Insulation through imitation

Sarina Tracy, sarina.tracy@photonics.com

When temperatures drop, we humans do our best Arctic animal impressions: We don the puffiest coats we can find, insulate our homes with thick mineral wool – even migrate. But we aren’t wild animals, and our bodies cannot insulate themselves enough to keep out the chill, so thousands of dollars are spent to heat our homes every winter. Maybe if we imitated the polar bear after actually studying its biological insulation, we might save a little money and energy.

The 1000-pound Arctic animal can insulate its body to 37 ºC (98.6 ºF), even when the temperature plummets to –40 ºC. Compared with the 2-in.-thick fur coat of a polar bear, a couple of feet of man-made building insulation might as well be tissue paper.

“Why do we need at least 60 cm of rockwool or glasswool [building insulation] to get a temperature of 20 ºC inside from about negative 5 ºC outside?” said Dr. Priscilla Simonis, a researcher at the University of Namur in Belgium. “Why is the polar bear fur much more efficient than what we can develop for our housing?”

To answer this question, she and her team re-examined two ways heat can travel: radiation, which transfers thermal energy through electromagnetic waves, and conduction, which transfers thermal energy through the vibrations of neighboring atoms and molecules. Most people assume that fur and feathers keep animals warm by trapping a layer of air that slows thermal conduction, she said. However, initial calculations showed that radiation dominated the heat loss between two bodies separated by air.

In a computer model, the team placed radiative shields to represent individual hairs in a fur coat between a hot and a cold thermostat, meant to simulate an animal’s warm body and a cold environment. In one version of the model, the researchers incorporated blackbody shields that absorbed the radiation that strikes them. In a second version, opaque gray-body shields were used to allow transmission and reflection. As the reflectivity of the radiative shields increased, the rate of heat transfer between the hot and cold thermostat was reduced. Adding more shields dramatically decreased the loss of energy.

This finding suggests that the repeated reflected backscattering of IR light between radiative shields – or, say, individual hairs – could be the primary mechanism for fur’s thermal insulation properties.

Polar bear fur contains several sizes of hair with a large density of interfaces, producing the scattering needed to retrodiffuse heat. This control of thermal radiation seems to be the dominant factor for biological adaptation of controlled heat exchange in mammals and birds living in cold environments, Simonis said.

This is true of feathers, too. A white peacock feather, for example, appears as a flat, bulky blade with longitudinal segmentation and long appendages. This adds to the bird’s ability to diffuse visible and far-IR radiation. Its feathers appear a bright white color because of its barbs and lack of melanin. A dense fleece of feathers with segmented barbules close to the skin provides effective thermal insulation.

By focusing on animal biology to minimize radiative heat loss, humans could develop new, more efficient types of ultrathin insulation. A stack of thin metal layers separated by thin homogeneous or fibrous spacing layers, for example, might provide enough reflection to outperform existing insulating systems.

“The idea is to multiply the interaction of electromagnetic waves with gray 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.”