Sunday, January 17, 2010

Cling-Film Diaper

In our first demonstration of radiative cooling by a water film, we applied the water film to the top surface of a brass disk, which we heated from above with a lamp. The experiment requires a thin water film, or else the film will interfere with dissipation of heat by convection, thus slowing down the cooling effect of the film. In this second demonstration, we cover the bottom surface of the brass with a water film, and observe the radiative cooling more easily. We hold the water on the bottom surface of the disk with a diaper made out of kitchen cling-film.

We allow the disk to warm up beneath our lamp. It warms to 49.4°C. We inject water into the diaper. The disk cools to 42°C at first because the water we added was at 42°C. After that, the brass warms up again, reaching a new equilibrium temperature of 47.0°C. The new equilibrium is 2.4°C cooler than the equilibrium without the water film. For a more detailed description of the experiment, see here.

The diaper stops the evaporation of water. Water is a poor conductor compared to brass, so covering the bottom surface with water will only decrease the heat lost by convection and conduction. Water is transparent to the 1-μm infrared radiated by the lamp, but it is black to the 10μm infrared radiated by warm bodies. A black body radiates heat more efficiently than a shiny body. We cover the brass with water and it radiates more heat, so the disk cools down.

And so we obtain the same result again: water acts as a radiative cooler for warm objects that are heated by visible and near-infrared light.

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