Why blowing small bubbles is so difficult

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By John

Bubble inflation still has some surprises in store, as Marc Grosjean and Elise Lorenceau, from the Interdisciplinary Physics Laboratory in Grenoble, tell us in an article by Physical Review Fluids of May 22. So, depending on the conditions of blowing air through a straw to get bubbles, some sizes will be impossible to achieve. Large or even very large ones will form but not small ones. The fault is due to a phenomenon of instability analogous to that felt when blowing in a balloon: in vain you toil, the elastic membrane resists and no longer grows, until, suddenly, a threshold is crossed and the balloon inflates.

The calculations are in fact simpler in the case of a bubble whose membrane does not have the elastic properties of that of a balloon. But the phenomenon was poorly documented until now. “, emphasizes Elise Lorenceau. His student Marc Grosjean, at the time in master’s degree, now in thesis, therefore built an experiment using a straw connected on one side to a syringe with variable volume to push the air and on the other to a film liquid “soap”.

At the beginning, when the air arrives, the flat liquid membrane deposited at one end of the duct swells regularly and the gas pressure increases. There comes a time when the membrane forms a half-sphere with a radius identical to that of the straw. This shape corresponds to the maximum pressure that the interface can sustain. However, as air continues to arrive, the size of the bubble increases and exceeds this critical hemispherical shape. This has the effect of expanding the gas which expands in the bubble, increasing its volume even more… “It’s like a pressure cooker, you have to release gas and it goes through a sudden increase in the volume of the bubble to rebalance the pressures”summarizes Elise Lorenceau.

A very simple experience

In five milliseconds, the size can go from one millimeter, the radius of the straw, to almost five, the researchers observed. But this “jump” is not always present. This essentially depends on the size of the syringe and therefore on the volume of the air reserve upstream of the bubble. If it is too small, the swelling is regular and all sizes of bubbles will be allowed. But if it is too big, instability appears and only big bubbles are formed.

“Now, if someone asks us how to make bubbles of a certain size, we will know the answer, notes Elise Lorenceau. Previously, many experiments hid their lack of control over this aspect. » However, the issue is not insignificant. Systems, called microfluidics, made of tiny channels in which fluids circulate, need to create small drops or bubbles to trap chemical reagents, transport them, make them react… for analysis or production purposes. The presence of instability in these mini-reservoirs is therefore troublesome.