From the Wikipedia reference: "A venturi can also be used to mix a fluid with air. If a pump forces the fluid through a tube connected to a system consisting of a venturi to increase the water speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), air will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of fluid and air will appear."
Or, to paraphrase: A venturi works to mix fluid with air by working to mix fluid with air. That isn't an answer. My question is why does a venturi produce microbubbles of CO2 which is introduced where in the venturi? I know very well how a venturi is designed and how it works as a venturi, having designed several many years ago to measure flow rates of both air and water, but I don' t know why it works to make microbubbles of CO2, and not knowing why it works, means I have no idea how to make it work better or best or well enough, etc. In principle, the lower pressure at the venturi throat should result in big bubbles at that point, which become smaller bubbles when the pressure goes back up in the downstream part of the venturi. But, that wouldn't by itself produce micro bubbles. In fact if you introduced milimeter size bubbles in the approach pipiing to the venturi, the bubble size in the outlet tube from the venturi should be almost exactly the same, because the static pressure will be almost the same.