Monday, March 3, 2014

pH indicating soap bubbles - more work needed

I thought it would be cool to have a colored soap bubble that responded to changes in pH.  I concentrated some phenol red (pH indicator), and mixed it with commercial bubble solution.  Unfortunately, it was not dense enough to actually show its color. While investigating this, I noticed that normal soap bubbles lose their color as they become thinner (due to evaporation) and eventually burst.  I tried to get footage of this color loss, but the bubble will not stay in the same place!  I started to develop a method to partially fill the bubble with helium to make it neutrally buoyant.


  1. The trick I learned as a kid from a book that came with a bubble apparatus was to add glycerin.

    The apparatus was a loop of wick-like fabric, a plastic tube to support it, and a slide to open and close the loop, and a weight at the bottom of the loop. Using a image search I found this which is pretty close. That site has a recipe page, which lists glycerin among other things. It also mentions humidity makes the best bubbles. I was doing this in dry Colorado (although it was summertime in lawn covered suburbia, so better than the rest of the year) and with the glycerin I'd still get bubbles that lasted minutes and in some cases could land on things without popping.

  2. Could a bubble be suspended electrostatic attraction? If so, an heavier than air bubble could be suspended, and a He bubble could be held down.. Closed loop control using ultrasonic sensor? OpenCV tracking reflections? Would the field distort the bubble?

    Well you've got plenty of high-voltage sources so only the control (position sensing) part might be a small challenge.

  3. Or maybe just an e-force ring the bubble can sit in?

  4. Looks like making coloured bubbles is hard. Or, at least someone spent 11 years on it, according to this article:

    Quoting from the article:
    "It turns out that coloring a bubble is an exceptionally difficult bit of chemistry. A bubble wall is mostly water held in place by two layers of surfactant molecules, spaced just millionths of an inch apart. If you add, say, food coloring to the bubble solution, the heavy dye molecules float freely in the water, bonding to neither the water nor the surfactants, and cascade almost immediately down the sides. You'll have a clear bubble with a dot of color at the bottom. What you need is a dye that attaches to the surfactant molecules and disperses evenly in that water layer. Pack in more dye molecules, get a deeper, richer hue. Simple. Well, on paper anyway."

  5. I'd worry that the soap molecules would get protonated at low pH, since they are slightly alkaline salts of fatty acids. There are detergents that are not charged, but I've never tried to blow a bubble with one.

    The whole surfactant thing is a matter of polar head and non-polar tail. I wonder if anything like a dye has been connected to one end or the other. I would ask someone who does cell biology- some of these guys attach fluorescent molecules to stuff that will work its way into the lipid bi-layer around a cell.

  6. Membrane stains exist but they aren't cheap. E.g. is $2/uL

    On the other hand, it might be fluorescent. Fluorescent bubbles would be cool :)

  7. Can't you "hang" a bubble off a smooth, wet surface? Maybe a convex lens hung from some fishing line?