One morning I came back to my prototype bubble display to find this:

Obviously my final bubble display will need to have a screened mesh at the tops of the tubes to allow air to escape, but keep insects out. (But I will also need a way to re-fill the tubes, so it can’t be a permanent screen…so acrylic welded solutions are not on the table….)

After leaving a prototype bubble display running for a week (in 80-100 degree weather) I returned to find the following clear case of evaporation:

The two tubes on the outside have glycerin, while the two tubes in the center (which are several inches lower) contain water. I didn’t leave a marker of what level the tubes were filled to at the beginning of the week, but the water tubes have obviously lost several inches due to evaporation. (The continuous bubbling action helps evaporation along quite a bit…) It looks like the glycerin suffers less from evaporation, but I expect that even a glycerin filled bubble display will need periodic refills. Note that the 80-100 degree (and dry) weather may have made this problem more pronounced.

Once I had five six foot tall tubes, I wrote some code that would attempt to render text using a 5×7 font. If you can read the three letters I scroll (vertically) in the movie below I have been successful:

So far I have been using 1 and 2 foot acrylic tubes to try out different nozzle types, acrylic welding techniques, and liquid bubble mediums. Now that I’ve selected my nozzle (a 1/4″ coupler that joins the 1/8″ vinyl tubing in polycarbonate) it’s time to scale up to full six foot tubes.

Although all of my calculations told me that everything should work with a six foot acrylic tube, you never really know if something is going to scale up correctly until you try it. For example, here is a list of possible problems with scaling up (ordered by probability):

• The air pumps, which are rated to a max pressure of 350mmHg (basically meaning they can just barely blow a bubble into a column of mercury 0.35 meters high, or deliver 6.7 psi), won’t actually achieve their rated pressure, and be unable to blow a bubble into a 6 foot high column of water (4.7 meters of water is equivalent to 350mmHg…but still….)
• The bond holding the nozzle and bottom of the acrylic tube which worked fine for 1 foot of water would spectacularly fail when loaded with 6 times the pressure.
• The Acrylic column won’t be able to support the water pressure, and will burst. (Even 1/8″ acrylic is relatively strong, if brittle, so I wasn’t really worried about this one…)
• The cyclic pumping, combined with flashing RGB LED’s would interact in a heretofore unknown way to produce fusion, a la Chain Reaction (1996).

Fortunately, everything worked just fine. What does an air bubble look like rising thorugh six feet of water? I’m glad you asked. Watch this video:

The nozzle size has an effect on the amount of air (expressed in milliseconds of pump activation) that will form into a single bubble. In water, even if you launch a stream of several bubbles, they typically join together relatively quickly to form a single bubble. However, with Glycerin the bubbles are more likely to separate, and the smaller bubbles rise slower, eventually getting caught and integrated into a larger following bubble. By increasing the nozzle size from a 1/8 inch hose barb to a 1/4 inch hose barb, I found that I could create larger more regular bubbles, and launch them closer together (temporally and vertically).

I also switched my nozzle material from Nylon (which acrylic weld won’t weld, and required the use of epoxy) to Clear Polycarbonate, which acrylic weld WILL weld. This allows me to use a single welding solvent to construct the entire tube assembly, and gives me more confidence in the bond. (Although the Epoxy appears to be working well….) Because they are clear I can visually inspect the weld between the bottom of the tube and the nozzle. Also, they blend into the acrylic bottoms of the tubes and give a much nicer visual appearance. The specific part I used to get both polycarbonate and a 1/4″ output was a Single-Barbed Reducing Coupling for 1/4″ X 1/8″ ID Tubes. The nylon 1/8″ nozzle cost 0.26 USD, the same part in clear polycarbonate is 0.47 USD, and if you want a 1/8″ to 1/4″ coupling it costs 0.70 USD, so the cost for the larger clear nozzle is nearly triple the plain old nylon part, but as a percentage of the total tube cost it is negligible.