French painters inspire new insights into the physics of soap bubbles

French painters Jean Simeon Chardin and Edouard Manet both created well-known paintings that depicted children blowing bubbles through straw-like tubes, albeit more than a century apart. Those realistic depictions of bubble dynamics have now inspired two physicists at the Universite Grenoble Alpes in France, who conducted their own soap bubble experiments to learn more about the early formation stages of bubble dynamics. They describe their experimental results in a forthcoming paper to be published on May 22 in the journal Physical Review Fluids.

Bubbles may seem frivolous, but there is some complex underlying physics, and their study has long been serious science. In the 1800s, Belgian physicist Joseph Plateau outlined four basic laws of surface tension that determine the structure of soapy films. Surface tension is why bubbles are round; that shape has the least surface area for a given volume, so it requires the least energy to maintain. Over time, that shape will look more like a soccer ball than a perfect sphere as gravity pulls the liquid downward ("coarsening").

More recently, French physicists in 2016 worked out a theoretical model for the exact mechanism for how soap bubbles form when jets of air hit a soapy film. The researchers found that bubbles only formed above a certain speed, which depends on the width of the jet of air. In 2018, we reported how mathematicians at New York University's Applied Math Lab had fine-tuned the method for blowing the perfect bubble based on a series of experiments with thin, soapy films. In 2019, physicists at MIT and Princeton University demonstrated how to develop spherical bubbles uniformly by confining them in a narrow tube. Something about the interactions between the walls of the tube and the bubble makes the whole system less sensitive to irregularities in the initial conditions.

In 2020, physicists determined that a key ingredient for creating gigantic bubbles is mixing in polymers of varying strand lengths. That produces a soap film able to stretch sufficiently thin to make a giant bubble without breaking. Varying the length of the polymer strands resulted in a sturdier soap film. Last year, French physicists created "everlasting bubbles" from plastic particles, glycerol, and water. The longest-lasting bubble survived for 465 days.

Ideally, scientists would like to be able to form bubbles with a uniform size and shape, like the liquid droplets that form from a dripping faucet. That could help control the formation of drops and bubbles in applications such as microfluidics, inkjet printing, or medical imaging. Droplets and bubbles start with the flowing water or air elongating into a neck, then pinching off from the main flow to collapse into spheres. It's what happens every time you dip a bubble-blowing wand into the bubble solution and gently blow through the film that forms along the ring.

But droplet and bubble formation are inverse physical processes. Droplets forming off a dripping faucet is a universal process from a physics standpoint. That means they will be fairly uniform in size and spacing, even if there are differences in initial conditions, such as the viscosity of the water, surface tension, or in the size of the faucet opening. However, mix air or gas into a large vat of liquid (like injecting air into a Jacuzzi tub), and bubbles will form in a more random, scattershot fashion.

Marc Grosjean and Elise Lorenceau, co-authors of this latest paper, wanted to learn more about the factors underlying bubble formation, particularly in the less-studied early stages. They were particularly intrigued by bubbles that linger when attached to a straw-like tube instead of pinching off, as depicted in paintings by Chardin and Manet. The latter's Boy Blowing Bubbles (1867) depicts a boy--modeled by the illegitimate son of Manet's future wife, who appears in several other works by the artist--blowing soap bubbles, at the time a common symbol for the ephemeral nature of life. Among Manet's influences was Chardin's Soap Bubbles paintings; there are three versions painted between 1733 and 1734. There is also a small boy blowing bubbles in Chardin's The Washerwoman.