Variation of the frictional anisotropy on ventral scales of snakes caused by nanoscale steps

Effects of surface morphology and chemical composition on friction properties of Xenopeltis hainanensis scales

The scales of Xenopeltis hainanensis, a snake that can crawl in fields, valleys, and other places, can serve as inspiration for the design of scale-like bionic materials. We present a systematic morphological, microstructural, chemical, and mechanical analysis, including elastic modulus, hardness, and wear morphology of the scales to understand the friction basis for achieving the reptile requirements. At the surface level, a comb-like arrangement of microstructures on the ventral scales provides more surface area and reduces pressure. The separation of microstructures, along with the bending and delamination of collagen fibrils could contribute to energy dissipation, which helps prevent catastrophic failure at deeper structural levels. At the cross-sectional level, a greater thickness provides more distribution of stresses over a larger volume, reducing local deformation and increasing the resistance to damage. At the material level, the ventral scales show higher modulus (E = 384.65 ± 19.03 MPa, H = 58.67 ± 6.15 MPa) than other regions of snake scales, which is attributed to the increased thickness of the scales and the higher concentration of sulfur (S). The experimental results, combined with Energy-dispersive X-ray spectroscopy and SEM characterization, provide a complete picture of the fiction properties influenced by surface morphology and chemical composition during scratch extension of the Xenopeltis hainanensis scales.

Bidirectional Underwater Drag Reduction on Bionic Flounder Two-Tier Structural Surfaces

Engineering marvels found throughout the exclusive structural features of biological surfaces have given rise to the progressive development of skin friction drag reduction. However, despite many previous works reporting forward drag reduction where the bio-inspired surface features are aligned with the flow direction, it is still challenging to achieve bidirectional drag reduction for non-morphable surface structures. Inspired by the flounder ctenoid scales characterized by tilted, millimeter-sized oval fins embedded with sub-millimeter spikes, we fabricate a bionic flounder two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by ηdr = 19%. Even in the backwards direction, where the flow is completely against the tilting direction of surface structures, BFTSS still exhibits a considerable drag reduction of ηdr = 4.2%. Experiments and numerical simulations reveal that this unique bidirectional drag reduction is attributed to synergistic effects of the two-tier structures of BFTSS. The array of oval fins can distort the boundary layer flow and mitigate the viscous shear, whilst the microscale spikes act to promote the flow separation to relieve the pressure gradient in the viscous sublayer. Notably, the pressure gradient relief effect of microscale spikes remains invariant to the flow direction and is responsible for the backward drag reduction as well. The bidirectional drag reduction of BFTSS can be extensively applied in minimizing the energy consumption of ships and underwater vessels, as well as in pipeline transport.

Efficient bending and lifting patterns in snake locomotion

We optimize three-dimensional snake kinematics for locomotor efficiency. We assume a general space-curve representation of the snake backbone with small-to-moderate lifting off the ground and negligible body inertia. The cost of locomotion includes work against friction and internal viscous dissipation. When restricted to planar kinematics, our population-based optimization method finds the same types of optima as a previous Newton-based method. With lifting, a few types of optimal motions prevail. We have an s-shaped body with alternating lifting of the middle and ends at small-to-moderate transverse friction. With large transverse friction, curling and sliding motions are typical at small viscous dissipation, replaced by large-amplitude bending at large viscous dissipation. With small viscous dissipation, we find local optima that resemble sidewinding motions across friction coefficient space. They are always suboptimal to alternating lifting motions, with average input power 10–100% higher.

The Tribological Adaptability for Ventral Scales of Dinodon rufozonatum in Dry/Wet/Rough Environments

The ventral scales of Dinodon rufozonatum were investigated to understand the outstanding tribological adaptability in various environments. The coefficient of friction (COF) of ventral scales was measured and changed with the contact conditions. It was discovered that the COF of scales under water-lubrication conditions (WLC) was larger than that under dry conditions (DC). More interestingly, the COF increased first and then decreased as the substrate roughness reduced. The abrasion marks on scales were then observed. The results indicated that the scales in DC wore more gently than that in WLC. Moreover, the degree of wear reduced with the decrease of substrate roughness. The frictional performance of ventral scales enabled the snakes to move more efficiently, quickly, and flexibly in multiple environments.

Functional consequences of convergently evolved microscopic skin features on snake locomotion

The small structures that decorate biological surfaces can significantly affect behavior, yet the diversity of animal-environment interactions essential for survival makes ascribing functions to structures challenging. Microscopic skin textures may be particularly important for snakes and other limbless locomotors, where substrate interactions are mediated solely through body contact. While previous studies have characterized ventral surface features of some snake species, the functional consequences of these textures are not fully understood. Here, we perform a comparative study, combining atomic force microscopy measurements with mathematical modeling to generate predictions that link microscopic textures to locomotor performance. We discover an evolutionary convergence in the ventral skin structures of a few sidewinding specialist vipers that inhabit sandy deserts-an isotropic texture that is distinct from the head-to-tail-oriented, micrometer-sized spikes observed on a phylogenetically broad sampling of nonsidewinding vipers and other snakes from diverse habitats and wide geographic range. A mathematical model that relates structural directionality to frictional anisotropy reveals that isotropy enhances movement during sidewinding, whereas anisotropy improves movement during slithering via lateral undulation of the body. Our results highlight how an integrated approach can provide quantitative predictions for structure-function relationships and insights into behavioral and evolutionary adaptations in biological systems.