Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 21

of lakes and oceans is also another common experience. Scientists have investigated
bubble formation extensively (Boys, 1959; Lovett, 1994; Birdi, 1997). It is also known
that soap bubbles are extremely thin and unstable. In spite of the latter, under special
conditions, soap bubble can be kept intact for long periods of time, which thus allows
one to study its physical properties (such as thickness, composition, conductivity,
spectral reflection, etc.).
The thickness of a bubble is, in most cases, over hundreds of micrometer in the
initial state. The film consists of a bilayer of detergent that contains the solution. The
film thickness decreases with time due to following reasons:
Drainage of fluid away from the film
Evaporation
Therefore, the stability and lifetime of such thin films will be dependent on these
different characteristics. This is evident from the fact that, as an air bubble is blown
under the surface of a soap or detergent solution, it will rise up to the surface. It may
remain at the surface if the speed is slow, or it may escape into the air as a soap
bubble. Experiments show that a soap bubble consists of a very thin liquid film with
an iridescent surface. But, as the fluid drains away and the thickness decreases, the
bubble approaches the equivalent of barely two surfactant molecules plus a few mol-
ecules of water. It is worth noting that the limiting thickness is of the order of two
or more surfactant molecules . This means that one can see with the naked eye the
molecular-size structures of thin liquid films (TLFs) (if curved).
As the air bubble enters the surface region, the soap molecules are pushed up and,
as the bubble is detached, it leaves as a TLF with the following characteristics (as
found from various measurements):
A bilayer of soap (approximately 200 Å thick) on the outer region contains the
aqueous phase.
The thickness of the initial soap bubble is of a certain micrometer.
The thickness decreases with time, and one starts to observe rainbow colors as
the reflected light is of the same wavelength as the thickness of the bubble
(a few hundred Angstroms).
The thinnest liquid film consist mainly of the bilayer of surface-active sub-
stance (such as soap = 50 Å) and some layers of water. The light interfer-
ence and reflection studies show many aspects of these TLFs.
The iridescent colors of the soap bubble arise from the interference of reflected light
waves. Reflected light from the outer surface and the inner layer cause this inter-
ference effect (Figure 2.9). Rainbow colors are observed as the bubble thickness
decreases due to the evaporation of water. Thicker films reflect red light, and blue-
green colors are observed. Lesser thin films cancel out the yellow wavelength, and
blue color is observed. As the thickness approaches the wavelength of light, all col-
ors are canceled out and a black (or gray) film is observed. This corresponds to 25
nm (250 Å).