Film on a Liquid Substrate

Suppose we place a drop of oil A (for instance, octane) on a surface of water B kept in a vessel. The spreading coefficient S — Jb — ia is negative, and the oil does not spread. Rather, it remains coalesced in a drop floating on the water s surface. As we keep adding more and more oil, lenses flattened by gravity (of thickness e Cc) form and expand until the entire surface is covered by a single stable film of thickness e e. At 

The energy of a liquid substrate B covered with a floating film A is given 

To make a thick, metastable film, all we need to do is siphon out an initially stable film down to a thickness less than Cc. To make an even thinner film, one technique is to deposit a thin film of liquid A in solution in a solvent C that is wetting and volatile. Left behind after the evaporation of (7 is a thin, unstable pellicle of A, 

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Bodiguel, H., and Eretigny, C., Viscoelastic dewetting of a polymer film on a liquid substrate, Eur. Phys. J. E, 19,185-193 (2006a). 

The previous discussion can be extended to the case of a liquid film on a liquid substrate. X. Noblin has studied the inertial dewetting of a film of water deposited on carbon tetrachloride, which is a highly hydrophobic surface, as evidenced by the large thickness of the puddles Cc = 7 mm. " The dewetting velocity is extremely high, of the order of a m/s for thin films. Since a liquid substrate constitutes an ideal surface, one can create much thinner metastable films than is possible on a solid and, therefore, attain larger velocities. Under such conditions, Culick s law is perfectly verified. 

We consider films that arc cither suspended in air or depo.sited on a low-viscosity substrate. The only difference between these two situations is the nature of the driving force Fa/ responsible for opening up the hole. Fm = 27 for a suspended film, whereas Fm = S for a film on a liquid substrate, the only purpost of which is to control the wettability and to provide an opportunity to create thicker films. 

FIG. 10 Formation of a dry patch (zone) in an unstable liquid film on a rigid substrate. 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

Figure 6-1. A sketch of the generic thin-film configuration for a liquid film on a solid substrate that is tilted to an angle a from the horizontal. The upper surface of the film is an interface.

The concept for the rupture of a free liquid film or a thin liquid film on a solid substrate is used in some applications, for example in the flotation process. Scheludko et al. (1968) have published first contact angle measurements for liquid films. 

Andrieu, Sykes), which can generate a shallow water shock (Buguin, Vorelle). On a liquid substrate, viscous dissipation no longer takes place in the rim, but rather in the substrate, and in a much reduced form. If the substrate has low viscosity, the behaviour resembles that in suspended films (Martin, Buguin) (see Fig. 1.30). 

FIGURE 7.12. Images of the spinodal dewetting of a nanoscopic film of PDMS deposited on a silicon wafer (a) for short times, showing the amplification of the thickness fluctuations (courtesy O. Rossier) (b) for long times (courtesy G. Reiter) (c) also for long times, but on a liquid substrate (courtesy A. Buguin). 

R. Yerushalml-Rozen and J. Klein, Stabilization of nonwetting thin liquid films on a solid substrate by polymeric additives, Langmuir, 11, 2806... 

A simplified schematics for three main separation techniques, namely GC, high-performance or high-pressure liquid chromatography (HPLC), and supercritical fluid chromatography (SFC), are shown in Fig. 3. In all cases, the analyte molecules in a mixture (such as Ml and M2) are partitioned between a liquid-phase film on a sohd substrate and a carrier flow (mobile phase). In GC, the carrier is a gas, most commonly helium (He), in HPLC, the carrier flow is a combination of common solvents, such as water, methanol, acetonitrile, etc., and in SFC the mobile phase is a supercritical fluid, usually CO2. The partition of analyte molecules between the carrier phase and the liquid (stationary) phase depends on many factors, such as volatility, polarity, and hydrophobicity/hydrophilicity of the analyte, the chemical composition of the liquid phase, the flow rate, and the temperature applied. This partition can be visualized as a flooding river carrying debris of different sizes and shapes (analyte... 

In systems of particles at a thin liquid film on a solid substrate, the relevant disjoining pressure can be formally approximated by an effective gravitational field [36] (see Section 2.3.1.1.). 

Before proceeding to the main subject of this chapter—namely, the behavior and properties of spread films on liquid substrates—it is of interest to consider the somewhat wider topic of the spreading of a substance on a liquid surface. Certain general statements can be made as to whether spreading will occur, and the phenomenon itself is of some interest. 

Films spread at liquid-liquid interfaces or on liquids other than water are discussed followed by the important effects of charged monolayers on water. Finally, the most technologically important application of Langmuir films, the Langmuir-Blodgett film deposited on a solid substrate, is reviewed. 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

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