Physicists have worked out the optimal recipe for making giant, long-lasting soap bubbles. It should allow anyone to reliably blow bubbles at least 12 centimetres across and potentially create some large enough to envelop a person.
, who carried out the research with her colleagues at the University of Paris-Saclay, started the project by investigating what liquids artists and performers were using to make giant soap bubbles. 鈥淭here are a lot of recipes out there,鈥� says Pasquet, who is now at the University of California, Berkeley.
These bubbles are essentially packages of air surrounded by an iridescent liquid film. A phenomenon known as the , which influences the flow of molecules on the bubble鈥檚 surface, helps keep their surface intact.
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Typically, mixtures for blowing bubbles are mainly water, with a small amount of soapy detergent, like those used to wash dishes. These act as surfactants 鈥� chemicals that lower surface tension 鈥� allowing the Marangoni effect to keep the bubble surface from popping, the team found.
The amount of detergent has to strike a balance: with more detergent, the bubbles become easier to blow, but their lifespan also drops, says Pasquet.
That is probably all that most people would add at home, but the researchers found that two extra ingredients can lift your bubbles to another level. Chemicals called long aqueous polymers increase a characteristic known as elongational viscosity, keeping the film of the bubble stable as you blow it, says Pasquet. Common examples include J-Lube 鈥� an obstetrical lubricant used by vets 鈥� and the food additives guar gum and xanthan gum.
The final key ingredient is a little glycerol, also known as glycerine, which helps the ingredients mix well so the solution consistently forms bubbles. The glycerol can also slow evaporation of the water in the bubble鈥檚 film, which allows it to last for longer, says the team.
Some bubble artists also add ingredients such as citric acid, baking powder or yeast to alter the pH of the solution, but Pasquet鈥檚 team found that this didn鈥檛 help generate bubbles if you have the other ingredients.
The team tested about 20 promising bubble mixes and timed how long 50 bubbles from each lasted, blowing about 1000 large bubbles. 鈥淲e tried using our mouths to blow the bubbles at first, but as you blow, you change the flow rate,鈥� says Pasquet. So they used a pressure controller to blow air at a constant velocity through a bubble wand instead.
The team says the ideal recipe is:
- 85.9 per cent water (or 85.95 if using J-Lube)
- 4 per cent washing-up liquid (Pasquet鈥檚 team used the Fairy brand in their tests, but others should also work)
- 0.1 per cent guar gum (or 0.05 per cent J-Lube)
- 10 per cent glycerol
This mix enabled the team to reliably create big, long-lasting bubbles. One bubble about 12 centimetres across lasted for 24 hours in the lab, says Pasquet. Using a very similar mix, the team made a bubble large enough to completely envelop her.
The recipe could prove fun for children鈥檚 parties, but a reliable and standardised recipe would also be useful in physics labs for studying turbulence and surface waves in bubbles, says Pasquet.
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