How to make square soap bubbles

Anyone remember the show “That's Incredible!”? It was on in the early 80's. I watched it as a kid (and yes I'm dating myself as the show went off the air in 1984). One episode I remember vividly: The hosts hyped how this man could make a square bubble. Since all the bubbles I had ever seen were round, I was really curious how a square bubble could be made (and I wasn't yet jaded about TV). I expected the square bubble to be free-floating by itself--which isn’t how the demonstration played out leaving me disappointed.

In the show, the “performance artist” blew a series of bubbles, probably while era appropriate electronic music played. He then formed the bubbles into a connected blob. Next he took a long inhale from a cigarette, before sticking a straw between the connected bubbles and filling the space with the smoke. The filled space was in the form of a cube, and I felt tricked.

A soap bubble is really just a thin film of soapy water containing a volume of air--the film is so thin (in the 100s of nanometre range) that light bouncing around inside interacts with itself creating iridescent effects.

Surface tension holds the bubble together in a more-or-less spherical shape because a sphere has the smallest surface area that can contain a specific volume of air (large bubbles end up elongated due to air currents). The soap film is flexible enough that waves can travel along the surface--but the bubble can’t hold itself together for long.

Surprisingly, soapy water has less surface tension than water alone, but is needed to keep the bubble stable. As a bubble forms, the soap film stretches decreasing the concentration of soap which increases the surface tension--a mechanism called the ‘Marangoni Effect’.

The 'Marangoni Effect' occurs when a surface tension gradient (that is regions of greater and lesser surface tension) causes liquid to move away from areas of low surface tension. The soap acts as a stabilizer by letting the thinnest parts of the film to have the strongest surface tension thus keeping the bubble together.

What happens when two bubbles stick together? Well, they will arrange themselves in such a way that minimizes the surface area. Bubbles of different sizes will end up with a bulging internal wall into the larger one as smaller bubbles have higher internal pressure. If they are the same size, the internal wall will be flat - a phenomenon exploited by the cube-making bubble performer.

So sneaky internal bubble-wall cubes aren't so impressive.