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Why and how do birds as small as a hummingbird and as large as a condor fly? Research reveals new secrets.

Whale baleen is a keratin‐based biological material; it provides life‐long (40–100 years) filter‐feeding for baleen whales in place of teeth. This study reveals new aspects of the contribution of the baleen's hierarchical structure to its fracture toughness and connects it to the unique performance requirements, which require anisotropy of fracture resistance. Baleen plates are subjected to competing external effects of hydration and varying loading rates and demonstrate a high fracture toughness in transverse loading, which is the most important direction in the filtering function; in the longitudinal direction, the toughness is much lower since delamination and controlled flexure are expected and desirable. The compressive strength is also established and results support the fracture toughness measurements: it is also highly anisotropic, and exhibits a ductile‐to‐brittle transition with increasing strain rate in the dry condition, which is absent in the hydrated condition, conferring impact resistance to the baleen. Using 3D‐printing prototypes that replicate the three principal structural features of the baleen plate (hollow medulla, mineralized tubules, and sandwich‐tubular structure) are created, and the role of its structure in determining its mechanical behavior is demonstrated. These findings suggest new bioinspired engineering materials.

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Data are presented which show the effect of dilute addition of polymer on the ribbing instability in coating flows. A qualitative model which treats the growth of a disturbance as an extensional flow suggests that elasticity is a destabilizing factor, in agreement with the observations.

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In bird flight, the majority of the wing surface consists of highly refined and hierarchically organized feathers. They are composed of barbs that stem from the feather shaft and barbules that branch from barbs, forming a rigid feather vane. Barbules provide adhesion within the vane through an interlocking hook-and-groove mechanism to allow for the effective capture of air. This functional adhesive can reattach if structures unfasten from one another, preventing catastrophic damage of the vane. Here, using pelican primary feathers as a model material, we investigate the in-plane adhesion and stiffness of barbules. With guineafowl, pelican, and dove feathers, we determine the effect of barbules on the feather vane's ability to capture air. The vane is found to have directional permeability, and the effect of detaching barbules on the feather's competency is determined to be a function of barb dimensions. Interestingly, barbule spacing is found to vary within a narrow 8-16 µm range for birds weighing from 4-11 000 g (hummingbird to condor). Additionally, bioinspired barbules are fabricated through additive manufacturing to study the complexities of the vane. Barbules are underexplored structures imperative to the adeptness of the feather in flight, with the potential to provide bioinspired aerospace materials.

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