 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.

Peptide Length and Dopa Determine Iron‐Mediated Cohesion of Mussel Foot Proteins

“…[29] The measured adhesion (or cohesion) force F ad (or F c ) is related to the adhesion (or cohesion) energy per unit area by W ad = F ad /2π R for rigid surfaces with weak adhesive interactions, and by W ad = F ad /1.5π R (used in this study) for soft deformable surfaces with strong adhesion or cohesion. [29, 32] …”

Stick–slip friction of gecko-mimetic flaps on smooth and rough surfaces

“…The mechanisms for attachment and high adhesive forces of gecko spatula and setae have been measured and modelled by Johnson-Kendall-Roberts (JKR)-type theories [4,5], whereas the ease of detachment from surfaces requires a peel-off theory [6][7][8]. The effects of end-shape and size of microfibres on adhesion have been investigated experimentally [9,10] and theoretically [11].…”

“…It has been found that the frictional forces (parallel to the surface) also contribute to the adhesive force (perpendicular to the surface), giving rise to the model of frictional adhesion. According to this model, the adhesion of a gecko footpad [12] or its mimic to a substrate depends on the applied shear force [4,6] and explains the very low detachment forces observed in climbing geckos. Anisotropic fibrillar synthetic adhesives mimicking the gecko footpad functionality have been previously fabricated [5,[13][14][15][16][17][18][19][20][21][22][23][24] and were used to study adhesion and frictional properties on silica surfaces.…”

Tough Coating Proteins: Subtle Sequence Variation Modulates Cohesion

“…The measured adhesion (or cohesion) force F ad (or F c ) is related to the adhesion (or cohesion) energy per unit area by W ad = F ad /2 πR for rigid surfaces with weak adhesive interactions and by W ad = F ad /1.5 πR (used in this study) for soft deformable surfaces with strong adhesion or cohesion. 34,35 …”