Fragile, ephemeral, fascinating, soap bubbles and in particular giant soap bubbles are often used by artists to accompany their acts. Each artist has a recipe that allows him to optimize his number, to obtain bigger, more beautiful, more stable bubbles.
One of our research questions, worked on in collaboration with artists, is to better understand how to make big bubbles, which fixes their size and the interest of each of the ingredients used by the artists. We will be able to give you all the tips and tricks to create giant bubbles at home.
VIDEO : Making bubbles, a whole art, a whole science (Frédéric Restagno)
The first question is to understand what sets the size of soap bubbles. To form a bubble, you soak an object in a soap solution, which creates a film of soap when you remove the object. It will then be necessary to stretch the film, which requires thinning it while preventing the bubble from bursting. The size of the bubbles is basically set by the initial size of the soap film.
Children’s bubbles
Thus, children’s bubble toys make it possible to form a film of about two centimeters and the bubbles formed are a few centimeters in diameter. In fact, when you blow on the film, you make a cylindrical tube of soap. Instead of closing and forming a single large bubble, this tube destabilizes to form several small spherical bubbles, because of the so-called “Rayleigh-Plateau” instability : Creating soapy film surface costs energy, and many bubbles have a smaller total surface area than a long tube.
The soap bubbles made with the commercial children’s tanks have a limited size because the plastic circle is small © Giu Vicente / Unsplash
If we blow hard on a film of a given diameter, we will thus obtain bubbles twice as big than the diameter of the film. We recently showed that if we blew slowly on the film in reverse, we could make bubbles.
significantly larger (you must then blow a little sharp blow to detach the large bubble obtained).
How to make giant bubbles
But to make giant bubbles, no question of subtlety on the air flow sent to the film: the wind must be able to take care of it. We therefore simply manufacture the largest possible films, which will make it possible to manufacture bubbles of comparable size.
To make larger bubbles, you have to enlarge the size of the object on which the film initially rests © Maxime Bhm / Unsplash
In principle, the size of the bubbles therefore only depends on the size of the film formed and the flow rate with which it is blown on: the soapy solution used has very little importance. However, everyone knows that simply replacing the commercial solution contained in a toy with dishwashing liquid is not always effective.
To understand why the “bubble soap” recipe is so important, you have to look at the mechanisms that allow the thinning of the film when the bubble forms, and those that slow down this thinning.
A soap bubble is air surrounded by an extremely fine film of liquid, itself surrounded by air. This film initially has a thickness of a few micrometers and will tend to thin over time until it becomes extremely brittle, and shatters. When we form a giant bubble, we stretch the initial film very strongly, which tends to thin it.
Formation of a soap bubble. At the molecular level, the water-compatible part turns towards the inside of the film, while the more air-compatible part looks towards the outside of the film © authors / The Conversation
To prevent the film from bursting, soap is needed. At the molecular level, soap molecules are “amphiphilic” molecules : they include a part that is more energy compatible with water and another part that is more compatible with air. They are not quite comfortable in either water or air, and they tend to split at the interface between the two. The soap films thus have two interfaces with the air, each delimited by a layer of soap molecules which protects the film.
A little soap, but not too much
These molecules are very important in stabilizing soap films for several reasons. On the one hand, when the films become very thin, the molecules located on each of the air / liquid interfaces start to touch each other. They then repel each other, either because they cannot interpenetrate, or because they are positively or negatively charged and the identical charges repel each other.
On the other hand, these soapy molecules make it possible to resist the thinning of the film thanks to theMarangoni effect which is due to the presence of a little soapy molecules, but not too much. This is why the concentrations of dishwashing liquid used to blow giant bubbles are only a few percent.
Understanding the behavior of the film on a local scale to improve recipes: we are looking for a balance between molecules of water, soap, polymers, glycerol… © Lanju Fotografie / Unsplash
But soap molecules are not enough to make giant bubbles. One of the difficulties is that the soap film must be greatly lengthened. This step, which requires the film to be very strongly locally thin without breaking, depends very much on the soapy solution used.
Soap … and other molecules so as not to crack
For this, it is necessary to slow down the thinning by increasing the ” elongational viscosity “. Just as viscosity slows down the flow of liquids, so does elongation viscosity slow film elongation. A high elongational viscosity therefore makes it possible to lengthen the films strongly, but slowly, in order to prevent them from breaking.
This parameter can be increased by adding polymers to the solution. They are long molecules, made up of millions of small molecules all identical and attached to each other. Like spaghetti on an overturning plate, these molecules will tend to align themselves during a flow and to rub very strongly against each other, thus preventing thinning and too rapid breaking of the films.
It is for this reason that artists’ soap solutions contain lubricant containing a polymer with high elongation viscosity (polyethylene oxide) – for example a commercial veterinary lubricant.
Long live the bubbles
To have time to see them disappear on the horizon, or even to manipulate them for the most talented artists, the bubble must not explode immediately.
Giant bubbles survive better in humid weather, as this limits evaporation © Alfred Kenneally / Unsplash
This requires avoiding their thinning as much as possible, because the thinner the films, the more fragile they are. However, the bubbles get thinner over time because of two mechanisms. The first is drainage: due to gravity, the liquid in the thin liquid film tends to fall out. The question of the impact of the recipe on drainage is today a question of research which is still open.
The second mechanism that affects the longevity of the bubbles is theevaporation. Indeed, even at room temperature, any liquid evaporates. The film of the soap bubbles is so thin that this evaporation is very rapid.
To limit it, you can place yourself in a humid environment. That’s why Sylvain Létuvée, “bubbling artist”, is placed in the forest in the early morning to take advantage of the dew when he wants to beat his bubble size record.
Another solution is to play on the recipe by adding glycerol, a liquid soluble in water and which tends to absorb moisture (it is said to be ” hygroscopic “). Thus, a solution sufficiently concentrated in glycerol
no longer evaporates. This is why we often find glycerol in giant bubble recipes.
It’s your turn !
To make giant films, you need two large rigid rods between 1 and 3 meters long. Then, connect these two chopsticks with two pieces of string, one of about 1 meter and the other of about 2 meters. They will form a surface on which to make a large film. It is then necessary to tighten the chopsticks, soak the strings in soapy water, remove them and move the chopsticks away from each other to form a large film. Then step back, holding the chopsticks far in front of you to inflate the film and close the chopsticks to close the bubble… As for the recipe, we offer the following, with additional information available here, because the ingredients are not all equally effective .
Mix, in order:
- 20 milliliters of water
- 40 milliliters of dishwashing liquid
- 1 gram of lubricant (mixture of polyethylene oxide – PEO – and sucrose)
- 100 milliliters of glycerin
- 40 milliliters of water
You can keep this mixture. Add 800 milliliters of water before use to obtain 1 liter of solution!
This analysis was written by Marina Pasquet, doctoral student in physics, Anniina Salonen, teacher-researcher in soft matter physics and Emmanuelle Rio, teacher-researcher in physics (all three at the University of Paris-Saclay).
The original article was published on the website of The Conversation.
