Scientific Method / Science & Exploration

Shorebirds use surface tension for easier eating

Some classes of shorebirds open and close their beaks rapidly to use capillary …

A Phalarope

A Phalarope feeding Image: Mike Baird

Have you ever wondered just what a bird was chewing on when you saw it incessantly opening and closing its beak? Well some shorebirds, including the Phalarope pictured here, are using surface tension to pull food into their mouths. Unlike rose petals, which use surface tension to create a superhydrophobic surface, these birds rely another property that results from surface tension: the capillary effect. A recent article in Science showed how birds can use the capillary effect to overcome both gravity and hysteresis.

Capillarity results from water having an affinity for the surface it is in contact with. Essentially, water bonds more readily to the surface than it does to other water molecules. By moving into a smaller diameter tube, the contact area between water and the material is increased, leading to a lower energy state.

A drop of fluid can move up the length of a bird's beak by capillary action. The "tube" formed by the beak has a larger diameter closer to the tip. The researchers showed that, with a nearly perfectly wetting fluid (silicone oil), the fluid would move freely along a mechanical beak they had created for testing purposes. However, when they used water, hysteresis prevented the water from moving along the beak.

Hysteresis refers to the fact that a process behaves differently depending on its history. Water behaves differently if it is moving onto or off of a material. This is immediately evident if you watch a drop of water run down a plate of glass. The bottom end of the drop forms a high contact angle with the plate of glass, while the top of the drop forms a very low contact angle, even when the drop is no longer moving. As a result of this change in contact angle, hysteresis tends to prevent water from flowing onto a new surface. For a bird trying to get a drop of water into its mouth, hysteresis prevents the water droplet from moving.

By opening and closing its beak, the bird forces the water drop to move. When the bird is closing its beak, it forces the water to wet the surface in both directions. Because both sides are wetting, hysteresis is acting the same on both sides of the drop. As a result, the only preferential force is the capillary effect mentioned previously. While the water moves in both directions, on average, it moves further from the tip of the beak. Finally, when the bird opens its beak, both sides are drying. Again, hysteresis acts the same on both ends of the water drop, while the capillary effect draws the water away from the beak tip. The researchers refer to this process as a capillary ratchet.

When the researchers mimicked the opening and closing beak with their mechanical one, they found that water flowed away from the tip. By filming their mechanical beak with a high speed camera, they were able to show that water moves in both directions almost evenly when the beak closes, but it exhibits a highly preferential flow away from the beak tip when the beak opens. Movies are available from Science.

The researchers also point out that changing the surface tension of water (e.g. with petroleum or detergent pollutants) will decrease the efficiency of the capillary ratchet, or prevent it entirely. On a final note, researchers are studying analogous processes in microfluidic systems.

Science, 2008. DOI: 10.1126/science.1156023
Movies are now available.

Expand full story

You must to comment.