January 24, 2014
Unexpected Optical Mechanism Behind Insulating Power Of Polar Bear Fur
[ Watch the Video: Looking To Polar Bears For Insulation Tips ]
Brett Smith for redOrbit.com - Your Universe Online
In work which has major connotations for enhancing the performance of artificial insulation, a team of international researchers has determined hairs and feathers that reflect infrared light might contribute considerable insulating capacity to the remarkably warm winter coats of polar bears and other Arctic animals.
Published by the journal Optics Express, the study was partially motivated by observations of polar bears to maintaining their bodies at temperatures over 98 degrees F, even during very long, cold winter months when outdoor temperatures can reach a frigid -40 degrees F, particularly impressive provided that the bears have a covering of hair that is only 2 inches thick.)
“Why do we need to have at least 60 cm of rockwool or glasswool to obtain a temperature of (68 degrees F) inside from about (23 degrees F) outdoors?” asked study author Priscilla Simonis, a researcher at the University of Namur. “Why is the polar bear hair much more successful than what we could develop for the housing?”
The study team looked at how heat moves thermal energy via electromagnetic waves, also referred to as radiation. Additionally, they considered how thermal energy is moved via the vibrations of nearby molecules and atoms – a principle known as conduction. While feathers and fur keep animals warm by trapping a layer of warm air below, the study team suspected that radiation might play a bigger role. The scientists performed some initial calculations that showed heat loss between two bodies separated by air would be mostly by radiation, not conduction.
To further explore the heat loss from radiation, the team created a simple computer model consisting of a hot and a cold thermostat that roughly simulated an animal’s warm body and the outside, colder environment. The two thermostats were separated by an empty space into which were added “radiative shields” that could mimic the individual hairs in a fur coat.
In one edition of the model, the researchers incorporated so-called black-body shields, which absorb all of the radiation that strikes them. In a second version, solid grey-body shields were used.
“A grey body has some reflection and transmission as well,” Simonis said.
The researchers established that as the reflectivity of the radiative shields increased, the rate of heat transfer between the hot and cold thermostat was markedly lowered. Inserting more shields also significantly reduced the energy loss. The model indicated that the continued backscattering of infrared light between the shields, acting like individual hairs and feathers, could be the chief mechanism for the thermal insulation advantage of fur and feathers.
The light scattering features of animals’ coats also have another purpose, Simonis said. These coats can generate capable thermal insulation in the far infrared range while also giving a white appearance in the visible wavelength range.
“This is particularly useful to animals, such as mammals and birds, that live in snowy areas,” Simonis said, noting that the bears’ fur provides both warmth and camouflage against the white snow.
For our purposes, looking at ways to reduce radiative heat loss could lead to new types of insulation.
“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.”