Soap film interference effects

Interferometry is based on the fact that light reflected from the front and back interfaces of a film travels different distances, producing interference effects. The method has been applied to Langmuir-Blodgett films (Section XV-7) and to soap films (Section XrV-8) [147-149]. 

Let us now turn our attention to the effect of a thin monolayer of soap molecules present on both film surfaces. In the first place it was found (see Section III.B) by Bouchiat and Langevin that density and orientation fluctuations of the soap molecules (in the case of a single interface) make only a negligible contribution to the scattering process, except possibly at a (two-dimensional) critical point of the monolayer, or by addition of a few fluorescent soap molecules. Thus we are left with the optical effect of the monolayers on the interference of the light waves. It turns out (see Section VI) that this interference effect can be taken into account by redefining the (optical) film thickness. 

Polyferrocenylsilanes can be fabricated into films, shapes, and fibers using conventional polymer processing techniques. The dimethyl derivative 3.22 (R=R = Me), which has been studied in the most detail, is an amber, film-forming thermoplastic (Fig. 3.7a) which shows a Tg at 33°C and melt transitions (T ) in the range 122-145 °C. The multiple melt transitions arise from the presence of crystallites of different size, which melt at slightly different temperatures [65, 100). Poly(ferrocenyldimethylsilane) 3.22 (R=R =Me) can be melt-processed above 150°C (Fig. 3.7b) and can be used to prepare crystalline, nanoscale fibers (diameter 100 nm to 1 pm) by electrospinning. In this method, an electric potential is used to produce an ejected jet from a solution of the polymer in THF, which subsequently stretches, splays, and dries. The nanofihers of different thickness show different colors due to interference effects simUar to those seen in soap bubbles... 

The films are thinner at the top than at the bottom. After a while, the upper part often ends up becoming extremely thin, reducing to two soap monolayers sandwiching just a few molecules of water. Such films no longer produce optical interference effects instead, they look quite black. Newton studied these black films. Long before, the Assyrians had observed them and even used their unpredictable shapes (in horizontal geometries) to divine the future. 

---