They found that bubbles only form above a certain speed threshold for air velocity. As Emily Conover writes at Physics Focus:

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.


That threshold speed varies with the width of the jet, not the thickness of the soap film. Wider jets have a lower speed threshold and produce larger bubbles than narrower ones. (If the jet is wider than the soap film, the width of the bubble wand will determine the threshold speed.) The French physicists repeated the experiment with a simple bubble wand, just to demonstrate that these principles hold for actual everyday bubble-blowing.

This isn’t just child’s play. If you want to avoid bubbles forming in glassmaking and other industrial processes, for instance, having a better understanding of the underlying physics is very helpful. Bubble physics is also relevant to building lab-on-a-chip bubble logic devices, and artificial viruses for vaccines. Mostly, though, “This paper is really about explaining an everyday-life experiment,” Courbin told Physics Focus.


[Physical Review Letters]