Surface Tension Transport of Prey by Feeding Shorebirds: The Capillary Ratchet

Abstract

The variability of bird beak morphology reflects diverse foraging strategies. One such feeding mechanism in shorebirds involves surface tension-induced transport of prey in millimetric droplets: By repeatedly opening and closing its beak in a tweezering motion, the bird moves the drop from the tip of its beak to its mouth in a stepwise ratcheting fashion. We have analyzed the subtle physical mechanism responsible for drop transport and demonstrated experimentally that the beak geometry and the dynamics of tweezering may be tuned to optimize transport efficiency. We also highlight the critical dependence of the capillary ratchet on the beak's wetting properties, thus making clear the vulnerability of capillary feeders to surface pollutants.

Bioinspired Dynamic Wetting on Multiple Fibers

“…Bush and co-workers revealed the feeding mechanism of the shorebird, which included the cooperation of surface tension and the characteristic tweezering action of the beak. [13] When using an artificial smooth stainless-steel beak with a constant opening angle (3.4°), wetting droplets (silicone oil) could spontaneously propagate toward the narrower region, which indicated that the asymmetric Laplace pressure caused by the conical-shaped liquid channel could drive liquid motion (Figure 3b 2 ,b 3 ). Further research indicated that the dynamic beak morphology provided a non-negligible driving force for the drop motion in the capillary feeding of the shorebird.…”

“…b 1 ) Feeding of a shorebird. [13] b 2 ) Schematic of the shorebird beak with a trapped droplet. b 3 ) A drop of silicone oil self-propelled toward the apex of an artificial beak with an opening angle of 3.4°.…”

“…b 1 -b 3 ) Reproduced with permission. [13] www.advmat.de www.advancedsciencenews.com created by radially distributed fibers with periodic spindleknots. [21] When the membranes were placed in a mist environment, tiny droplets condensed, moved to the spindle-knots via the Laplace pressure difference on the spindle-knots on each fiber, and then, formed big drops, as shown in Figure 3b 4 ,b 5 .…”

“…[2] The cactus spine was recently reported to be capable of collecting and directionally transporting droplets, [3] and spider silk can efficiently harvest water from humid air. [4] The distinctive conical structure of the individual natural fiber www.advmat.de www.advancedsciencenews.com on water strider legs, [12] directional liquid transport inspired by the shorebird beak, [13] controllable liquid transfer inspired by the Chinese brush, [14] highly efficient liquid encapsulation inspired by dandelion pappi, [15] and capillary-induced fiber self-assembly inspired by wet hairs. [16] We focus on multiple fibrous systems with different spatial distributions, which are more complicated and challenging but offer more possibilities for practical applications.…”

External‐Field‐Induced Gradient Wetting for Controllable Liquid Transport: From Movement on the Surface to Penetration into the Surface

“…[9][10][11][12][13][14][15][16] The wettability gradient of a surface for a liquid droplet with an asymmetrical contact angle (CA) can produce a driving force for liquid motion, which is generally generated by introducing a chemical or structure gradient. [17][18][19][20][21][22][23][24][25][26][27][28] External-field-responsive liquid transport has received extensive research interest owing to its important applications in microfluidic devices, biological medical, liquid printing, separation, and so forth. To realize different levels of liquid transport on surfaces, the balance of the dynamic competing processes of gradient wetting and dewetting should be controlled to achieve good directionality, confined range, and selectivity of liquid wetting.…”