Penguin Tales

Gentoo penguins — residents of the Antarctic and parts of the coldest, southernmost tip of South America — jump in and out of frigid ocean water all day without accumulating ice on their feathers. Researchers say they now know why the penguins are ice-free, and they theorize that this knowledge could lead to better ways of keeping ice off airplane wings.

The explanation involves both the structure of the penguins’ feathers and the characteristics of an oil secreted by their glands.

Pirouz Kavehpour, a professor of mechanical and aerospace engineering at the University of California, Los Angeles, said the research was inspired by his thoughts while watching a documentary film on penguins.

“I noticed the penguins were coming out of very cold water and sitting in very cold temperatures, and it was curious that no ice formed on their feathers,” Kavehpour said in a statement issued by the American Physical Society (APS) in late November in advance of the annual meeting of its Division of Fluid Dynamics. Kavehpour and several colleagues, including Judy St. Leger, corporate veterinarian for pathology and research for SeaWorld Parks and Entertainment and an expert on penguins, presented their findings during the session.

“Antarctic penguins reside in a harsh environment where air temperature may reach minus 40 degrees C [minus 40 degrees F], with wind speed of 40 meters per second [about 90 mph], and water temperature remains around minus 2.2 degrees C [28 degrees F],” the researchers said in papers accompanying their presentation. “Penguins are constantly in and out of the water and splashed by waves, yet even in sub-freezing conditions, the formation of macroscopic ice is not observed on their feathers.”^1,2

The scientists already knew that the feathers of many bird species, including penguins, repel water. Penguin feathers have a hydrophobic (water-repelling) coating to “reinforce their non-wetting properties,” as well as their resistance to ice buildup, they said.

Their research centered on the examination — “in extreme detail,” under a scanning electron microscope — of penguin feathers donated by San Diego SeaWorld.

They found that the feathers contained “tiny pores that trap air,” thereby making the surface hydrophobic. In addition, a gland near the base of the penguins’ tails produces preen oil, which the birds apply to their feathers. Together, the tiny pores and the preen oil make the penguins’ feathers superhydrophobic.

When a surface is superhydrophobic, droplets of water “bead up and sit on the surface, almost like spheres,” the APS statement said, adding that the researchers believe that “it’s the sphere-like geometry that delays ice formation, since heat has a hard time flowing out of the water droplet if the droplet does not make much contact with the surface.”

This means that water flows off the surface before it freezes.

“Heat flow could be compared to traffic,” Kavehpour said. “If you have a freeway that turns into a tiny, two-lane road, the traffic will back up. Similarly, heat does not flow well from the large cross-section of the middle of the [water] drop to the small cross-section where the drop makes contact with the feather.”

The research showed that feathers of some species of penguins were better than others at repelling water and inhibiting ice formation.

The researchers focused on gentoo penguins, which live in Antarctica and extreme southern points of South America, and discovered that they were more water repellent than those of warm-water species such as the Magellanic penguins living farther north in Chile, Argentina and Brazil. The warm-water species do not have the same small pores in their feathers, and they produce a less hydrophobic type of preen oil.

Further research into the anti-icing properties of penguin feathers could yield results of practical interest to the aviation industry in preventing airframe icing and associated accidents, Kavehpour said.

A 2010 report by the U.S. Government Accountability Office (GAO) cited data from 1998 through 2009 that showed that no fatal accidents involving large commercial airplanes in the United States had been attributed to airframe icing during that period.³

However, six nonfatal accidents and numerous incidents were blamed on icing — both in flight and on the runway — during that period (see “Icing Incident Reports”).

“Although large commercial airplanes have experienced few icing-related accidents in the last decade,” the report said, “the several hundred icing-related incidents involving these airplanes contained in [U.S. Federal Aviation Administration and U.S. National Aeronautics and Space Administration] databases suggest that they face ongoing risks from icing.”

Methods of dealing with those risks during flight typically involve the use of pneumatic deicers on the leading edges of the wings and tail, a “weeping wing” that pumps anti-icing fluid to the wings and tail, or thermal systems that electronically heat pitot/static tubes, propeller blades and other areas to eliminate existing ice and prevent new buildup.

On the ground, the primary protection is anti-icing or deicing fluid, which is applied to aircraft surfaces before takeoff. Some operators also rely in part on other methods, including:

• Infrared heat deicing — in use at only a few airports — which requires aircraft to be positioned inside a specially equipped hangar where infrared waves of electromagnetic energy (radiation) are used to quickly melt any frost, ice or snow on the airframe; and,
• Forced air deicing, which uses high-^­pressure air to blow snow and ice off aircraft surfaces.

Both infrared heat and forced air deicing are sometimes used in combination with the application of deicing or anti-icing fluids.

“Airlines spend lots of time and money applying chemical deicers to planes that fly in winter weather,” Kavehpour said.

He said that superhydrophobic surfaces “inspired by penguins” might ultimately be less expensive, longer lasting and friendlier to the environment than methods that rely on deicing or anti-icing fluids.

“It’s a little ironic,” he added, “that a bird that doesn’t fly could one day help airplanes fly more safely.”

Notes

1. Alizadeh-Birjandi, Elaheh; Tavakoli-Dastjerdi, Faryar; St. Leger, Judy; Davis, Stephen H.; Rothstein, Jonathan P.; Kavehpour, H. Pirouz. Ice Formation Delay on Penguin Feathers. Abstract submitted for the DFD15 meeting of the APS. Aug. 2, 2015.
2. APS. The Anti-Icing Tricks of Penguins.
3. GAO. GAO-10-678, Aviation Safety: Improved Planning Could Help FAA Address Challenges Related to Winter Weather Operations. July 2010.