Lucy joins Katherine and I as our special-guest scientist as we study the effects of surface tension on floating paper circles.
There are a few points we gloss over in this video, so I’d like to correct for those now.
First, we mention toothpicks, but never explain what we do with them. We used them to herd the circles to the center of the half-full cup, and to the edges of the overfilled cup. That allowed us to repeatedly observe their movements in the other direction.
Second, I mention that the blue cup was "too big" for the second half of the experiment, where we overfilled it. This experiment, which is #23 in Janice VanCleave’s book, "Chemistry for Every Kid" (Copyright 1989 by John Wiley & Sons, Inc.), advises to use a cup that is no more than 2 inches in diameter at the top. The blue cups we were using were wider than that, so we switched to the smaller ones. That allowed us to get a higher bubble at the top, and see the effects more easily.
Finally, the paper circles didn’t really move to the center of the overfilled cups. "Center" in this case was really "not snuggled up next to the lip of the cup." I will say this – what the pictures don’t show is that the circles moved toward the edge, and then rebounded slightly before coming to rest where you see them in the video. It seemed clear to us that there was a force moving them to the outside, but that force was then overcome by another moving it in the opposite direction (to the center). At the very least, the overfilled cup shows a distinct difference from the half-full cup in the resting point of the paper circles.