Challenges for Synthetic Gecko Adhesives: Roughness, Fouling, and Wear

Author(s): Gillies, Andrew George | Advisor(s): Fearing, Ronald S; Keaveny, Tony | Abstract: The past decade has seen rapid advancement in gecko synthetic adhesives (GSAs) and their performance has also steadily increased. However, there still remains a gap between the capabilities of current GSAs, and the properties required for GSAs to perform as the gecko does: on natural undulating surfaces with several scales of roughness, in dirty environments where particle contamination is the norm, and for thousands or even tens of thousands of cycles. For continued progress to be made in GSAs, focus must shift from trying to attain high adhesive values under ideal conditions, to exploring the weaknesses in current GSAs and contrasting those with the principles that underpin the success of the natural gecko systems in real world challenging conditions. Here we show results from the testing and simulation of various GSA systems in rough environments, with contaminating particles of varying size and for repeated cycling. We report that with careful geometry and material consideration, large increases in 'real world' performance can be obtained, and in some cases active control can be utilized to increase controllability.To better understand adhesion on macroscopic rough surfaces, we studied the ability of live Tokay Geckos to adhere to an engineered substrate constructed with sinusoidal patterns of varying amplitudes and wavelengths in sizes similar to the dimensions of the toes and lamellae structures (0.5 to 6 mm). We found shear adhesion was significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. We discovered that the toes are capable of adhering to surfaces with amplitudes much larger than their dimensions even without engaging claws, maintaining 60% of shear adhesion on surfaces with amplitudes of 3 mm. As well, Gecko adhesion can be predicted by the ratio of the lamella dimensions to surface feature dimensions. In addition to setae, remarkable macroscopic-scale features of gecko toes and lamellae that include compliance and passive conformation are necessary to maintain contact, and consequently, generate shear adhesion on macroscopically rough surfaces.Similarly, we sought to understand the impact of surface roughness on the adhesion of two types of GSA arrays: those with hemispherical shaped tips and those with spatula shaped tips. Our simulations showed that the nanoscale geometry of the tip shape dramatically alters the macroscale adhesion of the array, and that on sinusoidal surfaces with roughness much larger than the nanoscale features, there is still a clear benefit to having spatula shaped features. Similar to experimental results found with the macroscale features of the gecko adhesive system, when roughness approaches the size of the fiber features, adhesion drops dramatically.We have also investigated the impact of two design parameters on the dry self-cleaning capability of GSAs by experimentally testing two GSAs after fouling with small (1 micron), medium (3-10 microns) and large (40-50 microns) particles. We found that a GSA made from a hard thermoplastic with nanoscopic fibers was able to recover 96-115% of its shear adhesion after fouling with small and large but not medium particles, while a GSA made from a soft polymer and microscopic fibers recovered 40-55% on medium and large particles. Further examination by scanning electron microscopy (SEM) revealed that the soft polymer structures were not shedding the smaller particles during recovery steps, but were instead being absorbed into the surface, and that, regardless of their size, particles did not release from the soft polymer surface.An analysis of the contact strength between fibers, particles and substrates of various dimensions and elasticity reveals that dry self-cleaning will be more effective for GSAs fabricated with smaller fiber diameters and for GSAs fabricated from materials with smaller loss functions, such as hard thermoplastics. This has important implications on the choice of materials and geometries used for GSAs when dry self-cleaning capability is a desired function in the material, and indicates that hard polymer GSAs with smaller fiber diameters are less prone to fouling.As indicated by results of dry self-cleaning on a passive soft polymer fibrillar adhesive, we set out to design a system with active control and release of particles. We have demonstrated controllable adhesion to glass spheres with a new magnetically actuated synthetic gecko adhesive made from a magnetoelastomer composite. Capable of controlling adhesion to glass spheres 500 microns to 1 mm, this represents an important step in realizing an adhesive with dry self-cleaning capabilities across a wide range of particle sizes.We also examined the behavior of high density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with normal preload less than 40 kPa. We found that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10,000 cycles, despite showing marked plastic deformation of fiber tips. This deformation was attributed to shear induced plastic creep of the fiber tips from high adhesion forces , adhesive wear or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays, and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko. The discovery of this wear mechanism has uncovered a possible pathway to the fabrication of nanoscale spatula shaped tips. Spatula tips have been shown by the rough surface simulation to greatly improve adhesion strength. Several possible fabrication pathways are proposed and preliminary results on these fabrication techniques are presented.