Design Of Gecko-Inspired PDMS Composite Pillar Arrays And Their Friction Properties

Geckos’ remarkable ability to stick to and climb just about any surfaces found in nature has been attributed to a complex array of hairy structures, which have motivated research interest in fabrication and testing of synthetic adhesives. To have a truly gecko-like adhesive, the gecko-inspired adhesives should be able to adhere to surfaces with a wide range of surface energy. In designing such adhesives, the stiffness of the material(indicated by Young’s modulus) is a critical consideration as it determines the flexibility of the fiber elements and ultimately the aspect ratio of the fibers required to achieve the desired adhesive strength. In this study, the design of gecko inspired fibrillar adhesives with different shapes, aspect ratios and Young’s modulus is investigated by numerical simulation method. The adhesion, unstick and instability models are built. Polydimethylsilxane(PDMS) fiber arrays are fabricated by moulding method. The effects of fabrication technics on morphology and wetting of PDMS fiber arrays are studied. The effect of reinforced particles on surface topography is investigated. The macroscale friction behavior of PDMS fiber arrays is conducted with friction testing setup. The Young’s modulus and wetting behavior of PDMS fiber arrays with various curing technics and reinforced particles are also studied.The deformation of fiber with various aspect ratios and Young’s modulus are studied. The results show when the aspect ratio increases, the friction force increases because of the increasing deformation of fiber. The deformation of fiber decreases with increasing Young’s modulus. Fiber stick model is derived from cantilever beam theory, which indicates that adhesion force increases with increasing effective number of fiber and maximum contact areas of counter-surface. Unstick model is derived from equation of deflection curve, which reveals when the fiber debonding, maximum deflection increases with increasing aspect ratio and decreasing Young’s modulus. Instability model is built, which reveals maximum critical force increases with increasing Young’s modulus and decreasing aspect ratios. The buildup of model provides theoretical basis for fiber design. Based on above models results, the technics and structure which are sutiable for this test are found.According to design and model analysis of gecko-inspired adhesives, PDMS fiber arrays are fabricated by moulding method. The effect s of technological parameters, such as oxygen plasma etching and chemical etching on size of nanofiber are studied. It is indicated when the power is 30 W, the flow is 100 sccm, the diameter of silicon nanowires(Si NW) decreases with increasing etching time of oxygen plasma, and the etching velocity becomes faster when the etching time is more than 8 min. The length of Si NW increases with increasing etching time of chemical etching, and the etching velocity becomes slower when the etching time is more than 16 min. The one-fluid wetting behaviors of surfaces textured by an array of Si NW are investigated systematically. The results show the hydrophobic abilities are inhanced with the increasing length of PFTS and ODTS coated silicon nanowires, and when the length of Si NW is more than 0.5 μm,contact angle remains at 160 o, which is beneficial to demould PDMS and obtain integrated sample. The Wenzel/CB model is built to describe near 0 contact angle even at low roughness approximating the receding angle of the droplet in a non-wetting environment. The CB/CB model is built to describe intermediate angle closer to the advancing angle. The effect of curing methods on sample geometry size and morphology are researched. It is shown that curing at room temperature(20 oC) for 12 h and high temperature(100 oC) for 1 h makes sample in good condition. The effect of reinforced particles on morphology indicates the top surface roughness of fiber is affected by particle size, content and dispersion.The friction behavior of PDMS nanofiber arrays is investigated. Compared to non-fiber structure, the friction coefficient of PDMS fiber is enhanced 2 times. The friction coefficient increases first and then decreases with the increasing length of fiber, and the friction coefficient of fiber arrays with the diameter of 700 nm and the length of 1 μm is maximum, about 59. When the surface energy of countersurface is higher, the friction coefficient of fiber is larger. The relationship between friction behavior and Young’s modulus of PDMS microfiber arrays are researched. When increasing content of curing agent into PDMS, Young ’s modulus increases while the friction coefficient decreases. When adding particles into PDMS, Young’s modulus is enhanced 67% and the friction coefficient remains high, at 18. The effects of particle types, size and contents on wetting behavior of PDMS micro fiber arrays are investigated. It is concluded that the hydrophobicity of PDMS arrays depends on its fiber structure. The contact angle of fiber arrays increases 27% compared to non-fiber structure. This provides general solution for the fabrications and applications of gecko-inspierd pilliar arrays in the future.