|MadSci Network: Physics|
When you are outside on a cold day, there are two main methods of losing heat. The first is conduction, where heat is transferred from a higher temperature to a lower temperature by physical contact with a solid, liquid, or gas. The second mechanism of heat loss is by convection. Convection is the process of transferring heat to a moving fluid, whether the fluid is liquid or gas. Conductive heat transfer depends upon the thermal conductivity of the material that you are in contact with. For example, steel is a pretty good conductor (certainly compared with air). So, just imagine how you would feel if you sat on a steel chair while not wearing any clothes. Even if the chair is at room temperature, it still feels cold because it transfers more heat away from you than air. Because steel is such a good conductor, if you touch your tongue against a steel pipe when the temperature is low enough, the moisture on the surface of your tongue will freeze against the steel. Air, on the other hand, is a pretty poor thermal conductor. So most of the clothes we wear to keep warm are designed to keep air trapped against our bodies in order to take advantage of the insulating properties of air. More trapped air keeps you warmer; less, cooler. Convective heat transfer is involved in the process we call windchill. In convective heat transfer, let's say that there is a little bit of air around your body that you have heated up. Because of movement of the air, that warm air is removed from being close to your body, replaced instead with air that has yet to be warmed. So you can see that the more often that warm air layer next to your body is replaced, the more heat is going to be lost from your body. Windchill comes from the idea that when you expose bare skin to temperatures below freezing, sooner or later you will get frostbite. If the air is still, there is not a lot of convective heat transfer taking place. If the wind is blowing, there is a lot more convective heat transfer, so your skin will freeze faster. There is a windchill chart at this link (http://www.nws.noaa.gov/om/windchill/index.shtml) which shows the relationship between temperature, windspeed, and windchill. It is pretty easy to understand a windchill graph. For example, in calm air, you will start to get frostbite after 30 minutes when the temperature is about -13F. In a 30 mile per hour wind, you will get frostbite in 30 minutes if the outside temperature is 5F. Your question is which material is the best for protecting against windchill. Since windchill is caused by the movement of air, replacing the warm air layer next to your skin with cold air, the best material for preventing windchill is a material that prevents outside air movement from disturbing the warm layer of air next to your skin. From that point of view, a material that doesn't let air move through it would work to prevent windchill. A plastic bag, for example, provides good protection from air movement. So, if you are in a survival situation, a plastic trashbag that you can put over your clothes may help you trap enough heat to help save your life. Space blankets are sheets of aluminized mylar that also prevent air movement through them keep you warm by limiting convective heat transfer. In your question, you suggest cotton, denim, and felt as three materials under investigation. Denim is one type of fabric made from cotton; the only thing different is how tightly the cotton is woven and how much fiber is included per square yard. Felt is often made from wool, although felting is a process, not a specific material. Yes, you can get cotton felt. So, let's concentrate on specific fibers rather than differences in the structure of the fabric. When designing clothes for cold weather wear, however, you also have to keep in mind that your body regulates it's heat by releasing moisture, which when if evaporates, produces localized cooling. Dealing with perspiration is a significant problem. Water, whether in the form of perspiration or whether it comes from rain or snow, has different effects on different fibers. Cotton, for example, adsorbs quite a bit of water (about 7%). As soon as cotton adsorbs moisture, it's thermal conductivity goes higher (since the air pockets within the cotton fiber are replaced with water pockets). You can read a lot about the structure and properties of cotton here: http://www.engr.utk.edu/mse/pages/Textiles/Cotton%20fibers.htm Wool has long been known as a fiber that retains much of it's insulating properties, even when wet. The surface of wool fibers consists of an outer layer of overlapping scales, called cuticles, which are water repellant. The interior of the wool fiber is similar to cotton so that it adsorbs moisture. Also wool fibers seem to have more natural bending than cotton. As best as I understand it, because the outerlayers of wool fibers are not hydrophillic or wetted by water, there is less tendency for water to be trapped between fibers. Instead, the gaps between fibers will tend to hold some air between them. Cotton, on the otherhand, has a hydrophillic surface, so water is trapped between fibers, which will push out the insulating air between fibers. Synthetic fibers, such as polypropylene, have a number of interesting features which are making them more popular for cold weather ware. Polypropylene does not adsorb moisture, it has low thermal conductivity (see http://www.engr.utk.edu/mse/pages/Textiles/Olefin%20fibers.htm , about half way down the page for a chart that shows thermal conductivity of different fibers). And you can get different surface treatments on polypropylene so that it can be either hydrophillic (loves water) or hydrophobic (hates water). So, in the process of trying to stay warm, you want a fabric that will prevent the movement of air from the outside in, yet you want a fabric that can handle perspiration that your body generates and resists moisture from the outside. The W. L. Gore company has made a science out of trying to balance the properties of fabrics for cold weather protection. For example, their GORE-TEX® Fabrics use a special membrane that has holes that are too small for air movement to penetrate, is naturally hydrophobic so water won't get through, yet have small enouth pores that water molecules can get through. The have other fabrics such as WINDSTOPPER® Fabrics that attempt to further reduce windchill, along with AIRVANTAGE™ Adjustable Insulation, where the wearer can vary the amount of trapped air retained within the structure, allowing you to change how much heat you retain. See http:\\www.gore-tex.com. 3M has also worked to improve the insulating properties of clothes by using much finer fibers than normal textile fibers. Trapping a large number of very small air pockets seems to make their THINSULATE® insulation work better than most other insulating fibers. You might want to check out: http://cms.3m.com/cms/US/en/2-147/crzuRFW/view.jhtml So, as you might guess, there is a lot of technology involved in keeping you warm on a cold, windy day. And the process is one of optimization, answering questions such as how heavy should the clothes be, how thick and bulky, how concerned should we be about water, and how much will a consumer pay for a technologically superior solution. I hope this gives you a bit of direction for your science fair project.
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