Polar bear fur could hold secret to ultrathin insulation
By Grant Banks
February 2, 2014
Put a polar bear and a biophotonics expert together in a chilly room and what do you get? Potentially, better insulation. When looking to uncover the secrets of the impressive insulation properties of polar bear fur, researchers at the University of Namur in Belgium and the University of Hassan I in Morocco found that radiation plays a larger role than conduction in the insulation of polar animals, such as penguins and polar bears, than previously believed.
It was previously thought that feathers and fur work to keep polar animals such as polar bears and penguins warm by trapping a layer of air that slows thermal conduction and loss of body heat. Instead biophotonics scientist Priscilla Simonis and her team found that heat loss between two bodies separated by air was more affected by radiation than by conduction.
Using computer models, the researchers showed how radiative shields, used to mimic individual hairs in a polar bear's fur, effectively backscattered heat. Backscattering is the deflection of energy at an angle of up to 90 degrees from the source. This deflection was not only limited to the infrared spectrum, or heat, but also reflected visible light, helping in animal camouflage in the snowy conditions by making their coats look white (despite appearances, polar bear fur is actually pigment-free and transparent). By adding more shields (hairs) the loss of energy was greatly reduced.
The computer model involved a hot and cold thermostat, to replicate the animal’s body and the cold environment, separated by a pocket of air in which the radiative shields were added. In a control test, so-called black-body shields, which absorb all heat and light radiation, were placed in the space between the thermostats to see how much the air aided the transmission of temperature via conduction.
They were replaced with opaque grey-bodied shields in further tests to allow some transmission and also reflection. These grey-bodied shields were said to simulate the hairs or feathers of polar animals. What the researchers found was that by increasing the reflectivity of the grey-bodied shields or by adding more shields, the rate of heat transfer was dramatically reduced.
"This is particularly useful to animals, such as mammals and birds, that live in snowy areas," Simonis said.
This led Simonis to question why building insulation materials are so inefficient when a polar bear with only a couple inches of fur is able to insulate its body to temperatures of 98.6° F (37° C) when outside temperatures get as low as -40° F (-40° C)
"Why do we need at least 60 cm (24 in) of rockwool or glasswool to get a temperature of 20 degrees Celsius (68° F) inside from about -5 degrees Celsius (23° F) outside?" she asked. "Why is the polar bear fur much more efficient than what we can develop for our housing?"
She suggested the study may have a number of applications for humans and could lead to the development of new types of ultrathin insulation. This could not only find use in insulating buildings, but might also result in a new generation of snow clothing, sleeping bags and survival gear.
"The idea is to multiply the interaction of electromagnetic waves with grey bodies – reflecting bodies, like metals, with very low emissivity and no transparency – in a very thin material," Simonis said. "It can be done by either a multilayer or a kind of 'fur' optimized for that purpose."
The research was published in The Optical Society’s (OSA) open-access journal, Optics Express.
Source: The Optical Society