Reversible Attachment with Tailored Permeability: The Feather Vane and Bioinspired Designs

Abstract

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.

Spontaneous Directional Self‐Cleaning on the Feathers of the Aquatic Bird Anser cygnoides domesticus Induced by a Transient Superhydrophilicity

“…[ 6 ] As shown in Figure 1d, a single feather on goose wings is about 15–20 cm in length, composing a main shaft (rachis and calamus) and a vane in which feather barbs branch from the shaft. [ 8,10 ] The single feather barb is ≈4–5 cm in length and ≈800 µm in width (Figure 1e), showing a typical asymmetric microstructure on both sides. Notably, the water transport directionally on the superhydrophilic feather from the shaft side to the edge side in a preferred direction B , while was pinned in the opposite direction A (Figure 1f; Movie S1, Supporting Information).…”

“…As shown, the feather vane was composed of numerous over‐lapped feather barbs, and each barb is composed of both grooved barbules and hooked barbules ( Figure a), as has been reported. [ 8,10,12 ] Considering the similar liquid transport behavior shared on both grooved and hooked barbules, as well as the similar structure, here we focused on the grooved barbules which is ≈275 µm in width. The grooved barbules show curved laminas structures with dorsally curved margins, forming a typical half‐closed concave‐shaped microchannels with width of ≈20 µm by overlapping with the neighboring barbules (Figure 3b).…”

Engineering with keratin: A functional material and a source of bioinspiration

“…The feather vane is directionally permeable, which effectively helps it capture air for lift ( Alibardi, 2007 ; Sullivan et al., 2017a ). This mechanism is controlled by the branching barbs' geometry and stiffness and interconnecting barbule network, which ultimately forms the feather vane ( Alibardi, 2007 ; Sullivan et al., 2017a ). Barbs, which branch from the rachis, are further branched into barbules.…”

“…Having multiple hooklets increases both the adhesion and the probability that two neighboring barbs will stay connected. The interconnected network of the feather vane, provided by this adhesive mechanism between the barbs and barbules, is credited as the essential element that allows birds to achieve flight ( Sullivan et al., 2017a ).

Figure 16 Progression of bioinspired designs based on the attachment mechanism found in the feather vane (A) SEM micrograph of the feather vane showing a branched network of barbs, barbules, and hooklets.

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The meaning of purely structural colour: white plumage reflectance indicates feather condition

“…As barbules play important roles in maintaining the mechanical integrity of flight feathers (e.g. Chen et al 2016, Sullivan et al 2017, it is possible that a reduction in the number of barbules could result in lower mechanical integrity, causing the apical sections of the barbs to 'bend together' or break off at the edge of the vane and reducing its surface area (Fig. 1).…”