Color changing beads under cellophane

[c.s.]

I've got some beads that will change color under sunlight. The stronger the intensity of sunlight, the darker (purple) the beads will turn. If the beads are put back indoor, they will resume their original color (white) after a while. The beads will also change color if they're under the light from a UV torch. The beads are plastic like.

I tried to cover the beads with cellophane of different colours (red, yellow, green, and blue) and put them under the sun. After a while, when I uncovered the cellophane, I found that the beads covered with the blue cellophane became the darkest while the beads covered with yellow/ green cellophane had become only light purple, and the color of the beads covered with red cellophane were somewhat in between. I did have a control with beads not covered, the beads didn't become as dark as those covered by blue cellophane but were slightly darker than those covered by red cellophane.

Can anyone help me explain my observation? Is it because there are different ranges of light penetrating the cellophane? Or is it because cellophane of different colors absorb different ranges of light in sunlight?

 

[Andy Resnick]

Interesting experiment. What conclusions can you draw, given the spectrum of sunlight? What can you hypothesize regarding the color agent in the beads? How do the results of irradiation with the UV light fit with these?

Can you control for the intensity? What about the rate of change- does it vary, and can you control the variation?

 

[c.s.]

Thanks Andy, for your reply and questions.

Since the beads are not sensitive to visible light (as stated on the package), the color change should be caused by UV. It seems that the cellophane of different colors allow different amount of UV to pass through. However, UV is not part of the visible light spectrum, its penetration into cellophane of different colors should be the same. Therefore, I speculate that the color change of the beads covered by cellophane of different colors are caused by HEV present in sunlight. I know that HEV is also called blue light, so when sunlight shines on the blue cellophane, only blue light (including HEV) passes through, and this is also why the blue cellophane is blue as perceived by our eyes. Since similar color change is observed when the beads covered by cellophane of different colors are placed under UV torch light which has a visible obvious blue color, I think the color agent in the beads is sensitive to HEV. However, am I ignoring the possibility that the color agent is also sensitive to other wavelengths in the spectrum of sunlight?

Well, I can't control the intensity of sunlight. But when I put all the beads covered with cellophane of different colors under the sun all at once for the same amount of time, I guess the intensity variable is controlled.

I've never consider the rate of color change, probably because the color change can not be easily observed. I can record the color of the beads every 1 minute for example but everytime I do this I have to move the beads back into a shaded area nearby and remove the cellophane. You know, once the beads are not under the sun/ UV torch, they return to their original color very quickly. Can you think of any ways I can measure the rate of change? I think it would be interesting to know.

Sorry, I don't quite understand the question "How do the results of irradiation with the UV light fit with these?". Can you explain it a bit.

 

[Andy Resnick]

I'm not familiar with the beads- I am just trying to stimulate your thinking.

What does 'HEV' mean?

Ok- so you have at least one assumption, that the UV transmission of the cellophane is not related to the visible color. That's most likely true, but you could perhaps check it by using multiple layers of cellophane (same colors, or different colors). If you can measure the spectral transmission with a spectrometer, that would be definitive evidence.

You could also, instead of the sun, use an incandescent bulb- they have nearly no UV output. For controlling the intensity of sunlight, you could use a scrim or neutral density filter (if you have access to one). or wait for a cloudy day... :)

In terms of the dynamics, I was going to suggest exactly what you thought of. Perhaps there is a way you can take a picture of the beads without taking them out of the sun, maybe by making a 'exposure chamber', opaque on all sides except the top where the cellophane is. Then if you can peek in a side (a moveable peephole), you may be able to measure the dynamics.

Basically, it sounds like you are on your way to getting answers. I'm just suggesting ways to make predictions and test them.

What are these beads, anyway?

 

[alxm]

Since the beads are not sensitive to visible light (as stated on the package), the color change should be caused by UV. It seems that the cellophane of different colors allow different amount of UV to pass through. However, UV is not part of the visible light spectrum, its penetration into cellophane of different colors should be the same.

I wouldn't be so sure of this.. But if the film is blue, it has absorption in the yellow/orange area (since that's the complementary color), whereas the yellow, for instance, would be absorbing in the blue/violet area. Typically things absorb fairly broadly, so it's quite possible the blue film would absorb less UV light.

You could try putting them under yellow film, under a tanning bed lamp (if you can find one), or UV any other kind of blacklight.

The 'secret' here:

Spoiler

The beads contain a UV dye. Probably similar to the one http://i29.tinypic.com/23m070z.jpg". What happens is that you have molecules (or parts of the big plastic molecules) which react to UV light by changing their conformation (shape), which in turn causes them to absorb light that they didn't absorb before. After a while, if they're not subjected to UV light, they regain their original conformations. The one in the image regains its original conformation when subjected to visual light without UV. (The ovals and squiggly lines represent different parts of the molecule. The change in this case is caused by a neutral part of the molecule splitting into positively and negatively charged parts.)

 

[texasblitzem]

I have a quick question, is the color change in these beads only on the surface? Or if someone were to crack the bead in half, would the interior color have changed too? just curious

 

[Andy Resnick]

That's an excellent question too! It gets to the mechanism.

 

[alxm]

is the color change in these beads only on the surface? Or if someone were to crack the bead in half, would the interior color have changed too?

Either the dye is a covalently-bonded part of the polymer molecules themselves, or it's been mixed into the polymer.
Either way, the pigmentation almost certainly goes all the way through. (how often do you see painted plastic?)

But its anyone's guess how far into the beads the UV light will typically penetrate.

 

[c.s.]

Thanks everyone for the questions and ideas.

I've taken a few photos to help you guys understand the experiment.

Original colors of the beads: http://dc182.4shared.com/img/336849520/50116542/01022010091.jpg

Covered by cellophane paper of different colors: http://dc237.4shared.com/img/336849715/8d21660/01022010092.jpg

After putting under the sun and cellophane paper removed: http://dc236.4shared.com/img/336849792/5e6f09cb/01022010096.jpg

The beads are from Carolina Biological Supply Company:
http://www.carolina.com/product/solar+energy+beads+pack-1000.do

 

[c.s.]

But if the film is blue, it has absorption in the yellow/orange area (since that's the complementary color), whereas the yellow, for instance, would be absorbing in the blue/violet area. Typically things absorb fairly broadly, so it's quite possible the blue film would absorb less UV light.

Do you mean if the film is blue, it absorbs yellow/ orange area light and allows blue light to pass through?

And if the blue film absorbs less UV light, does that mean it allows more UV light to pass through so the beads turn darker purple?

 

[c.s.]

What does 'HEV' mean?

HEV means high energy visible (HEV) light?

 

[c.s.]

I have a quick question, is the color change in these beads only on the surface? Or if someone were to crack the bead in half, would the interior color have changed too? just curious

Interesting!
I tried to crack the bead in half and found that the color change started from the surface (where the light from the UV torch shone on) and then to the interior.
However, I forgot to take a picture of it.
Will do it when I return to my office.

 

[Claude Bile]

Short wavelength radiation can induce colour centers in "transparent" media. Colour centers are defects in the solid-state structure that are optically active and exhibit (among other things) absorption and emission bands (Look up "Nitrogen-Vacancy" centers in diamond for a good example).

Exposure to short-wavelength radiation can therefore induce absorption bands in the material, a process called photo-darkening. This is a possible mechanism for the change in colour that is observed.

It would make sense then that beads exposed to shorter wavelengths get darker, as higher energy photons are more likely to induce colour centers. It is interesting that exposing the beads to all wavelengths (no filtering) yields less photo-darkening, this could be due to the presence of photo-bleaching (light removing colour centers).

There may be other mechanisms at play, but I think it is worth understanding terms like colour centers and photo-darkening when it comes to seeing how light can change the transmission properties of dielectrics.

Claude.

 

[Andy Resnick]

Thanks everyone for the questions and ideas.

I've taken a few photos to help you guys understand the experiment.

The beads are from Carolina Biological Supply Company:
http://www.carolina.com/product/solar+energy+beads+pack-1000.do

Thanks for the photos and link- now you know that whatever is in the beads, they respond to UV light. You could verify this by illuminating the beads with light from an incandescent bulb or a fluorescent bulb- ones that are made for interior lighting so the UV is filtered out. White LEDs could be used also.

The UV portion of the spectrum is around 380-220 nm; at shorter wavelengths atmospheric absorption (ozone, mostly) prevents meaningful amounts from reaching the earth:

http://www.udel.edu/Geography/DeLiberty/Geog474/spectrum.jpg

So your experiments really checked the UV transmission of the different cellophanes, and that the beads can (possibly) be used as a UV detector. You could use the beads to measure the optical density of various filters (by measuring how long it takes them to change color), and if you calibrate the bead response time against a known source (a UV lamp/blacklight is a good source), you can make quantitative measurements. Water is a good UV filter.

Since you don't know what's in the beads, some simple things to look for are phosphorescence (do the 'charged' beads glow in the dark?) or fluorescence- that can be trickier to setup without a lab. Crush up a bead and see what happens then- you could get a larger effect. And then you could used the crushed up bead(s) as a filter on top of other beads.

Fun toy!

 

[DaveC426913]

Well, I can't control the intensity of sunlight.

Sure you can. Use layers of something partially opaque. You could use something as simple as screening like on a screen door. The advantage of using screening is that it physically cuts down on the intensity of light without altering any properties of the light that does get through.

No screening = 100% intensity
Max screening (opaque) = 0% intensity

(Carefeul though: the increments from 100% to 0% will not be linear. Each additional layer will block less light than the layer before.)