Interference colors, soap bubble

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

The shapes of soap bubbles are due to surface tension, an important physical property of liquids. White light striking the bubbles gives brightly colored interference patterns. 

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 iridescent colors of the soap bubble arise from the interference of reflected light waves. The reflected light from the outer surface and the inner layer gives rise to this interference effect (Figure 1.17). The rainbow colors are observed as the bubble thickness decreases (and reaches magnitudes corresponding to that in the light waves) due to the evaporation of water. 

The variation of reflection from thin films (e.g., soap bubbles) of light of different wavelengths results in the perception of colors and is a fanuliar example of scattering interference. Less familiar is the variation in reflection of a particle beam, outlined above. However, once we recognize the wave nature of matter, we must expect particles to manifest the same sort of wave properties we associate with light. 

The usual soap bubbles observed in white light show bright interference colors. Their thickness ranges from 0.1 to 10 pm, and they are called "thick" films. When such thick films placed in vertical frames, their thickness varies. This variation can be easily calculated for insoluble surfactants, where Tg 0 and c = 0. In this case n= r RT(see (2.11)) with (2.17) gives that ... 

We can estimate the thickness of the fluid films in a soapy shampoo foam, or even a single soap bubble, by noting the beautiful reflected colors. This phenomenon is due to thin-film interference wavelengths comparable to the thickness of the film ( 300-700 nm) constructively interfere to produce distinct reflection colors. 

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 (few hundreds of Angstroms). The thinnest liquid film consists mainly of the bilayer of surface-active substance (SAS) (such as soap = 50 A) and some layers of water. The light interference and reflection studies show many aspects of these TLFs. 

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