Switchable Friction Coefficient on Shape Memory Photonic Crystals
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.182
Switchable Friction Coefficient on Shape Memory Photonic Crystals Yifan Zhang1, Xingyi Lyu2, Yongliang Ni1, Diyang Li2, Sin-Yen Leo2, Yinong Chen1, Peng Jiang2, Curtis R. Taylor1 1
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, 32611, U.S.A.
2
Department of Chemical Engineering, University of Florida, Gainesville, FL, 32611, U.S.A.
ABSTRACT
Intelligent control of friction and adhesion has attracted much attention for use in soft robotics, human-sensor interfaces, and bionics. Here we introduce a shape memory photonic crystal (SMPC) polymer that can be programmed and recovered by solvent to realize switchable surface friction. Micro sliding test show that the friction coefficient on this SMPC in the programmed and recovered state can vary by three times. We also show that the mechanism behind this switchable friction coefficient is the surface roughness related adhesion.
INTRODUCTION As tribology has been well studied in recent years, people can realize ultra-high or ultra-low friction and adhesion by thin film surface coating [1,2], surface nano texture design [3,4], or tuning the nanocomposite structure of materials [5,6]. However, the intelligent control of friction and adhesion is desired for potential applications as soft robotic fingers [7,8], body tissues interfaces [9,10], gecko feet bionics [11-14], and so on. Most popular ways to realize this goal are fabricating nano textures from thermally triggered shape memory polymers (SMP) [15-18], but unfortunately, they are inoperable at room temperature. Here, we introduce an athermal shape memory nano texture made of ordered polymeric nano pores. As the pore diameter is about 300 nm, and very close to visible light wavelength, it’s also a photonic crystal whose photonic band gaps (PBGs) can be predicted by Bragg’s law of diffraction [19-21]. The original ordered porous structure 757
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can be programmed to a deformed state by water swelling and evaporating at room temperature. Once the deformed pores are swelled in ethanol and dried, they will recover to its original state. The mechanism behind this shape memory cycle is the competing relationship between the capillary force of the solvent and the elasticity of pores [22]. In this paper, we investigate the changes in friction on nano textured SMPCs and their associated mechanisms. The surface roughness will increase three times once the (SMPC) is programmed to deformed state. During the contact, the adhesion force will decrease because of the less real contact area on the rough surface [23-26]. As the friction force at micro scales is dominated by adhesion or surface energy of the interface [27-29], the surface roughness change will result in a switching of the friction
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