I used to have a version of this experiment myself -- there was a desk toy that was a transparent tube with dark caps at either end and a somewhat viscous liquid inside. Because of refraction, bubbles in the tube looked much larger than they were, and so perhaps 30 of them would flow up, in about a minute, each time the tube was flipped. I bought one in 1996 or so, and picked the one that was the slowest -- it took maybe two minutes at the time.
This turned out to be because the liquid was -- very slowly -- evaporating out of the tube, and getting thicker.
Within a year, it took about 5 to 10 minutes.
Within five years, it took over an hour.
After ten years it took weeks.
After fifteen years, it took most of a year.
I last flipped it over in something like 2015, and after several years it appeared to be pretty much unmoving, although I assume it was still making progress. I used to joke that I wanted it to be flipped one last time and placed beside me in my coffin, but I lost it in a move several years ago.
This turned out to be because the liquid was -- very slowly -- evaporating out of the tube, and getting thicker.
Within a year, it took about 5 to 10 minutes.
Within five years, it took over an hour.
After ten years it took weeks.
After fifteen years, it took most of a year.
I last flipped it over in something like 2015, and after several years it appeared to be pretty much unmoving, although I assume it was still making progress. I used to joke that I wanted it to be flipped one last time and placed beside me in my coffin, but I lost it in a move several years ago.