Strong Adhesion and Friction Coupling in Hierarchical Carbon Nanotube Arrays for Dry Adhesive Applications

Abstract

The adhesion and friction coupling of hierarchical carbon nanotube arrays was investigated with a hierarchical multiscale modeling approach. At device level, vertically aligned carbon nanotube (VA-CNT) arrays with laterally distributed segments on top were analyzed via finite element methods to determine the macroscopic adhesion and friction force coupling. At the nanoscale, molecular dynamics simulation was performed to explore the origin of the adhesion enhancement due to the existence of the laterally distributed CNTs. The results show interfacial adhesion force is drastically promoted by interfacial friction force when a single lateral CNT is being peeled from an amorphous carbon substrate. By fitting with experiments, we find that under shearing loadings the maximum interfacial adhesion force is increased by a factor of ~5, compared to that under normal loadings. Pre-existing surface asperities of the substrate have proven to be the source of generating large interfacial friction, which in turn results in an enhanced adhesion. The critical peeling angles derived from the continuum and nano- levels are comparable to those of geckos and other synthetic adhesives. Our analysis indicates that the adhesion enhancement factor of the hierarchically structured VA-CNT arrays could be further increased by uniformly orienting the laterally distributed CNTs on top. Most importantly, a significant buckling of the lateral CNT at peeling front is captured on the molecular level, which provides a basis for the fundamental understanding of local deformation, and failure mechanisms of nanofibrillar structures. This work gives an insight into the durability issues that prevent the success of artificial dry adhesives.

Tribological properties of vertically aligned carbon nanotube arrays

“…The presence of pull-off forces found by other authors can be explained with different testing methods and VACNT arrangements [19,20,22,38]. Testing of adhesion of VACNTs by pressing with a plate led to an anisotropic alignment of the tips of the 200-500 µm long CNTs in parallel to the test surface [20].…”

“…This kind of deformation led to attachment of the CNTs to the test substrate by their sides, and that is why normal pull-off forces, presumably caused by shear adhesion between the single CNTs and the substrate, increased with preload. The latter was later predicted by Finite Element Analysis [38]. Chen et al [22] reported significant pull-off forces from micro-patterned 170-230 µm high VACNT pillar-like arrays, and an influence of pillar aspect ratio and spacing on both EEM and adhesion strength.…”

Progress in Bioinspired Dry and Wet Gradient Materials from Design Principles to Engineering Applications

“… (C) Schematic diagram of carbon nanotube modifications with polydopamine ( Li et al., 2019 ). (D) Schematics of vertical aligned CNT arrays with laterally distributed CNT segments adhering on a target surface; FEA model for simulating the macroscopic adhesive behaviors of vertical aligned CNT arrays and a snapshot of a lateral CNT being peeled from substrate in molecular dynamics ( Hu et al., 2012 ). (E) Schematic diagram of the self-healing principle of a mussel thread ( Xu et al., 2019 ).…”

“… (D) Schematics of vertical aligned CNT arrays with laterally distributed CNT segments adhering on a target surface; FEA model for simulating the macroscopic adhesive behaviors of vertical aligned CNT arrays and a snapshot of a lateral CNT being peeled from substrate in molecular dynamics ( Hu et al., 2012 ). …”

“…The hierarchical structure of geckos' feet, which include the toe pads, setal arrays, and spatulae, allows the gecko to generate strong adhesion and friction to climb on various surfaces (Figure 5A) (Yu et al, 2011 for the pillars: simple fiber arrays, fiber ends with different geometries (such as mushroom shaped microfibers, fiber arrays with asymmetric spatulas, and fiber arrays with concaved pillar ends), titled fiber arrays, hierarchical fiber arrays, and fiber arrays combined with a lamellar structure (del Campo and Del Campo et al, 2007b;del Campo et al, 2007a;Haefliger and Boisen, 2006;Sameoto and Menon, 2009). As for the materials classification, polymer materials such as polyimide (Yoon et al, 2009), PVS (Gorb et al, 2007), PDMS (Dong et al, 2019;Greiner et al, 2007;Menon et al, 2004), poly (methyl methacrylate) (PMMA) (Yu et al, 2011), polystyrene (PS) (Kustandi et al, 2007), polyethylene (Lee and Fearing, 2008), and polyurethane (Kim et al, 2007;Jin et al, 2012) have been reported to be suitable for making gecko inspired (D) Schematics of vertical aligned CNT arrays with laterally distributed CNT segments adhering on a target surface; FEA model for simulating the macroscopic adhesive behaviors of vertical aligned CNT arrays and a snapshot of a lateral CNT being peeled from substrate in molecular dynamics (Hu et al, 2012). (E) Schematic diagram of the self-healing principle of a mussel thread (Xu et al, 2019).…”