The necessity of cool down quickly beverageslike beer, is common during hot summer days or in the most varied social events. Among the various methods that are in circulation onlinea particularly interesting one suggests wrapping the bottle with a napkin Of wet paper and put it directly into the freezerWe made a experiment to verify the effectiveness of this technique. In this video-article, therefore, we see how we conducted the experiment and give a possible conceptual explanation of the phenomenon, taking into account that the phenomenon is much more complex than it may seem and the variables at play are many.
The method used and the results of our experiment
We used two bottles of beer initially at the same temperature (about 23 °C), as confirmed by the use of a room thermal. One of the bottles was wrapped with a wet paper towel, while the other was left without any wrapping. Both bottles were then placed in a freezer for a period of 30 minutes. During the experiment, a timer to accurately monitor the cooling time.

After 30 minutes the bottles were removed from the freezer and the temperature of the beers was measured using two identical thermometers. The beer wrapped with the wet napkin reached a temperature of 11.4 °Cwhile the one without coating has cooled down to 13.7 °C. This result indicates a cooling down more effective for the bottle with the napkin wet.

A possible scientific explanation for the different cooling rates
A possible conceptual explanation of the phenomenon can be attributed to two main factors: the contact surface and the capacity Of transmission of heat from the material part (which in jargon is called conductivity thermal). The speed of heat transfer between two bodies, in fact, also depends on the contact surface: a larger and more irregular surface allows for greater heat transfer. In the case of the experiment, the wet napkin considerably increases the contact surface between the bottle and the cold air of the freezer, facilitating a greater quick transfer of heat compared to the smooth surface of bare glass.
As we were saying, another important factor is the material itself and its thermal conductivity. In general, Water is less efficient than glass at exchanging heat with other materials. So we have two opposite effects that occur simultaneously: on the one hand, the increase in the heat exchange surface causes a faster cooling, on the other hand, the use of water slows down the cooling. Depending on the specific conditions in which the experiment is carried out, one effect can dominate over the other. The result we obtained suggests that in our case the predominant factor would be that related to the contact surface.
If we want to go into more detail, the law that describes the speed with which heat is transferred by thermal conduction (i.e. by direct contact and without movement) is the Fourier’s law. This states that the heat transfer rate (let’s call her R) through a material – regardless of the direction of heat propagation – is directly proportional to the thermal conductivity of the material (k), to the area of the contact surface (TO), at the temperature difference between one side of the material and the other (ΔT) and it’s inversely proportional to the thickness of the material (d) along which heat is transferred:
R = k ⋅ TO ⋅ ΔT/d
In our case we do not expect any effects related to the temperature difference and the thickness, since they are the same in both bottles. One way to interpret our result is that the wet napkin increases the area TO more than the conductivity decreases kthus increasing the rate of heat transfer R.
In conclusion we can say that our experiment has obtained data that suggest that the use of a wet napkin can actually speed up the cooling process of beer. However, it is important to consider that variables such as the initial temperature of the beers, the power of the freezer and the size of the bottles can to influence the results.