The soap in the bottom is colder than the air around it.

Jan. 2 () –

Physicists have measured the temperature of the films that form soap bubbles to show that they can be up to 8°C cooler than ambient temperature.

These results obtained by scientists from the Laboratoire de Physique des Solides (CNRS / Paris-Saclay University) agree with a model that attributes this cooling to evaporation, published in the journal Physical Review Letters.

We have all experienced in our childhood the fragility of soap bubbles and other bubbles. This observation is under investigation because the mechanisms that cause its explosion are not yet fully understood. However, it is known that the reasons lie on the borders between physics and chemistry.

In the new study, the researchers used a mixture of dishwashing liquid, water, and glycerol, a nonvolatile solvent. This last compound makes it possible to limit the rate of evaporation of the mixture and, therefore, to control it. The soap film is formed on a filiform circular frame with a diameter between 4 and 12 mm. The specificity of this frame is that it is made up of two different metal wires, each of which forms a half periphery.

Known as the Seebeck effect, it is possible to measure the temperature of the soap film using an electrical voltage measurement. The temperatures measured in this way could be successfully compared with a model that combines the energy required by evaporation and heat transfer by diffusion and radiation. Thus, the lower the initial glycerol content of the films or the drier the atmosphere, the greater the cooling, according to a statement the laboratory where the study was carried out.

The temperature variations that were measured are significant enough to suggest possible induced effects, such as variations in viscosity, surface tension, or crystallization of certain soap molecules. It is conceivable that these induced effects play a role in the life of movies and bubbles, whose importance should be clarified in future research.