My self siphoning beads video presented a puzzle... Why do the beads rise up above the rim of the pot before they fall to the floor?
The problem eventually came to the attention of academics at Cambridge University, John Biggins and Mark Warner. And I'm happy to say, we have an answer! In a published scientific paper no less.
It was published in the Proceedings Of The Royal Society A and can be read here.
They also put together an explainer video:
My favourite part is where they call it the Mould Effect. I've never had an effect named after me before!
The surprising top line explanation is that the pot pushes the beads up. It sounds crazy but the way to reach this conclusion is to consider an idealised version of the chain; a chain of rods. As each rod is pulled into motion, the opposite end of the rod is forced downwards (as the rod rotates around it's centre of mass). The reaction force from the pot is where the extra upward force comes from. The links between beads make the chain somewhat ridged and in that way it acts like a chain of rods. Anyway, watch the video or read the paper or both, their explanation is great.
Like all good scientific ideas, this explanation makes testable predictions. Firstly, that a larger drop will result in a higher fountain. This is certainly the case. It also predicts that a chain built from more loosely joined beads would not produce the fountain effect because they would not act at all like rods. Watch their video to the end to see this tested.
Finally, Biggins and Warner have used the bead chain fountain problem to inspire young physicists to look at real world practical problems. You can find their resources here.