No abstract

Geckos, which are capable of walking on walls and hanging from ceilings with the help of micro-/nano-scale hierarchical fibrils (setae) on their toe pads, have become the main prototype in the design and fabrication of fibrillar dry adhesives. As the unique fibrillar feature of the toe pads of geckos allows them to develop an intimate contact with the substrate the animal is walking on or clinging to, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification (CE) phenomenon. Even so, the possibility of the occurrence of CE and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos have been overlooked for several decades. In this study, by measuring the magnitude of the electric charges, together with the adhesion forces, that gecko foot pads develop in contact with different materials, we have clarified for the first time that CE does contribute effectively to gecko adhesion. More importantly, we have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.

The superior material properties of b-keratin along with the hierarchical high-aspect-ratio structure of geckos' foot pad have enabled geckos to stick readily and rapidly to almost any surfaces in both dry and wet conditions. In this research, nonsticky fluoropolymer (Teflon AF) resembling b-keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of high-aspect-ratio nanopillars terminated with a ''fluffy'' top layer. Both the nanopillars and the terminating layer are fabricated concurrently by replica molding using a nanoporous anodic aluminum oxide membrane as the mold. These Teflon AF hierarchical nanostructures are shown to have an exceptional capacity to generate strong adhesion in both dry conditions and under water because of combined actions of van der Waals forces, electrostatic attractions, and hydrophobic effects.

In this research, ultrahigh-molecular-weight polyethylene (UHMWPE)/multiwalled carbon nanotube (MWCNT) nanocomposites with different nanotube concentrations (0.5, 1.5, 2.5, and 3.5 wt %) were prepared via in situ polymerization with a novel, bisupported Ziegler-Natta catalytic system. Magnesium ethoxide [Mg(OEt) 2 ] and surface-functionalized MWCNTs were used as the support of the catalyst. Titanium tetrachloride (TiCl 4 ) accompanied by triethylaluminum constituted the Ziegler-Natta catalytic system. Preparation of the catalyst and the polymerization were carried out in the slurry phase under an argon atmosphere. Support of the catalyst on the MWCNTs was investigated with Fourier transform infrared spectroscopy. The results confirmed the interaction between the catalyst and the MWCNT hydroxyl groups. Intrinsic viscosity measurements showed an ultrahigh molecular weight in the produced samples. Scanning electron microscopy images confirmed the good dispersion of MWCNTs throughout the polyethylene (PE) matrix. The crystallization behavior of the samples was examined with differential scanning calorimetry. Its results showed that the crystal content of the samples increased with increasing MWCNT concentration up to 1.5 wt %. The same trend was observed for the crystallization temperature, whereas the melting temperature did not change with increasing MWCNT concentration up to 1.5 wt %, but it decreased beyond this concentration. In addition, thermogravimetric analysis results showed that the addition of MWCNTs noticeably improved all of the investigated thermal stability factors of the UHMWPE/MWCNT nanocomposites compared to those of pure PE. The results obtained from tensile testing revealed significant increases in the Young's modulus, yield stress, and ultimate tensile strength. This indicated a tremendous improvement in the mechanical properties of the PE/MWCNT nanocomposites compared to those of pure PE. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: E453-E461, 2012

With respect to the dry adhesion mechanism that geckos employ for their locomotion, it is commonly accepted that the adhesive performance of synthetic bio‐inspired dry adhesives results from the formation of van der Waals interactions at the tip or side of the dry adhesive fibrils with the substrate they are brought into contact with. However, what has been usually neglected in this connection is that electrostatic interactions may also be developed at the contact between any two materials via the familiar contact electrification (CE) phenomenon. Although CE is common and can have a large influence on interfacial interaction forces, its impact on adhesive properties of synthetic dry adhesives has been overlooked. Even so, recent reports on fabrication of polymeric dry adhesives, which can generate strong adhesion forces relying on electrostatic interactions coming from CE, have brought to light again the idea that charging the surface of dry adhesives, specifically polymeric ones, can play a very crucial role in their adhesive behavior. From this perspective, the main reasons that have caused the lack of attention to this concept and the possible contributions of CE in interfacial interactions of polymeric dry adhesives are thoroughly discussed in this current critical review.