A catecholamine freestanding film is discovered to be spontaneously formed at the air–water interface, and the film has unique properties of robust surface adhesiveness, self‐healing, and ...stimuli‐responsive properties. The interfacial film‐producing procedure is a simple single step containing polyamines and catechol(amine)s. It is found that oxygen‐rich regions existing at an air–water interface greatly accelerate the catecholamine crosslinking reaction.
A specific feature of fibrous surfaces is the dependence of their mechanical properties on the alignment of the fibers. Vertically aligned fibers enhance friction and adhesion, whereas horizontal ...fibers are known to act as a lubricant reducing the friction. Many plants form a specific fibrous mucilage cover around their seeds upon hydration. This mucilage consists of cellulose, hemicelluloses, and strongly hydrophilic pectins. We show that the controlled critical-point drying of hydrated seed mucilage of three exemplary seed mucilage-rich plant species results in the exposure of free-standing cellulose nanofibers with a very high aspect ratio and anchored to the seed surface. The structural dimensions of the cellulose nanofibers are similar to the vertically aligned carbon nanotubes and the contact elements in the adhesion system of the gecko that show an outstanding high dry friction and adhesion. Tribological experiments demonstrate very high average friction coefficients when sliding a smooth and stiff probe over the surface of such arrays of dry free-standing cellulose nanofibrils in the range from 1.4 to 1.8. The high friction values most likely arise from bending of the single cellulose fibers and their alignment with the counterpart surface in close contact. We suggest the potential of free-standing cellulose nanofibrils of plant seed mucilage as a natural and ecologically friendly material where high contact forces to surfaces in dry environments are desired.
Wandering spiders climb vertically and walk upside‐down on rough and smooth surfaces using a nanostructured attachment system on their feet. The spiders are assumed to adhere by intermolecular van ...der Waals forces between the adhesive structures and the substrate. The adhesive elements are arranged highly ordered on the hierarchically structured attachment hair (setae). While walking, it has been suggested that the spiders apply a shear force on their legs to increase friction. However, the detailed mechanical behavior of the hair's structures during attachment and detachment remains unknown. Here, gradients of the mechanical properties of the attachment hair on different length scales that have evolved to support attachment, stabilize adhesion in contact, and withstand high stress at detachment, examined by in situ experiments, are shown. Shearing helps to self‐align the adhesive elements with the substrate. The study is anticipated to contribute to the development of optimized artificial dry adhesives.
Nanostructured adhesive hair on the feet of spiders is responsible for strong adhesion, enabling the animals to walk upside down. Using scanning nanofocus X‐ray scattering under force control, the mechanism of alignment and consequently intimate contact, leading to adhesion of the nanostructures to a surface, is examined. The results show excellent adaptation of the biological structures for their specific function.
X-ray phase contrast nanotomography enables imaging of a wide range of samples with high spatial resolution in 3D. Near-field holography, as one of the major phase contrast techniques, is often ...implemented using X-ray optics such as Kirkpatrick-Baez mirrors, waveguides and compound refractive lenses. However, these optics are often tailor-made for a specific beamline and challenging to implement and align. Here, we present a near-field holography setup based on Fresnel zone plates which is fast and easy to align and provides a smooth illumination and flat field. The imaging quality of different types of Fresnel zone plates is compared in terms of the flat-field quality, the achievable resolution and exposure efficiency i.e. the photons arriving at the detector. Overall, this setup is capable of imaging different types of samples at high spatial resolution of below 100 nm in 3D with access to the quantitative phase information.
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•Long-term friction of snake skin using pure sliding reciprocating motion is studied.•Snake skin shows low and stable μ (≈0.2) over several thousands of sliding cycles.•Stick-slip ...behavior is not observed as compared to an epoxy resin of similar hardness.•Hierarchical microstructure and stiffness gradient probably contribute to reduce wear.
Snake skins evolved to withstand permanent friction and wear during sliding. Here, the microstructure of ventral scales of the snake Lampropeltis getula californiae was analyzed using scanning electron microscopy, and the long-term dynamic friction behavior was investigated by reciprocating sliding friction tests. A smooth epoxy resin with similar elasticity modulus and hardness was used for comparison purposes. Strong differences in frictional and wear mechanisms between the two materials were revealed in spite of similar mechanical properties. Snake skin showed a considerably lower frictional coefficient that kept stable over several thousands of sliding cycles. A reduction of the stick-slip behavior was also denoted by analyzing the variation of the friction coefficient in the forward and reverse motion influencing the wear mechanism. This frictional behavior can be explained by three different but complementary mechanisms: fibrous layered composite material of the skin with a gradient of material properties, surface microstructure, and the presence of ordered layers of lipid molecules at the skin surface.
To adhere their silk threads for the construction of webs and to fix the dragline, spiders produce attachment discs of piriform silk. Uniquely, the aquatic spider Argyroneta aquatica spends its ...entire life cycle underwater. Therefore, it has to glue its attachment discs to substrates underwater. Here we show that Argyroneta aquatica applies its thread anchors within an air layer around the spinnerets maintained by superhydrophobic setae. During spinning, symmetric movements of the spinnerets ensure retaining air in the contact area. The flat structure of the attachment discs is thought to facilitate fast curing of the piriform adhesive cement and improves the resistance against drag forces. Pull-off tests on draglines connected with attachment discs on different hydrophilic substrates point to dragline rupture as the failure mode. The Young´s modulus of the dragline (8.3 GPa) is within the range as in terrestrial spiders. The shown structural and behavioral adaptations can be the model for new artificial underwater gluing devices.
The Role of Vanes in the Damping of Bird Feathers Deng, Kai; Rajabi, Hamed; Kovalev, Alexander ...
Journal of bionic engineering/Journal of Bionics Engineering,
07/2023, Volume:
20, Issue:
4
Journal Article
Peer reviewed
Open access
Bird feathers sustain bending and vibrations during flight. Such unwanted vibrations could potentially cause noise and flight instabilities. Damping could alter the system response, resulting in ...improving quiet flight, stability, and controllability. Vanes of feathers are known to be indispensable for supporting the aerodynamic function of the wings. The relationship between the hierarchical structures of vanes and the mechanical properties of the feather has been previously studied. However, still little is known about their relationship with feathers’ damping properties. Here, the role of vanes in feathers’ damping properties was quantified. The vibrations of the feathers with vanes and the bare shaft without vanes after step deflections in the plane of the vanes and perpendicular to it were measured using high-speed video recording. The presence of several main natural vibration modes was observed in the feathers with vanes. After trimming vanes, more vibration modes were observed, the fundamental frequencies increased by 51–70%, and the damping ratio decreased by 38–60%. Therefore, we suggest that vanes largely increase feather damping properties. Damping mechanisms based on the morphology of feather vanes are discussed. The aerodynamic damping is connected with the planar vane surface, the structural damping is related to the interlocking between barbules and barbs, and the material damping is caused by the foamy medulla inside barbs.
Nanostructured adhesive hairs on the feet of spiders are responsible for strong adhesion, enabling the animals to walk vertically and upside down. In article number 2002758, Clemens F. Schaber, ...Martin Müller, and co‐workers use scanning nanofocus X‐ray scattering under force control to elucidate the mechanism of alignment and consequently strong attachment of the nanostructures to a glass surface. The spider attachment system is a promising model for the development of powerful residue‐free dry adhesives.
Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy ...adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.
For flying insects, stability is essential to maintain the orientation and direction of motion in flight. Flight instability is caused by a variety of factors, such as intended abrupt flight ...manoeuvres and unwanted environmental disturbances. Although wings play a key role in insect flight stability, little is known about their oscillatory behaviour. Here we present the first systematic study of insect wing damping. We show that different wing regions have almost identical damping properties. The mean damping ratio of fresh wings is noticeably higher than that previously thought. Flight muscles and hemolymph have almost no 'direct' influence on the wing damping. In contrast, the involvement of the wing hinge can significantly increase damping. We also show that although desiccation reduces the wing damping ratio, rehydration leads to full recovery of damping properties after desiccation. Hence, we expect hemolymph to influence the wing damping indirectly, by continuously hydrating the wing system.
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