Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Brilliant bubbles and disappearing dyes
A question I often get asked when dealing with the bleaching of coloured compounds is: why does the colour disappear? One of my go-to demonstrations to address this question uses a chemical curiosity that came about following an 11-year dream of inventor Tim Kehoe to create a coloured bubble-blowing mixture. The resulting product (currently sold as Zubbles) works using leuco dyes. For a long time, producing coloured bubbles seemed almost impossible, since bubble walls are so thin dyes tend to sink to the base of the bubble. Leuco dye molecules can exist in at least one coloured and one colourless form. The structures of these forms can be used to show students how organic dyes and pigments rely on delocalisation of electrons over extended molecular orbitals. This reduces the energy needed to promote electrons between energy levels to those corresponding to visible light frequencies. Any disruption to these extended delocalised systems can prevent a compound absorbing visible light, rendering it colourless.
Commercially produced leuco dye-based coloured bubble mix (Zubbles). Do not confuse Zubbles with other ‘washable’ coloured bubbles. These have attracted many complaints from users, after they found the product was anything but.
In front of the audience
© Declan Fleming
This demonstration just requires you to blow the bubbles in front of your students. Encourage them to watch the bubbles land. When the bubbles pop on a surface, they will leave a coloured ‘stain’ that rapidly disappears. If you have not had the opportunity to check an item of clothing for colour fastness, I would recommend blowing the bubbles up against a whiteboard. The dye decolorises very rapidly on the smooth, non-absorbent surface. You can even spread neat solution on the board and make it disappear with the rub of a finger. If you’re feeling brave, you can blow bubbles up against a student wearing a white shirt or a lab coat. As the bubbles pop, it will look as if there is an ink stain on the item of clothing, but with a quick wipe, the stain will disappear. Always check the item of clothing beforehand as some weaves will absorb the dye rapidly away from the surface and the colour can persist longer (I’ve found that normal school polyester or polycotton shirts work better than the heavier weave of a lab coat, but of course the stakes are higher!). If a mark remains, a dab with a damp cloth should remove the colour. Any stains that do remain will disappear over time or in a normal wash cycle.
Teaching goal
Leuco dyes have been also been used in carbonless copy papers, ‘pink-to-white’ wall paints and erasable pens.
While leuco-dye based bubbles can be produced from numerous dyes in a variety of colours, the Zubbles product uses a dye similar in structure to crystal violet lactone. Functional modification of this compound allows for a variety of colours (although so far only two have made it to market). The simplicity of the crystal violet lactone molecule’s operation makes it particularly appealing to illustrate to students how organic dyes work. Students will be familiar with the leuco and pink forms of phenolphthalein indicator; crystal violet dyes have similar structures, consisting of three benzene rings linked by a single carbon. Molecular modelling kits are particularly useful for showing students how the structures change. The structure of crystal violet is a little easier to interpret, having a planar resonance-stabilised carbocation that enables extended conjugation across all three rings (although steric clashing makes it unlikely that all three rings will be coplanar at once). There is a carboxyl group ortho to the bridging carbocation on one of the benzene rings. In the presence of oxygen, water or pressure, this closes a lactone (cyclic ester) to the central carbon to produce a tetrahedral centre and disrupt the conjugation. This also forces the benzene rings out of alignment. It’s important to point out that the compound is, of course, still present but no longer visible. The story behind the invention of these disappearing coloured bubbles is almost as entertaining and educational as the product itself: Tim Kehoe worked for many years to make the product a reality.
Materials
- Bubble solution
- Glow in the dark solution (can use washable glow paint or can make glow solution)
- Bubble wand
- Mix bubble solution with the glow solution.
- The only ‘trick’ is making sure you have enough bubble solution to make strong bubbles and enough glowing solution to get a good glow. Try a 50:50 mix to start. You can add more glow liquid or more bubble solution, depending on your results.
How to Make Glow Solution
If you use washable glowing paint and add that to the bubble solution, your bubbles will glow in the dark after the solution has been exposed to bright light.
Sometimes it can be difficult to find washable glowing paint, so you may wish to make glowing water using a highlighter pen. This solution mixes about 50:50 with bubble solution to make glowing bubbles.
The color of the glow depends on the highlighter that you use. Highlighter pens fluoresce, which means you will need to shine a black light on the bubbles to get them to glow.
Check your pen with a black light before you cut it open. Yellow almost always glows. Green and orange are good too, but a lot of blue and red pens don’t glow.
Here is how you make the glow solution:
- Use a knife to (carefully) cut a highlighter pen in half. It’s a pretty simple steak knife and cutting board procedure.
- Pull out the ink-soaked felt that is inside the pen.
- Soak the felt in a small quantity of water.
- Use the dyed water to make bubble solution or for other glowing projects.
Safety and Clean-Up
The glowing bubble solution is very safe, providing you used either non-toxic washing glow paint or a non-toxic highlighter pen.
If you blow the bubbles outdoors you don’t have to wash glowing liquid off of walls or furniture. Bubble solution is already pretty soapy, so clean up any spills with lots of water.
One nice thing about cleaning up glowing bubble solution is you can see the spots made by the bubble solution very easily.
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Helmenstine, Anne Marie, Ph.D. “Glowing Bubbles.” ThoughtCo, Apr. 5, 2023, thoughtco.com/how-to-make-glowing-bubbles-607625. Helmenstine, Anne Marie, Ph.D. (2023, April 5). Glowing Bubbles. Retrieved from https://www.thoughtco.com/how-to-make-glowing-bubbles-607625 Helmenstine, Anne Marie, Ph.D. “Glowing Bubbles.” ThoughtCo. https://www.thoughtco.com/how-to-make-glowing-bubbles-607625 (accessed November 7, 2023).
copy citation
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Methods for producing vibrant colored bubbles
Bubble Juice:
1 gallon water 2/3 cup dishwashing soap
Mix the ingredients together in a big bucket or dishpan. If you make your bubble juice the day before you want to use it, you’ll get bigger, stronger bubbles, but it’s pretty good right away, too.
Tape the plastic lid over the end of the flashlight the light shines from.
Turn the flashlight on and hold it so the light shines straight up.
Dip your finger in the bubble juice and wet the lid. Put a spoonful of bubble juice on the lid. With a straw, blow one big bubble to make a bubble dome that covers the whole lid.
Turn off the lights and hold the flashlight so that the bottom of the bubble dome is just above your eyebrows.
Watch the swirling colors. If you put the wet straw into the bubble dome and blow very gently, you can move the colors around.
Watch the colors. How many do you see? If you watch a bubble for a few minutes, do the colors change? What colors do you see right before the bubble pops? Do you ever see black and white polka dots?
Right before it pops, the skin of a soap bubble is only one-millionth of an inch thick!
Why are soap bubbles so colorful?
The colors of a soap bubble come from white light, which contains all the colors of the rainbow. When white light reflects from a soap film, some of the colors get brighter, and others disappear.
You can think of light as being made up of waves—like the waves in the ocean. When scientists talk about waves, they often talk about a wave’s frequency. Frequency is the number of times that a wave vibrates in a second. For ocean waves, frequency measures the number of times a passing wave makes a surfer bob up and down in a second. For light waves, frequency measures how many electromagnetic vibrations happen in a second.
The frequency of a light wave determines which color light you see. Violet light, for instance, is the highest frequency light that you can see; it vibrates 723,000 times in a billionth of a second. White light is made up of light waves of many different frequencies.
Two waves can be in the same place at the same time. Suppose two ocean waves of equal size meet. Each wave pushes up and down on the water in its path. Where the waves meet, there are two different forces acting on the water, one from each wave. If both waves push up on the water, the water moves twice as high as it would move if it were pushed by one wave alone. This is called constructive interference.
If one wave pushes up and the other pushes down, the two pushes cancel each other and the water doesn’t move at all. When this happens, it’s called destructive interference.
What does all this have to do with the colors of bubbles?
Light waves, like water waves, can interfere with each other. A bubble film is a sort of sandwich: a layer of soap molecules, a filling of water molecules, and then another layer of soap molecules. When light waves reflecting from one layer of soap molecules meet up with light waves reflecting from the second layer of soap molecules, the two sets of waves interfere. Some waves add together, making certain frequencies or colors of light brighter. Other waves cancel each other, removing a frequency or color from the mixture. The colors that you see are what’s left after the light waves interfere. They’re called interference colors.
The interference colors depend on how far the light waves have to travel before they meet up again–and that depends on the distance between the layers or the thickness of the soap film. Each color corresponds to a certain thickness of the soap film. By causing the liquid bubble film to flow and change in thickness, a puff of wind makes the bubble colors swirl and change.
The very thinnest film—one that’s only a few millionths of an inch thick—looks black because all the reflecting wavelengths of light cancel. When the soap film looks black, it’s just about to pop.
What’s the best set-up for seeing colors in a bubble?
Interference colors on a bubble look brightest when there’s white light shining on the bubble and a black background behind it. The colors come from light that’s reflecting from the soap film. You want to be on the same side of the bubble as the light source so that light will bounce back to your eyes. The black background keeps light that’s shining through from the other side of the bubble from washing out the colors.
Where else can I see shimmering colors like these?
You can experiment with interference colors using the Reflecting Rainbows experiment online, or the Rainbow Prints experiment in the Science Explorer Book.
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1996 & 1997
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