Fluid dynamics: The strangeness of the soap film
The wafer-thin film that surrounds a soap bubble is sometimes several degrees cooler than the surrounding air. This can have an effect on the stability of the bubbles.
Bubbles are everywhere. In the saliva between our teeth, in the foam on our beer, in bath water, in the sea and as party toys. They fascinate children and adults alike - and even researchers are still puzzling over some unanswered questions. Why are some bubbles more resistant and burst later than others? Researchers at the Université Paris-Saclay in France have analysed the thin skin that surrounds soap bubbles and made a fascinating discovery: In some cases, it is up to eight degrees cooler than the surrounding air.
This is apparently due to the fact that soap films release energy when they lose liquid through evaporation - in much the same way as our bodies cool down when they release energy into the environment through sweat. Previous studies have already shown that the evaporation of liquid films can trigger temperature changes and therefore play an important role in stability. "To our knowledge, however, the importance of evaporative cooling has not yet been explicitly mentioned in the literature on soap films and foams," the researchers write in the "Physical Review Letters".
In their experiments, François Boulogne and his team created soap bubbles from a mixture of washing-up liquid, water and glycerine. They then measured the temperature of the soap film under different environmental conditions. The experts found that the film was up to eight degrees colder than the ambient air in some cases. They also found that the glycerine content of the soap film influenced this temperature difference, with layers with a higher glycerine content exhibiting higher temperatures.
"We observed experimentally that the temperature initially decreases and then increases again until the ambient temperature is reached again," the researchers write. "The extent of the cooling effect depends on both the relative humidity and the initial glycerol concentration, with a reduction in the values of these two parameters leading to stronger effects."
The team then created a theoretical model that describes the temperature drop of soap films from the moment they form. "It represents a first approximation of the problem," write the physicists. "More sophisticated theoretical developments, including in particular the unstable state, the role of natural convection for large-scale films and forced convection, will be the subject of future work." They believe the result could help bubble manufacturers to better control and even increase the stability of the bubbles.
Spectrum of Science
We are a partner of Spektrum der Wissenschaft and want to make well-founded information more accessible to you. Follow Spektrum der Wissenschaft if you like the articles.
[[small:]]
Cover image: Shutterstock, Tadija Savic
Experts from science and research report on the latest findings in their fields – competent, authentic and comprehensible.