This is a quirky method of creating concentric bubbles invented by Japanese bubble artists, the Sugiyama Brothers.
I have been doing this without ever really understanding the logic behind it, but recently I have finally been able to sort out the logic of the process.
The image is a frontal spatial view from where the bubbler is standing, facing the bubble.
The first wand is at position a1/b1. Imagine the wand with the bubble film on it being held in the right hand and projecting toward the front. a1 is the left edge of the circle of wand and b1 is the right edge.
From position a1/b1, swing the wand up and lift it to position a2/b2. Once lifted, while holding b spatially fixed (b2=b3=b4), rotate a from position a2 to a3 to a4 (= rotating the right wrist holding the wand).
As soon as the wand reaches the a4 position, slide the fixed b inside the bubble (b4→b5), while lowering the wand.
Then, while pulling b out of the bubble (b5→b6), the wand is finally turned back and b is completely separated from the bubble (b6→b7).
The bubble thus created will contain an "inner bubble," consisting of the film pulled from b1 to b2 and the film created by folding back from b4 to b5 to b6.
The lower edge of the inner bubble converges at just one point (b6), splits cleanly, and begins to float inside the bubble.
There is probably no other way to create a double bubble with a single shake of a simple circle wand without using breath.
There are occasional cases where a double bubble is created by an accidental impact (e.g., when a small bubble pops inward as a reaction to detachment). However, this method does not rely on contingency. The process is quite logical.
It was only when I tried to diagram it that I understood its brilliant rationality. The whole process makes sense, and it makes me feel a kind of beauty of inevitability: "No other way will lead to this result" = "If you do it this way, you will surely get this result.
The Sugiyama brothers must have acquired this naturally through repeated bubbling of soap bubbles. And in the world of soap bubbles, there are many such things, that is, things that somehow always happen, even though we don't understand the logic behind them.
They are probably governed by some kind of logic, just like this method.
