Soap films are cool!

Evaporation is known to cause cooling, much like perspiration. Physicists have measured the temperature of soap films to show that they can be up to 8°C cooler than the temperature of the environment.

We have all experienced in our childhood the fragility of soap films and other bubbles. This observation is the subject of research because the mechanisms causing the bursting are not yet fully understood. However, we do know that the reasons are lying at the frontiers between physics and chemistry. On the one hand, film thinning is a crucial parameter in the rupture of soapy objects, which is a matter of fluid mechanics, and on the other hand, the interfaces are held together by surface-active molecules whose properties are also important. In order to make progress on these issues, physicists from the Laboratoire de Physique des Solides (CNRS / Univ. Paris-Saclay) were interested in the temperature of soap films to show that a cooling up to 8°C can be measured. These results are in agreement with a model attributing this cooling to evaporation, which has been published in the journal Physical Review Letters.

In this study, the researchers used a mixture of dishwashing liquid, water and glycerol, a non-volatile solvent. This last compound allows to limit the evaporation rate of the mixture and thus to control it. The soap film is formed on a circular filiform frame with a diameter between 4 and 12 mm. The specificity of this frame is that it is made of two different metal wires, each forming 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 combining 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. 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 on the life of the films and bubbles, the importance of which will have to be clarified by future research.

Article in Phys Rev Lett and on arxiv

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