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.