To unravel the mystery, Bush teamed up with Manu Prakash, a graduate student in MIT's Center for Bits and Atoms, and David Quere, of the Ecole Polytechnique in Paris, a visiting professor in MIT's math department at the time of the study. They built a mechanical model of the phalarope beak that allowed them to study the process in slow motion.
The process depends on a surface interaction known as contact angle hysteresis, typically an impediment to drop motion on solids. For example, raindrops stick to window panes due to contact angle hysteresis. In the case of the bird beak, the time-dependent beak geometry couples with contact angle hysteresis to propel the drop upward.
This may be the first known example where droplet motion is enabled rather than resisted by contact angle hysteresis, Bush said.
As the beak scissors open and shut, each movement propels the water droplet one step closer to the bird's mouth. Specifically, when the beak closes, the drop's leading edge proceeds toward the mouth, while the trailing edge stays put. When the beak opens, the leading edge stays in place while the trailing edge recedes toward the mouth.
In this stepwise ratcheting fashion, the drop travels along the beak at a speed of about 1 meter per second.
The efficiency of the process, which the authors dub the capillary ratchet, depends on the beak shape: Long, narrow beaks are best suited to this mode of feeding. The study highlights the sensitivity of this mechanism to the opening and closing angles of the beak: Varying these angles by a few degrees can change the drop speed by a factor of 10, Quere said.
The capillary ratchet also depends critically on the beak's wettability--a measure of a liquid's tendency to bead up into droplets or spread out to wet its surface. Oil is much more wetting than water, so if the beak is soaked in oil from a spill, this process won't work.
|Contact: Elizabeth Thomson|
Massachusetts Institute of Technology