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Synthetic Gecko Adhesives and Adhesion in Geckos

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2011, Doctor of Philosophy, University of Akron, Polymer Science.

Geckos’ feet consist of an array of millions of keratin hairs that are hierarchically split at their ends into hundreds of contact elements called “spatula(e)”. Spatulae make intimate contacts with surface and the attractive van der Waals (vdW) interactions are strong enough to support up to 100 times the animals’ bodyweight. Tremendous efforts have been made to mimic this adhesion with polymeric materials and carbon nanotubes (CNT). However, most of these fall short of the performance of geckos. “Contact splitting principle”, based on Johnson–Kendall–Roberts (JKR) theory, predicts that a vertically aligned carbon nanotube array (VA-CNT) will be at least 50 times stronger than gecko feet. Although 160 times higher adhesion was recorded in atomic force microscopy (AFM) measurements, macroscopic VA-CNT patches often show low or even no adhesion to substrates.

The behavior of VA-CNT hairs near the contact interface has been explored using a combination of mechanical, electrical contact resistance, and scanning electron microscopic (SEM) measurements. Instead of making the expected end contacts, carbon nanotubes make significant side-wall contacts that increase with preload. Adhesion of side-wall contact CNTs is determined by the balance of adhesion in the contact region and the bending stiffness of the CNTs, thus a compliant VA-CNT array is required to make adhesive patches.

Macroscopic patches of compliant VA-CNT array have been fabricated. Patches of uniform array have adhesive strength similar to that of geckos (10 N/cm2) on a variety of substrates and can be removed easily by peeling. When the array is patterned to mimic the hierarchical structures of gecko foot-hairs, strength increases up to four times. VA-CNT-based gecko adhesives are self-cleaning, non-viscoelasticity and give good strength in vacuum. These properties are desired in robotics, microelectronics, thermal management and outer space operations.

Current theory still cannot completely explain adhesion of gecko feet. A series of experiments have been carried out to measure adhesion at different temperatures using a single protocol with two species of gecko that had been previously studied (G. gecko and P. dubia). Strong evidence of an effect of temperature was found but the trend was counterintuitive given the thermal biology of geckos and it violated the prediction by van der Waals interactions. Consequently, other factors (e.g., humidity) that could explain the variation in the observed clinging performance were examined. Evidence was found, unexpectedly, that humidity is likely an important determinant of clinging force in geckos. Both van der Waals and capillary forces fail to explain the shear adhesion data at the whole animal scale. Resolution of this paradox will require examination of the physical and chemical interaction at the interface and particular way in which water interacts with substrate and setae at the nanometer scale.

Ali Dhinojwala, Dr. (Advisor)
Gary Hamed, Dr. (Committee Chair)
Li Jia, Dr. (Committee Member)
Darrell Reneker, Dr. (Committee Member)
Peter Niewiarowski, Dr. (Committee Member)
189 p.

Recommended Citations

Citations

  • Ge, L. (2011). Synthetic Gecko Adhesives and Adhesion in Geckos [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1294161826

    APA Style (7th edition)

  • Ge, Liehui. Synthetic Gecko Adhesives and Adhesion in Geckos. 2011. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1294161826.

    MLA Style (8th edition)

  • Ge, Liehui. "Synthetic Gecko Adhesives and Adhesion in Geckos." Doctoral dissertation, University of Akron, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=akron1294161826

    Chicago Manual of Style (17th edition)