Quantification of feather structure, wettability and resistance to liquid penetration

Abstract

Birds in the cormorant (Phalacrocoracidae) family dive tens of metres into water to prey on fish while entraining a thin layer of air (a plastron film) within the microstructures of their feathers. In addition, many species within the family spread their wings for long periods of time upon emerging from water. To investigate whether wetting and wing-spreading are related to feather structure, microscopy and photographic studies have previously been used to extract structural parameters for barbs and barbules. In this work, we describe a systematic methodology to characterize the quasihierarchical topography of bird feathers that is based on contact angle measurements using a set of polar and non-polar probing liquids. Contact angle measurements on dip-coated feathers of six aquatic bird species (including three from the Phalacrocoracidae family) are used to extract two distinguishing structural parameters, a dimensionless spacing ratio of the barbule (D*) and a characteristic length scale corresponding to the spacing of defect sites. The dimensionless spacing parameter can be used in conjunction with a model for the surface topography to enable us to predict a priori the apparent contact angles of water droplets on feathers as well as the water breakthrough pressure required for the disruption of the plastron on the feather barbules. The predicted values of breakthrough depths in water (1-4 m) are towards the lower end of typical diving depths for the aquatic bird species examined here, and therefore a representative feather is expected to be fully wetted in a typical deep dive. However, thermodynamic surface energy analysis based on a simple one-dimensional cylindrical model of the feathers using parameters extracted from the goniometric analysis reveals that for water droplets on feathers of all six species under consideration, the non-wetting 'Cassie-Baxter' composite state represents the global energy minimum of the system. By contrast, for other wetting liquids, such as alkanes and common oils, the global energy minimum corresponds to a fully wetted or Wenzel state. For diving birds, individual feathers therefore spontaneously dewet once the bird emerges out of water, and the 'wing-spreading' posture might assist in overcoming kinetic barriers associated with pinning of liquid droplets that retard the rate of drying of the wet plumage of diving birds.

Superhydrophobic materials for biomedical applications

“…They went on to discuss the roughness of feathers and fur and the resultant water repellent properties. Superhydrophobicity is a property of many naturally occurring substrates including plant leaves [4–8], many insect features including wings, legs, and eyes [9–16], feathers [17,18], fur [19], and beetle shells [20] ( Figure 1b ). Superhydrophobicity on these natural surfaces leads to improved function by providing water repellency or alternatively providing a self-cleaning surface where debris and pathogens are removed as water contacts and subsequently rolls off the surface.…”

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

“…In water birds, many additional and particularly specialized functions may occur. Examples are the buoyancy function in ducks, a differentiation between an isolating hydrophobic inner plumage and a water-repellent outer layer in the diving cormorant [133,134]. The most intriguing case may again be the penguin: in addition to thermal isolation, its superhydrophobicity functions as drag reduction and fulfils a particular function when the bird jumps from water onto an ice sheet.…”

“…On the other hand, the plumage forms the thermal isolation layer: a warm-blooded homoeothermic penguin with plumage soaked with freezing water could hardly survive in Antarctic conditions. Thus, plumage keratin is coated by fatty oily substances ('preening oil') and becomes waterrepellent [133]. In particular, this is essential for birds living on water or diving under water.…”

Designing Robust Hierarchically Textured Oleophobic Fabrics

“…This has been demonstrated both by direct visualization and finite element simulations of water on arrays of nonwetting cylinders. 38 However, upon decreasing the value of θ E , the meniscus descends into the texture until it transitions to resting on the individual fibers that comprise the warp (weft) yarn that lies underneath two adjacent weft (warp) yarns (see Figure 1c). Therefore, the eventual wetting transition to the fully wetted Wenzel state is determined by the smaller length (n = 2, fibers) on heirarchical structures such as multifilament woven fabrics.…”

“…Therefore, the eventual wetting transition to the fully wetted Wenzel state is determined by the smaller length (n = 2, fibers) on heirarchical structures such as multifilament woven fabrics. The critical (or breakthrough) pressure difference (∆P b ) across the liquid-air interface at which the drop irreversibly transitions to the fully-wetted Wenzel state at the largest defect site can be expressed for an array of parallel cylinders as: 37,38 …”