Asymmetric oil-water-air films

The experiments show, however, that many oils with positive B and E coefficients might have low antifoam efficiency. In these cases, the stability of the asymmetric oil-water-air films is very high, and the formation of unstable oil bridges becomes impossible for kinetic reasons (factor 2 is decisive). The repulsive interaction that should be overcome for effectuation of the antifoam globules entry on the solution surface is usually termed the entry barrier. A recently... 

FIGURE 5.63 Schematic explanation of the optimal hydrophobicity of the solid particles in oil-solid antifoam compounds, (a) When an antifoam globule approaches the foam film surface, an asymmetric oil-water-air film of thickness forms, (b) The zone of contact in an enlarged scale. If the protrusion depth, dj,, of the solid particle is larger than the particle should be sufficiently hydrophobic (9 + 9o > 180°) to pierce the air-water interface and induce a film rupture however, if the particle is insufficiently hydrophobic (9 + 9q < 180°), it would stabilize the film. (c) On the other side, if the solid particle is overhydrophobized, then < PR < As and the particle is again unable to pierce the asymmetric fihn. ... 

A third mechanism of liquid-film breakage is observed when there is a transport of solute across the film (see Fig. 10c). This mechanism, investigated experimentally and theoretically by Ivanov and coworkers (109—111), was observed with emulsion systems (transfer of alcohols, acetic acid, and acetone across liquid films), but it could appear also in some asymmetric oil-water-air films. The diffusion transport of some solute across the film leads to the development of Marangoni instability, which manifests itself as... 

From a theoretical analysis, it was shown that positive values of B correspond to bridge de-wetting. However, although values of E,S and B can be determined from measurements of the interfacial tension, these models do not account for the rupture barrier of asymmetrical oil-water-air films which appear when the oil approaches the foam film surface. 

FIG. 1 Formation of asymmetric oil-water-air films (shaded areas) in two of the possible mechanisms of foam destruction by oil drops or lenses bridging-stretching (a-c-d) and (b-c-d) [11,12] bridging-dewetting (a-c-e) and (b-c-e) [2-6],... 

FIG. 9 Foam destruction by oil drops (slow antifoams) [13,14]. (a and b) The oil drops are rapidly expelled from the foam films into the neighboring Gibbs-Plateau borders (GPBs) soon after the foam is formed, (c) The larger drops are strongly compressed in the narrowing GPBs and asymmetric oil-water-air films are formed, (d) Drops of radius smaller than remain noncompressed and cannot induce... 

The observed sharp increase of at Cs > 9 mM is probably related to the stabilizing effect of the surfactant micelles trapped in the asymmetric oil-water-air film [3,10]. One can estimate that the effective volmne frac-... 

FIG. 13 Determination of the disjoining pressure. Das, in the asymmetric oil-water-air film [17]. (a) Schematic presentation of an oil drop trapped in a wetting film Pw + Has = Pf is the pressure in the asymmetric film, (b) Calculated critical disjoining pressure 11 as a function of the inverse effective film radius [see... 

A systematic experimental study was performed to clarify further the role of the entry barrier in the foam destruction by oil-based antifoams. The critical capillary pressure, P, which leads to rupture of the asymmetric oil-water-air film (formed between a pre-emulsified oil drop and the solution surface) and to subsequent drop entry, was measured by the film trapping technique (Fig. 2)—for brevity, is denoted as the entry bar-... 

When an oil drop in an aqueous phase rises to the surface of the solution or an oil drop approaches a bubble inside a foam an asymmetrical, oil/water/oil film, the so-called pseudoemulsion film forms between the oil and air phases (Figure 8.) The importance of... 

Initially devised to measure interactions in single soap films (air/water/air) [8], the TFB technique has been progressively improved and its application has been broadened to emulsion films (oil/water/oil) [ 12] and asymmetric films (air/water/oil or air/water/solid) [13,14]. In a classical setup, a thin porous glass disk is fused on the side to a capillary tube and a small hole is drilled in the center of the disk. The liquid solution fills the disk, part of the capillary, and a thin horizontal film is formed across the hole. The disk is enclosed in a hermetically sealed box, with the capillary tube exposed to a constant reference pressure Pr. Under the effect of the pressure difference AP between the box and the reference, the... 

A stratification phenomenon in foam films from sodium oleate aqueous solutions has been described by Johonnott [314] and Perrin [318] in the beginning of the century. The stepwise thinning has been studied by many authors [e.g. 235,345], It became evident that this phenomenon is universal and has been observed also in emulsion films [346,347], asymmetric films of the air-water/oil type [348], films from latex suspensions [349] and liquid crystalline films [350],... 

As we will discuss at length in Chapters 3 and 4, the stability of the asymmetric air-water-oil films that separate oil antifoam drops from the relevant gas phase is a... 

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