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

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems.

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 ≈ 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.

Importance of loading and unloading procedures for gecko-inspired controllable adhesives.

The importance of loading and unloading procedures has been shown in a variety of different methods for biological dry adhesives, such as the fibers on the feet of the Tokay gecko, but biomimetic dry adhesives have yet to be explored in a similar manner. To date, little work has systematically varied multiple parameters to discern the influence of the testing procedure, and the effect of the approach angle remains uncertain. In this study, a synthetic adhesive is moved in 13 individual approach and retraction angles relative to a flat substrate as well as 9 different shear lengths to discern how loading and unloading procedures influence the preload, adhesion, and shear/friction forces supported. The synthetic adhesive, composed of vertical 10 µm diameter semicircular poly(dimethylsiloxane) fibers, is tested against a 4 mm diameter flat glass puck on a home-built microtribometer using both vertical approach and retraction tests and angled approach and retraction tests. The results show that near maximum adhesion and friction can be obtained for most approach and retraction angles, provided that a sufficient shear length is performed. The results also show that the reaction forces during adhesive placement can be significantly reduced by using specific approach angles, resulting for the vertical fibers in a 38-fold increase in the ratio of adhesion force to preload force, µ', when compared to that when using a vertical approach. These results can be of use to those currently researching gecko-inspired adhesives when designing their testing procedures and control algorithms for climbing and perching robots.

Friction and adhesion of gecko-inspired PDMS flaps on rough surfaces.

Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the extremely important parameters that affect the friction and adhesion forces between two surfaces. Although much attention has been paid on fabricating various structures that mimic the hierarchical structure of a gecko foot, yet no systematic effort, in experiment or theory, has been made to quantify the effect of surface roughness on the performance of the fabricated structures that mimic the hierarchical structure of geckos. Using a modified surface forces apparatus (SFA), we measured the adhesion and friction forces between microfabricated tilted PDMS flaps and optically smooth SiO(2) and rough SiO(2) surfaces created by plasma etching. Anisotropic adhesion and friction forces were measured when sliding the top glass surface along (+y) and against (-y) the tilted direction of the flaps. Increasing the surface roughness first increased the adhesion and friction forces measured between the flaps and the rough surface due to topological matching of the two surfaces but then led to a rapid decrease in both of these forces. Our results demonstrate that the surface roughness significantly affects the performance of gecko mimetic adhesives and that different surface textures can either increase or decrease the adhesion and friction forces of the fabricated adhesives.

Vertical anisotropic microfibers for a gecko-inspired adhesive.

Geckos are able to adhere strongly and release easily from surfaces because the structurally anisotropic fibers on their toes naturally exhibit force anisotropy based on the direction of articulation. Here, semicircular fibers, with varying amounts of contact area on the two faces, are investigated to ascertain whether fiber shape can be used to gain anisotropy in shear and shear adhesion forces. Testing of 10-µm-diameter polydimethylsiloxane (PDMS) fibers against a 4-mm-diameter flat glass puck show that shear and shear adhesion forces were two to five times greater when in-plane movement caused the flat face, rather than the curved face, of the fiber to come in contact with the glass puck. The directional adhesion and shear force anisotropy results are close to theoretical approximations using the Kendall peel model and clearly demonstrate how fiber shape may be used to influence the properties of the adhesive. This result has broad applicability, and by combining the results shown here with other current vertical and angled designs, synthetic adhesives can be further improved to behave more like their natural counterparts.