Chemical Physics Experiments

Thin Film Interference
Candy Triboluminescence


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Thin Film Interference

Oil on Water

When oil (or soap) sits on top of water, it takes on a rainbow like appearance. Colors swirl and mix as the surface of the water ripples.

Do these colors come from the oil directly?

No, in fact, the colors you see are not really "the colors of oil" in the normal sense of the word - if you look at oil in its usual form, it tends to only have one color. Depending on the type of oil, it might be dark or light brown, yellow or perhaps a solid bright unnatural color, like blue (bright colors are dyes added by the manufacturer). It is only when the oil lies on top of water or some smooth surface like cement or glass that these colors can be seen.

Why is that?

If you have ever poured oil onto water, you will have seen another important property of oil - it floats. Most oils are lighter (or more precisely, less dense) than water, and thus float on top of it. So, when oil is poured onto water, it floats on top. But, because gravity pulls the oil down and its buoyancy in water pushes it back up, it spreads out untill it forms a film over the water that is very, very thin.

These films tend to be only a few wavelengths of light thick - about 1 micrometer (one millionth of a meter) thick. When light hits the surface of the oil, some of it is reflected (the reflected wave) and some of it goes through the surface (the refracted wave), bounces back off the water and re-emerges out the top of the oil (see figure).

How the Film Determines its Colors

When the light bouncing off the surface of the oil combines with the light coming out of the surface, they will either cancel each other out or reinforce each other (make each other stronger).

This is called interference.

Whether they cancel out or reinforce one another depends on two things: the color of the light and the distance the light travels inside the oil.

Constructive Interference

The color determines the wavelength, and if the extra distance traveled by the refracted beam relative to the reflected beam - the path difference is a whole number of wavelengths, then the peaks and troughs of both waves align exactly, or are "in phase", and the waves reinforce each other (see figure). This is called constructive interference

Destructive Interference

At the other extreme, if the extra distance traveled is exactly between two whole multiples of the wavelength, the peaks of the reflected wave will align with the troughs of the refracted wave (or are "180o out of phase") and they will exactly cancel out so none of this light comes through (see figure). This is called destructive interference. For distances in between these two extremes, the waves partially cancel and partially reinforce, depending on how far "out of phase" they are.

However, some of the refracted wave will reflect off the oil/air interface and bounce back and forth in the oil extra times. The effect of these extra bounces is to cancel out any waves that are not exactly the right wavelength to be perfectly in phase as they emerge from the oil, vis:
If the first refracted beam is a small amount out of phase with the reflected beam, then after a large number of internal reflections, a beam the exact phase to cancel out the reflected beam will emerge. Then the next refracted beam will cancel out the first and so on. (Each refracted beam is slightly weaker than the previous beam, but summing over the very large number of reflection cycles, there will be enough reflection cycles for the destructive interference to occur.

In turn, the distance the light travels depend on the thickness of the oil and the angle that the light hits the surface.

At one angle, green light might be reinforced while the other colors are cancelled out, so it would appear green. At a different angle yellow light might be reinforced. Differences in thickness cause different areas of the film to appear different colors. As the water ripples, the oil flows slightly, causing dynamic variations in the thickness that lead to swirling rainbows.

Experiment - Making a Permanent Film

In this experiment we will make a thin film on a piece of cardboard that is far less transitory than oil on water. Then you will be able to investigate the properties of the film interference, like the color and angle dependence. Also, just in case you didn't believe me about the thin film and think that maybe the rainbows come from the colors in the colored oil separating somehow, we're going to use something that is definitely clear - nail polish. And, as we'll be allowing it to dry and harden, there is no chance that the changing colors can be caused by movement of different colored parts of the oil, say.

Things That You Will Need

1) A bowl of water.

2) Thin cardboard

3) Scissors

4) Clear nail polish

5) String or fishing line

6) Safety pin, hole punch or stapler.


1) Cut a shape from the cardboard.

It can be any shape you like, but try to make it only a few centimetres across.

2) Punch a hole in the card and tie the string securely to the shape through the hole.

3) Place the shape into the bowl, so it is completely submerged, with the string hanging out of the bowl. If it doesn't completely sink straight away, you should hold it under briefly while it absorbs enough water to weigh it down.

4) Place a drop of nail-polish onto the surface of the water. It will probably stay as a drop for a few seconds, then suddenly spread out to make a puddle on the surface of the water, as its surface tension lets go.

5) As soon as the nail polish has spread out, pull the shape straight up out of the water, through the nail polish. If it ends up with a slimy-looking trail hanging off it, cut this off after it has dried.

6) Hang the shape up to dry. As it's drying, look at the light reflecting off the surface. You should be able to see the thin film interference from the layer of wet nail polish.

7) As the polish dies, it will keep its thin film characteristic and remain locked in this shape forever. The irregularities in the film will lead to a swirly effect much like for oil on water - perhaps you can think of ways this proceedure can be modified to minimize the irregularity.

When it has dried there are several things you can think about:

1) hold it so you can see light reflecting off its surface. Try holding it at different angles and see how the colors change. As you tilt the sheet away from you, the path difference will increase. This means that the perfect wavelength for a particular part of the film will also get longer (untill the path difference once more corresponds to a short part of the visible spectrum). From the order of colors you see as you tilt the card, what can you infer about the wavelengths of the colors?

2) As each part of the film reflects only a specific color (at a fixed angle), you have made a crude interference filter - in essence a devide that tells you how much of each color light is in the light that strikes it. For example, in sun light or under reasonable white light illumination, you should get all the colors (with a perfect film - it is quite posisble that irregularities might make it hard to observe some colors with your particular film), where as when you look at the yellow sodium street lights, you might be able to see the narrow bands of yellow (and a few other colors) that distinguish sodium fom other elements.

3) What other experiments can you think of to utilize your film?

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Make Antibubbles in your own Kitchen

Antibubbles are the exact opposite of bubbles.

What you need

1) Water

2) Clear bowl or tank

3) Dishwashing liquid/detergent

4) Beaker, jug or squirt bottle (tomato sauce or mustard bottles tend to work well)

What to do

1) Fill the clear bowl to the very brim with water with a few squeezes of washing-up liquid in it)

2) Keep some more of this liquid in the jug for the next step

3) Gently pour (or squirt) the liquid from the jug (or bottle) onto the surface of the bowl.

4) Watch beneath the surface as you pour and vary the speed at which you pour. If you are using a squirt bottle, you can also vary the angle you squirt at.

5) When you find the right speed, you will see antibubbles form as the stream of water breaks up beneath the surface.

6) You can now watch these antibubbles move and sink downwards and they will eventually burst.

What to look for

Antibubbles are the exact opposite of bubbles: where bubbles are thin surface of fluid in air surrounding a pocket of air, an antibubble is a thin surface of air in fluid surrounding a pocket of fluid.

You can watch these antibubbles move and sink downwards and they will eventually burst.

Dr Stéphane Dorbolo (who recently published research into the mechanisms of antibubble formation) said: "Antibubbles are mysterious phenomena but we now understand them much better. We have come up with a good model describing how they form and move and have also learnt more about the type of liquids you can create them in. We tried to create them in beer for fun, and didn't think it would be possible, but were amazed when we magaged to create giant antibubbles which lasted for almost two minutes and that moved around a glass of beer before bursting. "You can't create antibubbles in pure water, alcohol or oil. But beer is a special case because it is very similar to dishwashing liquid and contains what we call surfactants which is what you need to be able to produce antibubbles" We also found that when they die, or burst, they morph into a form of structure which we have nicknamed the jellyfish form because it looks very like a jellyfish swimming through water. It slowly moves and fades away until it disappears altogether."

What happens

The dishwashing liquid is a surfactant that tends to form membranes separating fluids and air. As you pour the liquids together, at the right speed, a thin layer of air can be trapped between the two bodies of fluid. As the fluids combine, a part of this film can wrap around a pocket of the fluid, forming an antibubble.

Things you can do:

If you disolve salt in the water that you pour in, this water will be denser than the water around it, so the antibubbles will sink.
If you color the water int he jug, you can also make the antibubbles more visible, as well as assuring your self that the antibubbles actually contain water from the jug.

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Candy Triboluminescence

Glow-in-the-dark fun!

For several decades people have been playing in the dark with triboluminescence using wintergreen-flavored Lifesavers candy. The idea is to break the hard, donut-shaped candy in the dark. Usually a person looks in a mirror or peers into a partner's mouth while crunching the candy to see the resulting blue sparks.

Triboluminescence is light produced while striking or rubbing two pieces of a special material together. It is basically light from friction, as the term comes from the Greek tribein, meaning "to rub," and the Latin prefix lumin, meaning "light". In general, luminescence occurs when energy is input into atoms from heat, friction, electricity, or other sources. The electrons in the atom absorb this energy.

When the electrons return to their usual state, the energy is released in the form of light.

The spectrum of the light produced from the triboluminescence of sugar (sucrose) is the same as the spectrum of lightning. Lightning originates from a flow of electrons passing through air, exciting the electrons of nitrogen molecules (the primary component of air), which emit blue light as they release their energy. Triboluminescence of sugar can be thought of as lightning on a very small scale. When a sugar crystal is stressed, the positive and negative charges in the crystal are separated, generating an electric potential. When enough charge has accumulated, the electrons jump across a fracture in the crystal, colliding with and exciting electrons in the nitrogen molecules. Most of the light emitted by the nitrogen in the air is ultraviolet, but a small fraction is in the visible region. To most people the emission appears bluish-white, although some people discern a blue-green color (human color vision in the dark is not very good).

The emission from wintergreen candy is much brighter than that of sucrose alone because wintergreen flavor (methyl salicylate) is fluorescent. Methyl salicylate absorbs ultraviolet light in the same spectral region as the lightning emissions generated by the sugar. The methyl salicylate electrons become excited and emit blue light. Much more of the wintergreen emission than the original sugar emission is in the visible region of the spectrum, so wintergreen light seems brighter than sucrose light.

Triboluminescence is related to piezoelectricity. Piezoelectric materials generate an electrical voltage from separation of positive and negative charges when they are squeezed or stretched. Piezoelectric materials generally have an asymmetric (irregular) shape. Sucrose molecules and crystals are asymmetric. An asymmetric molecule changes its ability to hold electrons when squeezed or stretched, thus altering its electric charge distribution. Asymmetric, piezoelectric materials are more likely to be triboluminescent than symmetric substances. However, about a third of known triboluminescent materials are not piezoelectric and some piezoelectric materials are not triboluminescent. Therefore, an additional characteristic must determine triboluminescence. Impurities, disorder, and defects are also common in triboluminescent materials. These irregularities, or localized asymmetries, also allow for electrical charge to collect. The exact reasons why particular materials show triboluminescence can be different for different materials, but it is probable that crystal structure and impurities are primary determinants of whether or not a material is triboluminescent.

Glow-in-the-dark fun!

Wint-O-Green Lifesavers aren't the only candies that exhibits triboluminescence. Regular sugar cubes will work, as will just about any opaque candy made with sugar (sucrose). Transparent candy or candy made using artificial sweeteners will not work. Most adhesive tapes also emit light when they are ripped away. Amblygonite, calcite, feldspar, fluorite, lepidolite, mica, pectolite, quartz, and sphalerite are all minerals known to exhibit triboluminescence when struck, rubbed, or scratched. Triboluminescence varies widely from one mineral sample to another, such that it might be unobservable. Sphalerite and quartz specimens that are translucent rather than transparent, with small fractures throughout the rock, are the most reliable.

There are several ways to observe triboluminescence at home.

If you have wintergreen-flavored Lifesavers handy, get in a very dark room and crush the candy with pliers or a mortar and pestle. Chewing the candy while watching yourself in a mirror will work, but the moisture from saliva will lessen or eliminate the effect.
Rubbing two sugar cubes or pieces of quartz or rose quartz in the dark will also work. Scratching quartz with a steel pin may also demonstrate the effect. Also, sticking/unsticking most adhesive tapes will display triboluminescence.
For the most part, triboluminescence is an interesting effect with few practical applications. However, understanding its mechanisms may help explain other types of luminescence, including bioluminescence in bacteria and earthquake lights. Triboluminescent coatings could be used in remote sensing applications to signal mechanical failure. One reference states that research is underway to apply triboluminescent flashes to sense automobile crashes and inflate air bags.

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