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      Synchronous measurement of tribocharge and force at the footpads of freely moving animals


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          Hypothesis on electrostatic attraction mechanisms involving the hairy adhesion of climbing animals has been a matter of controversy for several years. The detection of tribocharge and forces at attachment organs of animals is a practical method of clarifying the dispute with respect to electrostatic attraction in the attachment of animals. Nonetheless, the tribo-electrification is rarely examined in the contact-adhesion of animals (especially in their free and autonomous attachment) due to the lack of available devices. Therefore, the present study involves establishing a method and an apparatus that enables synchronous detection of tribocharge and contact forces to study tribo-electrification in the free locomotion of geckos. A type of a combined sensor unit that consists of a three-dimensional force transducer and a capacitor-based charge probe is used to measure contact forces and tribocharge with a magnitude corresponding to several nano-Coulombs at a footpad of geckos when they climb vertically upward on an acrylic oligomer substrate. The experimental results indicate that tribocharge at the footpads of geckos is related to contact forces and contact areas. The measured charge allows the expectation of an exact attraction with magnitude corresponding to dozens of newtons per square meter and provides a probability of examining tribo-electrification in animal attachment from a macro level.

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          Carbon nanotube arrays with strong shear binding-on and easy normal lifting-off.

          The ability of gecko lizards to adhere to a vertical solid surface comes from their remarkable feet with aligned microscopic elastic hairs. By using carbon nanotube arrays that are dominated by a straight body segment but with curly entangled top, we have created gecko-foot-mimetic dry adhesives that show macroscopic adhesive forces of approximately 100 newtons per square centimeter, almost 10 times that of a gecko foot, and a much stronger shear adhesion force than the normal adhesion force, to ensure strong binding along the shear direction and easy lifting in the normal direction. This anisotropic force distribution is due to the shear-induced alignments of the curly segments of the nanotubes. The mimetic adhesives can be alternatively binding-on and lifting-off over various substrates for simulating the walking of a living gecko.
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            Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements.

            The hairy attachment system on a gecko's toes, consisting of one billion spatulae in the case of Gekko gecko [Ruibal, R. & Ernst, V. (1965) J. Morphol. 117, 271-294], allows it to adhere to nearly all surface topographies. The mechanistic basis for gecko adhesion has been intensely investigated, but the lowest hierarchical level, that of the spatula, has become experimentally accessible only recently. This report details measurements of the adhesion force exerted by a single gecko spatula for various atmospheric conditions and surface chemistries. Through judicious choice and modification of substrates, the short- and long-range adhesive forces are separated. In contrast to previous work [Autumn, K., Sitti, M., Liang, Y. C. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, J. N. & Full, R. J. (2002) Proc. Natl. Acad. Sci. USA 99, 12252-12256], our measurements clearly show that humidity contributes significantly to gecko adhesion on a nanoscopic level. These findings are crucial for the development of artificial biomimetic attachment systems.
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              Undulatory swimming in sand: subsurface locomotion of the sandfish lizard.

              The desert-dwelling sandfish (Scincus scincus) moves within dry sand, a material that displays solid and fluidlike behavior. High-speed x-ray imaging shows that below the surface, the lizard no longer uses limbs for propulsion but generates thrust to overcome drag by propagating an undulatory traveling wave down the body. Although viscous hydrodynamics can predict swimming speed in fluids such as water, an equivalent theory for granular drag is not available. To predict sandfish swimming speed, we developed an empirical model by measuring granular drag force on a small cylinder oriented at different angles relative to the displacement direction and summing these forces over the animal movement profile. The agreement between model and experiment implies that the noninertial swimming occurs in a frictional fluid.

                Author and article information

                Tsinghua Science and Technology
                Tsinghua University Press (Xueyuan Building, Tsinghua University, Beijing 100084, China )
                05 March 2018
                : 06
                : 01
                : 75-83 (pp. )
                [ 1 ] Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
                [ 2 ] College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
                [ 3 ] College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
                Author notes
                * Corresponding author: Zhendong DAI, E-mail: zddai@ 123456nuaa.edu.cn

                Yi SONG. He received his bachelor and master degrees in engineering mechanics from Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China, in 2012 and 2015, respectively. He became a Ph.D. student in mechanical design and theory at NUAA in 2015 and his research interests include bio-tribology, biomechanics, and bio-inspired adhesion.

                Zhendong DAI. He is a professor, a supervisor of PhD students, and the director of Institute of Bio- inspired Surface and Engineering in Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, China. He obtained his Ph.D. degree in 1999 from College of Mechanical and Electrical Engineering, NUAA. After completing his postdoctoral research in 2001 in Max Planck Institute for Developmental Biology, he joined NUAA as a professor. His research areas include bionics, light material, control of bionics, bio-robots, and biological robots.


                This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

                Page count
                Figures: 4, Tables: 1, References: 38, Pages: 9
                Research Article

                Materials technology,Materials properties,Thin films & surfaces,Mechanical engineering
                animal,synchronous measurement,free locomotion,tribocharge,forces


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