Monday, October 4, 2010

Tiny 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, which I suppose what hosts are supposed to do. 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 was expecting the square bubble to be free-floating by itself, so I was kinda disappointed in how it was done. The “performance artist” blew a bunch of connected bubbles (I forget how many). Next he took a long inhale from a cigarette, then stuck a straw between the connected bubbles and filled the space with the smoke. The filled space was in the form of a cube, and I felt tricked.

A short lived creation, soap bubbles hold a sphere of air with a thin film of soapy water which is formed by surface tension. Spherical shapes are preferred (really large bubbles can end up forming elongated shapes from air currents) because a sphere is the smallest surface area possible to contain a specific volume of air. The soap film surface tension is strong and flexible enough that waves can travel along the surface and is so thin the surface appears iridescent.

Surprisingly, soapy water has less surface tension that water alone and is needed to keep the bubble stable. As a bubble is formed, the soap film stretches decreasing the concentration of soap which increases the surface tension. This mechanism is called the 'Marangoni Effect' and 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 on 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. How about antibubbles ….

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