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When the thickness $ (t, h)$ of a fluid phase decreases sufficiently (below a few in most contexts) it becomes a fluid film.

The properties of fluid films depend on the nature of the film phase and on that of each of the two neighbouring bulk phases. By analogy with the accepted emulsion nomenclature (§1.3) these films should be described, where appropriate, by three capital letters such as A for air, W for `water', O for `oil', and S for solid, separated by solidi, the middle letter indicating the film phase. For symmetrical films the first and last symbols are the same, e.g. A/W/A: water film in air, or W/O/W/: oil film in water, whereas for unsymmetrical films these are different, e.g. W/O/A: oil film between water and air.

The term soap film has been established by usage for A/W/A films stabilized by surfactants although it is not a film of soap, nor is the stabilizing surfactant necessarily a soap (cf. §1.6). The term lipid film has been similarly established for W/O/W films.

A film element is a small homogeneous part of a film including the two interfaces and any fluid between them.

Except for free-floating bubbles, films have to be supported by frames, bulk surfaces or by other films. The transition zone separating these from the film proper, always containing some bulk liquid, is called a Plateau border.

A thin film is often, but not always, unstable with respect to rupture, that is, the formation of a hole which permits coalescence or direct contact of the two phases which it separates. There may also be a thickness, or thicknesses, at which the film is stable or metastable with respect to small thickness changes. Such a film is said to be an equilibrium film. Unless the area of the film is small, its composition may not be the same over its area and the (metastable) equilibrium thickness may be characteristic of the local condition only.

For films other than equilibrium films the thickness is often non-uniform and changes more (mobile film) or less (rigid film) rapidly with time. These differences are often associated with differences in surface shear rheology.

A film often thins gradually to a thickness at which it either ruptures or converts abruptly to an equilibrium film This thickness is sometimes well enough defined statistically to be considered a critical thickness, $ t_c$ or $ h_c$. Rupture under these conditions characterizes unstable films, whereas transition to an equilibrium film characterizes (meta)stable films. Liquids yielding the former give no foam or only a transient one, lasting generally less than twenty seconds, whereas liquids giving (meta) stable films form much longer-lasting foams under the same conditions.

When viewed in reflected white light against a black background, transparent films show the classical interference colours of thin plates which permit an estimate of their thickness to be made. When of the order of 100 (1000 ) in thickness they appear white (silver film) and when thinner they appear gradually less intensely white, then grey and finally black. Hence, the term black film is a general one to designate films thinner than about $ \frac{1}{4}$ wavelength of visible light. Black films are often equilibrium films, but equilibrium films may be considerably thicker under some conditions.

In soap films, two types of equilibrium film are often observed, sometimes successively in the same system: one characterized by thicknesses of the order of 7 or more which varies significantly with minor changes in composition such as ionic strength, and the other having a lesser thickness relatively independent of such changes. It is recommended, that when a distinction is needed, the former be designated as common black films, and the latter as Newton black films. The current use of first or secondary for the common black film and of second, primary or Perrin's for the Newton black film is discouraged.

Conditions and quantities relating to the transition between common and Newton black films should be identified by the subscript $ N$, thus $ c_N$ or $ t_N$ or $ {\Delta}H_N$.

Stratified films are films in which more than two thicknesses coexist in a fixed configuration over significant periods of time.

No pure liquid is known to give stable A/W/A films and many surfactant solutions give them only above a rather sharply defined concentration, $ c_$bl. Above this concentration, under given experimental conditions, the film does not burst after it has thinned to $ t_c$ but gives equilibrium, often black, films.

The film tension, $ \Sigma_f$, of an equilibrium film in contact with the bulk phase is measured by the contractile force per unit length exerted by this film.

For a symmetrical film $ \Sigma_f/2$ may be called the surface tension of the film, $ {\sigma}_f$ or $ {\gamma}_f$. $ {\sigma}_f$ is generally lower than the bulk surface tension, $ {\sigma}_0$. When $ {\sigma}_f<{\sigma}_0$, the film and bulk phase form a macroscopic film contact angle, $ {\theta}$, analogous to the three-phase contact angle (see §1.2.2) and measured in the bulk phase between the limiting directions of the film and of the bulk liquid. $ {\theta}$ is related to the surface tensions of the two surfaces in contact by

$\displaystyle {\sigma}_f={\sigma}_0\cos{\theta}.$

Gibbs film elasticity, $ E$, pertains to a film element of a soap film changing in area at constant mass and is the differential change of its surface tension with relative change in area, $ A({\partial}{\sigma}/{\partial}A)_{T,p,n_i}$. Here $ {\sigma }$ is half the tension of the film element.

Some of the physical properties of a film such as its reflectivity for light or its parallel capacitance (for W/O/W or Hg/W/Hg films) are related to the film thickness. Determination of thickness from the measurement of such properties involves assumptions about the structure and properties of the film which at present are always somewhat uncertain and arbitrary. Unless the basic experimental data are reported, it is recommended that the method used in deriving from them any reported thickness or structure be given in sufficient detail to permit recalculation of these data in future work.

Because of this difficulty in obtaining accurately the thickness of a film, one sometimes expresses the experimental measurements in terms of an equivalent film thickness which approximates to some extent the true film thickness and can be determined unambiguously. Such a thickness should be indicated by an appropriate subscript.

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