They are a nematode that grows in wine vats. They are used as food for fish, and grown by hobbyists in a 1/2 vinegar 1/2 water solution.
The fluid, teeming with nematodes (yum!) is poured into a volumetric flask, the kind with a large bottom and a long, slender neck. A piece of filter floss is stuck in the neck where the vinegar ends and water poured on top. In a day or two, the eels swim though the floss into water and may be collected.
Why doesn’t the water in the neck become as acidic as the vinegar below?
Diffusion.
The vinegar sol’n is denser than water and, while miscible, will tend to take a long time to mix. You can probably get a good visual of the diffusion process if you drop a little food coloring in either ‘phase’. There will be mixing at the interface and possibly a gradient of color migrating to the other solution. It is mostly a matter of kinetics.
I’m sure there is a nice equation that describes the process, and it is likely dependent on viscosity, density, temperature and other variables that aren’t coming to me off the top of my head.
The Diffusion Equation is NOT nice.
It is apple cider vinegar, and you can indeed see the color change in the floss.
Thanks.