Focus: Physics of Blowing Bubbles
Blowing soap bubbles is child’s play, but surprisingly, physicists haven’t worked out the details of the phenomenon. Now researchers have performed experiments and developed a complete theory of the process of soap bubble formation. The team aimed a jet of gas at a soap film and observed that bubbles appear only above a threshold gas speed. By measuring this threshold under varying conditions, the team showed that bubbles result from a competition between the pressure of the gas jet and the surface tension of the soap film. Understanding the physics of bubbles is important for a variety of industrial processes and scientific fields, from cosmology to foam science, and the new experiments may also be useful in the classroom.
Researchers have studied related processes, such as the popping of bubbles, and examined soap films being pierced by pellets or liquid droplets. But bubble blowing has mostly been overlooked, say Laurent Courbin and Pascal Panizza, both of the French National Centre for Scientific Research (CNRS) and the University of Rennes 1. While watching children blowing bubbles in a local park, they realized that the phenomenon hadn't been studied before and hurried back to the lab to tinker with soapsuds. Following the example of previous soap film research on fluid flows and turbulence, Courbin, Panizza, and their colleagues built a large apparatus capable of creating a meter-tall, long-lived, vertical sheet of soap solution. In this system, the soap film continually flows downward—unlike the stationary film in a standard bubble wand—and the liquid is collected at the bottom and pumped back to the top. This laboratory setup allows the film to remain stable indefinitely, and its thickness can be adjusted, as can the speed with which it falls.
The team placed a gas nozzle at the surface of the soap film and used a high-speed camera to capture the results. At low gas jet speeds, only a small dimple appeared in the soap film. The dimple became deeper as the team increased the jet’s speed, until bubbles finally formed.
The phenomenon, the researchers found, can be explained as a contest between the pressure the gas jet exerts on the film and the surface tension of the film, which resists any increase in curvature. Bubbles form when the jet’s pressure is large enough to deform the film into a hemispheric dimple of the same width as the jet. At that point, the film has reached its maximum curvature, and the bubble can fill with gas and float away.
The researchers found that wider jets, which produce larger bubbles, create them at lower gas speeds than narrower jets. These larger bubbles have less curvature, making it easier to overcome surface tension’s pull. Repeating the study with a simple bubble wand gave similar results, suggesting that the laboratory setup is a passable proxy for real-world bubble blowing. The thickness of the soap film had no effect on the gas speed at which bubbles formed.
When people blow bubbles, the jet is formed at their lips—some distance away from the soap film—and it may be wider than the wand. To test these more realistic conditions, the team moved their gas nozzle back from the film. A jet of gas will have a complex structure once it reaches the surface of the soap film, thanks to turbulence, but these details turned out not to matter. Simply assuming that the gas spreads out in a cone was enough to account for the data, the team found. And in cases where the jet was wider than the film, the threshold velocity was determined by the size of the wand, rather than the size of the jet.
Understanding how bubbles form is important for certain industrial processes, like those involving foam production, and avoiding bubble formation is necessary in glassmaking and coating solids with liquids, says Courbin. But “this paper is really about explaining an everyday-life experiment,” rather than real-world applications, he says. Still, says Hamid Kellay of the University of Bordeaux in France, “it's the first time that these types of ideas can be tested correctly, because of the well-controlled experiments.”
The results could be useful in physics education, serving as an example of a careful experiment clarifying a simple problem, says Howard Stone of Princeton University. “It’s a very nice example that many people can learn from.”
This research is published in Physical Review Letters.
Emily Conover is the science writer for APS News.