Dry self-cleaning properties of hard and soft fibrillar structures

Recently, gecko-inspired synthetic adhesives (GSAs) have been made using a variety of fabrication techniques and materials, with one made from a hard polymer having been reported to recover its shear adhesion after fouling by normal use, or "dry self-clean", a feature useful for applications in wall crawling robots, reusable adhesives, microfabrication and solar panel cleaning. This paper investigates the impact of two design parameters on the dry self-cleaning capability of GSAs by experimentally testing two GSAs after fouling with small (1 μm), medium (3-10 μm), and large (40-50 μm) 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, with SEM imaging revealing particles embedding within the polymer. 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. These results have important implications on the choice of materials and geometries used for GSAs when dry self-cleaning capability is a desired function in the material.