ie-Chemistry P1
Thickness of a Bubble
estimating the size of a molecule


Chemists have traditionally called the study of collections of molecules physical chemistry.

Communities of people have some properties which are the result of individuals but only become apparent in a collective group such as the wave of a crowd moving in unison.  Likewise communities of chemical substances have group properties such as pressure, electric current, and freezing temperature.  Physical chemists study such properties then use them to help understand individual events on the molecular scale.  Atoms and molecules are so small that our senses are generally unable to directly learn what occurs at that size.  So the use of these bulk properties provides a chest of tools to supplement our senses.

Oil Sheen Experiment

FranklinTo gain an appreciation for the size of molecules, this investigation attempts to measure the thickness of some of the thinner objects in our world, soap bubbles and oil films.  This first procedure was described by Benjamin Franklin (b1706, d1790, portrait at right→).  Franklin wrote in 1773 a letter to a friend, mentioning having read in Pliny's Natural History (Gaius Plinius Secundus, b23AD, d79, better known as Pliny the Elder) a description of the practice of early seaman of stilling the waves in a storm by pouring oil into the sea.  Franklin described confirming that at the pond at Clapham Common in England: At length at Clapman where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropped a little of it on the water.  I saw it spread itself with surprising swiftness upon the surface.. the oil, though not more than a teaspoonful, produced an instant calm over a space several yards square, which spread amazingly and extended itself gradually until it reached the leeside, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.  Franklin wrote he was struck by it with particular surprise.  Franklin realized that the spreading of the oil could reveal crucial information about be size of atoms which scientists of the time suspected were the smallest constituents of the oil.  As the oil spread more widely, the layer grew thinner.  But the oil layer can be no thinner than the size of its constituent particles.  By measuring the original volume of oil which is essentially conserved during the experiment, and then dividing by the area of the oil sheen, one could compute the largest possible size of an atom.  A century later, the physicist Lord Rayleigh (John William Strutt, b1842, d1919 who won the 1904 Nobel Prize in Physics for othe matters) use this procedure to measure the size of a molecule.  In the first decades of the 20th Century, Irwing Langmuir (b1881, d1957) did systematic studies on floating monolayers on water to learn about molecular shape and contortions, a study which led to his receiving the 1932 Nobel Prize in Chemistry.

Today we can gain similar understanding by repeating Franklin's experiment.  The amount of oil must be tiny enough and the water surface large enough that the oil can spread without encountering the edge of the water.  To measure the thickness of the oil layer, the surface of the water should be as calm and smooth as possible to avoid puddling.  In addition a means is needed to determine the extent of the spread of the oil.

Sheen Procedure

  1. If you don't own your own pond, be very careful and environmentally considerate if you decide to use one belonging to others.  You may want to use a biodegradable oil such as a vegetable oil, and then only use the minimum drop or two necessary.  You can measure the volume of a drop by counting the number of drops required to fill a known volume such as a cubic centimeter or two.  (drop volume = total volume / number of drops contained)

  2. If you cannot easily access a pond, it is possible to obtain reasonable measurements on a large but shallow tray of water if you dilute the oil with a known amount of alcohol (perhaps one drop of oil diluted with 100 drops of alcohol).  Once a drop of oil and alcohol mixture is put on the water's surface, the alcohol both evaporates and dissolves in the water leaving only the fractional drop of oil stranded on the surface.  If you wish to repeat the experiment, you need to remove the previous oil sheen.

  3. It is often difficult to see the edges of an oil sheen.  Placing an extremely light coating of very fine powder on a small patch of water before the oil often helps to make the edges visible.  If you use this trick, try to find the finest powder available such as the spores of Lycopodium moss.

  4. Calculate the thickness of the oil sheen by dividing the volume of oil by the estimated surface area.

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.

Bubble Experiment

The second procedure involves using the wave nature of light to estimate the thickness of a soap bubble.  Light is a transverse electromagnetic wave.  Both the electric field and the magnetic field increase and decrease perpendicular to the direction of the light's direction.  When the light encounters the electric field (due to surface electrons) in some materials, the interaction causes the emission (i.e., reflection) of a similar light wave.  Depending on the surface, this may be a mirror like reflection where the angle of reflected light matches the incidence angle, or scattered widely such as by the surface of white paper.

bubbleA bubble has both an inside and outside surface, both of which may reflect light.  If the bubble has the thickness equal half the wavelength of the light, the reflected waves from the inside will be one wavelength behind those reflected from the outside surfaces.  These waves will match and so will combine in such a way that the amplitudes of their electric and magnetic fields will add together making a light wave twice as bright as either separate reflection.  But in the surfaces are a quarter wavelength apart, the light wave reflected from the inside will be a half wavelength behind that reflected from the outside surface.  The waves will be exact opposites so that their combined wave intensities nearly cancel leaving almost no reflected light.  White light is a combination of all colors, such that each color of is a slightly different wavelength extending from red light with a wavelength of about 7 x 10-7m to violet light with a shorter wavelength of about 4 x 10-7m.  The result is that soap bubbles, depending on their thicknesses, may cause bright constructive combinations for some light colors and the destructive combination of light of other colors.  If the bubble is too thin, there may be no constructive reflection and so the surface will seem to disappear with no reflection at all.

Bubble Procedure

  1. Make bubble solution,
    1. mix 10 to 20 ml of Dawn®, Joy® or other dishwashing liquid with 5 ml of glycerol.
    2. Add this to 85 ml of water.
    3. Gently stir to combine but avoid prematurely making suds by not shaking.
    4. Increasing the amount of detergent may produce longer lasting bubbles.  If you are having difficulty forming bubbles, distilled or deionized water may improves results.  Metal ions often in drinking water react with the soap.

  2. bubble colorsDip straw in soap solution, enough to blow a bubble.  Keep making bubbles.  Watch the color patterns on the bubble.  The colored pattern may be easier to see if the background is dark or black.  The colors will slowly go around the bubble and form rings.  At some point a black area (or area with no color except the background appearing through the totally transparent bubble) may form somewhere, perhaps near the top of the bubble.  Eventually the bubble will pop.

  3. If a black (or totally transparent) area does form in the bubble, what does that imply about the thickness of the soap bubble at that point?

  4. From the progression of colors, is it possible to estimate the wedge shape of the bubble and estimate a reasonable thickness for the thinest spot on the bubble?  Why does the rainbow of colors repeat at in the photograph at right?  If you can't observe any transparent or black region, which of the colored patterns might you be seeing?  If that is the case, what conclusion can you make about the thickness of the bubble?

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.


John Nees, Using Bubbles to Learn about Light Interference


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created 3 May 2007
revised 7 July 2009
by D Trapp
Mac made