Ways to create controlled small bubbles in water within plastic tubing

Question

What is a practical way in an engineering or physics laboratory to create bubbles of a specified size in water within plastic tubing? The tubing is a few mm inner diameter. We'd like to make bubbles smaller in size than the diameter, for example 1/2 the diameter. We'd like to have control over the size, especially volume, and rate of bubble generation.

We have tried a metal hypodermic needle connected to a low-power air pump, but air accumulates over time to make one big bubble, which eventually breaks away. Maybe there is a way to knock the bubble off when it's the desired small size, instead of waiting for it to do as it pleases?

The water is at roughly 1 to 1.3 atm pressure, at room temperature.

We imagine possibilities of electrolysis, or acoustics, but haven't found any written experience through google searches.

Answer 1

Bubble nucleation is largely controlled by the surface features of the material, the type of gas being used, impurities, temperatures and partial pressures. A quick search came up with this reference, which explores bubble formation in order to understand the process well enough to prevent formation. My immediate thought would be that if you had a good flow rate around a well controlled defect (like a needle sitting perpendicular to the wall of the tube in the flow stream), that after some experimentation with pressure and flow rates you could get pretty good control over the bubble nucleation. However, since it looks like you are trying to inject the bubble, here is a paper regarding analysis of bubble breakup in turbulent flows. That might get you started.