Remove surface water with tissue. Start revolution immediately. Although we had worked out an incantation for the experiment, we still didn't know what was going on near the surface of the water. Looking for other causes, we discovered that glycerin is highly hygroscopic. It would appear that the water was not in fact evaporating, but rather the glycerin was drawing water in from the air.
Watery glycerin is light and has low viscosity, so it is very difficult to mix.
A picture is worth a thousand words One day, a student who had just finished writing his thesis came and saw the video of this experiment.