When you stop and think about bubbles, you realize they’re everywhere: in the dishwasher, on the top of your beer, on the crests of the waves, in the saliva between your teeth and, of course, in bubble guns.
This means that bubble physics are important in all sorts of scenarios. With that in mind, researchers from the University of Paris-Saclay in France have made an intriguing discovery about the film surrounding the bubbles.
This film can, in some cases, be up to 8°C (14.4°F) cooler than the environment around it, according to the researchers. The findings build on previous investigations of how temperature changes can trigger thinning and evaporation of a liquid film.
“Although this effect is often taken into account in studies devoted to the evaporation of drops, to our knowledge, the importance of evaporation induced by cooling is not mentioned in the literature on films and foams. of soap,” the researchers write in their published paper.
To take a closer look at these films and soap suds – bubbles, essentially – the team developed a mixture of washing-up liquid, water and glycerol, with variations in the final substance used to fine-tune the duration. of life and the rate of evaporation of the bubbles.
These bubbles have been tested under different temperature and humidity conditions. In some cases, the difference between the soap film and the ambient air was significant – reaching a maximum at this level of 8°C.
While it was already known that soap films lose liquid through evaporation in an effort to release energy (just as we do when we sweat to cool ourselves), the temperature of these films was assumed to correspond to the surrounding environment.
“Experimentally, we observed that the temperature first decreases and then increases until the ambient temperature is reached again,” the researchers write.
“We reported that the magnitude of the cooling effect depends on both relative humidity and initial glycerol concentration, decreasing the values of these two parameters leading to stronger effects.”
One of the ways this research could be useful is in industrial processes where bubble stability management is vital. Temperature variations between the bubble wrap and the outside world will affect these calculations.
The researchers say the viscosity and surface tension of soap films are two of the properties that are likely to be influenced by the temperature differential they found; this is because soapy objects may not have a uniform thermal field all around.
This is the first study of its kind, however, and much more research is needed before scientists can tell how much the temperature of the film that makes up a bubble can be affected.
“We propose a model describing the temperature drop of soap films after their formation which is in quantitative agreement with our experiments,” the researchers write.
“We emphasize that this cooling effect is important and should be carefully considered in future studies of soap film dynamics.”
The research has been published in Physical examination letters.