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Tribological Characterization of Reptile Scales and Their Application As Technical Surfaces with Tunable Friction Anisotropy

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Tribological characterization of reptile scales and their application as technical surfaces with tunable friction anisotropy Zur Erlangung des akademischen Grades Doktor der Ingenieurwissenschaften von der KIT-Fakultät für Maschinenbau des Karlsruher Instituts für Technologie (KIT) genehmigte Dissertation von M.Eng. Weibin Wu Tag der mündlichen Prüfung: 25. September 2020 Hauptreferent: apl. Prof. Dr. Hendrik Hölscher Korreferenten: Prof. Dr. Manfred Kohl Prof. Dr. André Schirmeisen Abstract Some reptiles surprise humans with excellent tribological properties in terms of adhesion, friction, and wear, which originate from the long time of evolution in their respective living environment. Mimicking these unique tribological properties enables researchers to develop reptile-inspired surfaces with similar or even more advanced abilities. Thus, studying the tribological properties of reptiles as well as transferring their design to the artificial surfaces are of great importance for finding new solutions to engineering prob- lems which are related to friction and wear. After a brief introduction into the topic and a short recall of the applied methodologies, four chapters of this thesis deal with different aspects of reptile scales. Chapter 3 focuses on the sandfish (Scincus scincus), which is widely known for its special ability to swim in loose, aeolian sand without noticeable wear on its scales. The microscopic structural and tribological properties of single sandfish scales are characterized by atomic force microscopy (AFM) with various types of probe including sharp silicon tips, spherical glass tips, and sand debris. Snake scales together with several technical surfaces are also analyzed for compari- son. The experimental analysis demonstrates that scales of sandfish and snakes show lower friction coeffi- cients than most technical surfaces. However, their adhesion and abrasion resistant properties are not supe- rior to those of technical surfaces. Sandfish scales do not show any exceptional tribological properties if compared to snake scales. Other reasons such as the elastic properties of its epidermis or the dynamics of its locomotion are supposed to be responsible for the observed low wear rates of sandfish scales during the sand swimming. The multi-functionality of the scales from different regions of the snake bodies (i.e. dorsal, ventral, and spectacle scales) are investigated in the next chapter. For an individual snake species, the micro-ornamen- tations on these three types of scales were found to be very different. The dorsal scales of different snake species feature diverse microstructures. However, the ventral scales of most investigated snakes show very similar oriented micro-fibril structures, and the spectacle scales demonstrate stitching polygon like struc- tures. Characterization of the cross sectional architecture indicates stacked, layered structures on snake scales of all body regions. Ventral scales are nearly three times as thick as dorsal scales of the same species. Wettability, adhesion force, and spectral transmittance property were characterized and correlated with the micro-ornamentations on the scales. Dorsal scales are considered as the self-cleaning surface and light scattering filter to protect snakes from contamination and solar radiation. Spectacle snake scales sacrifice self-cleaning capability for obtaining high spectral transmittance in visible light and near-infrared spectrum. They can also block harmful ultraviolet light. Although displaying similar wettability and spectral trans- mitting ability as spectacle scales, ventral scales are widely accepted as a frictional component for optimiz- ing the locomotion performances of snakes. The fifth chapter deals with the frictional anisotropy of snake and sandfish scales. A sharp AFM tip scan- ning along the micro-fibril structures on ventral snake scales (i.e. upward and downward the nano-steps at the fibril ends) reveals significant friction anisotropy. Scanning up and down the nano-step structures of dorsal sandfish scales shows the same effect. The friction for an AFM tip scanning upward a nano-step increases significantly with the step height, while the increase is not apparent for downward scans. This effect leads to a nano-step height dependent friction anisotropy. A larger step height causes a larger friction anisotropy. With the micro-fibrils pointing towards the tail, snakes move much easier in forward than back- ward direction, which is favorable for them when climbing slops. The same effect is also found on a poly- meric replica of N. atra ventral scale. Experiments with micro-tribometer coincide with the above results. More interestingly, snakes vary nano-step height and therefore the subsequent friction anisotropy in differ- ent segments of their bodies. A general distribution pattern of friction anisotropy is observed along the body i Abstract of the examined snakes, i.e., larger friction anisotropy in the middle and tail section than in the other areas. Snakes might benefit from such design to realize optimized frictional performances during locomotion. Inspired by the step height dependent friction anisotropy found on the ventral scales of snakes, a tunable friction anisotropy surface was fabricated with shape memory polymers (SMPs) via hot embossing in Chap- ter 6. The shape memory effect of the polymers enables the surface to reversibly switch between a flattened topography and a micro-fibril structured topography, resulting in tunable anisotropic friction along the mi- cro-fibrils. AFM characterization shows that the shape-changing capability in terms of nano-step height and friction anisotropy can reach up to nearly 1000% and 300%, respectively. Moreover, the SMP topog- raphy recovers continuously and controllably with heating time allowing it to achieve desired step height and friction anisotropy on the surface by abruptly terminating the heating before full recovery. Long-term stability and reusability tests reveal that, at any arbitrary stage of the SMP programming/recovery cycle, the step height at the micro-fibril ends is highly stable in three months or after eleven programming cycles. Finally, the tunable friction anisotropy surface was utilized as a functional platform for unidirectional transport of micro-particles. As an example, a single PDMS microsphere was unidirectionally transported for nearly 5 mm on the structured SMP surface, while no significant movement (less than 0.1 mm) was observed on the flattened surface. The structured surface can also simultaneously transport many micro- spheres or micro sand particles towards a desired direction. This mechanism provides a promising prospect for applications such as dry self-cleaning coatings without liquids or particles sorting or separation devices. ii Kurzfassung Einige Reptilien überraschen den Menschen mit hervorragenden tribologischen Eigenschaften in Bezug auf Adhäsion, Reibung und Verschleiß, die aus der langen Entwicklungszeit in ihrem jeweiligen Lebensumfeld stammen. Durch die Nachahmung dieser einzigartigen tribologischen Eigenschaften können Forscher von Reptilien inspirierte Oberflächen mit ähnlichen oder noch fortgeschritteneren Fähigkeiten entwickeln. Daher sind Versuche, die tribologischen Eigenschaften von Reptilien zu untersuchen und sie auf künstliche Oberflächen zu übertragen, von großer Bedeutung, um neue Lösungen für technische Probleme zu finden, die mit Reibung und Verschleiß zusammenhängen. Nach einer kurzen Einführung in das Thema und einem kurzen Rückblick auf die angewandten Methoden befassen sich vier Kapitel dieser Arbeit mit verschiedenen Aspekten der Reptilienschuppen. Kapitel 3 konzentriert sich auf den Sandfisch (Scincus scincus), welcher weithin bekannt ist für seine besondere Fähigkeit, in lockerem, äolischem Sand zu schwimmen, ohne dass sich die Schuppen merklich abnutzen. Diese Arbeit charakterisiert die mikroskopischen strukturellen und tribologischen Eigenschaften einzelner Sandfischschuppen durch Rasterkraftmikroskopie (AFM) mit verschiedenen Arten von Sonden, einschließlich scharfer Siliziumspitzen, kugelförmiger Glasspitzen und Sandtrümmern. Zum Vergleich wurden auch Schlangenschuppen zusammen mit mehreren technischen Oberflächen analysiert. Die experimentelle Analyse zeigt, dass Schuppen von Sandfischen und Schlangen niedrigere Reibungskoeffizienten aufweisen als die meisten technischen Oberflächen. Jedoch sind Ihre Haft- und Abriebfestigkeitseigenschaften denen technischer Oberflächen nicht überlegen. Sandfischschuppen zeigen im Vergleich zu Schlangenschuppen keine außergewöhnlichen tribologischen Eigenschaften. Andere Gründe wie die elastischen Eigenschaften der Epidermis oder die Dynamik der Fortbewegung sollen für die beobachteten geringen Verschleißraten der Sandfischschuppen während dem „schwimmen“ durch den Sand verantwortlich sein. Die Multifunktionalität der Schuppen aus verschiedenen Regionen der Schlangenkörper (d. h. dorsale, ventrale, und Brillenschuppen) werden im nächsten Kapitel untersucht. Die Mikroornamente auf diesen drei Arten von Schuppen war für eine Schlange sehr unterschiedlich. Die Rückenschuppen verschiedener Schlangenarten weisen unterschiedliche Mikrostrukturen auf. Jedoch zeigen die ventralen Schuppen der meisten untersuchten Schlangen sehr ähnlich ausgerichtete Mikrofibrillenstrukturen auf, und die Brillenschuppen besitzen nähende polygonartige Strukturen. Die Charakterisierung der Querschnittsarchitektur zeigt, dass gestapelte Schichtstrukturen auf Schlangenschuppen in allen Körperregionen
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