Uncoiled Barbules Enhance Water Absorption in Sandgrouse Feathers

Hydration-induced uncoiling is essential for mechanical functions in various biological systems, from pinecones and seed pods to wheat awns. Among these, the ventral feathers of male sandgrouse possess helical barbules that enable rapid water absorption and long-term water retention. Each barbule consists of both helical and straight segments; the helical region uncoils upon wetting, while the straight section remains unaffected. Here, we explore how uncoiled barbules at different scales contribute to enhanced water absorption. At the micro-scale, dry barbules typically have 3-4 curls, which uncoil to form L-shaped structures with a single curl when wet. Comparative analysis highlights that this programmable morphing characteristic is unique to sandgrouse. At the millimeter scale, ventral feathers with helical barbules can absorb up to 15 g water/g dry feather, compared to only 5 g/g in dorsal feathers lacking these structures. Minimal changes in the feather's top-view area indicate that the uncoiled L-shaped barbules significantly increase the height and volume of absorbed water. At the centimeter scale, inertial-driven water column formation is observed in the underbelly region of sandgrouse during take-off, a phenomenon not seen in other birds. Finally, we aim to fabricate artificial barbules that replicate the programmable morphing characteristics observed in sandgrouse feathers.