In summer, we enjoy walking barefoot on the tiles at home, feeling a cool breeze rising from our feet. In winter, we love snuggling into warm beds, even wearing thick cotton slippers on our feet after getting up. Although well-insulated homes have minimal temperature differences between winter and summer, why do we always feel that tiles are colder?
It turns out that, just like water and electricity flows, heat also flows in materials—from high-temperature areas to low-temperature areas. When humans perceive the warmth or coldness of objects, they rely on the heat dissipation at the point of contact with the object. For example, ice feels very cold, while hot water burns our hands. When humans come into contact with objects, heat is transferred through thermal conduction, with the amount of heat transfer depending on three factors: the temperature difference between the human body and the object, the contact area between them, and the object's thermal conductivity.
Thermal conductivity refers to a material's ability to conduct heat. The higher the thermal conductivity, the stronger the heat conduction, and vice versa. Generally, metal materials have the highest thermal conductivity, followed by liquids, and gases the least. For example, at 20°C, the thermal conductivity of pure copper is 398 W/(m·K), while that of water is only 0.599 W/(m·K).
When we stand barefoot on tiles and cotton quilts at the same temperature, the temperature difference and contact area are the same, but the thermal conductivity of tiles is about 22 times that of cotton quilts. Therefore, the heat loss from our feet on tiles is about 22 times that on cotton quilts. Tiles dissipate heat from our skin much faster than cotton quilts, making standing on tiles feel colder.
Furthermore, metals contain many freely moving electrons, which not only have high electrical conductivity but also high thermal conductivity, as electrons colliding with metal ions can quickly transfer heat. Insulators have very few free electrons, so they are generally good at insulating heat. Animal fur and human clothing are poor conductors of heat, and thermal underwear is often coated with a thin heat-melting film or other special insulating materials. However, the hardest material in nature, diamond, is also an insulator but has high thermal conductivity. This is because its carbon atoms are arranged very tightly, allowing rapid transfer of thermal vibrations. In comparison, the transfer of thermal vibrations in other insulators is much slower, and the number of electrons involved in heat conduction is also very small, resulting in much lower thermal conductivity.
Thermal conductivity is greatly affected by temperature. The thermal conductivity of pure metals and most liquids (except water) decreases with increasing temperature, while that of non-metals and gases increases with increasing temperature. Therefore, most insulating materials are made of plastics, rubber, glass, and ceramics. Asphalt shingles on roofs are used to block the heat from direct sunlight to prevent overheating. Some thermal insulation boards and insulating films also consider fire resistance to prevent fires. Polytetrafluoroethylene (PTFE) materials have the advantages of good insulation, poor thermal conductivity, high and low temperature resistance, and corrosion resistance. The blue "bubbles" on the Water Cube were made from it.