Thin-liquid-film elasticity

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

Thin-liquid-film stability. The effect of surfactants on film and foam stability. Surface elasticity. Froth flotation. The Langmuir trough and monolayer deposition. Laboratory project on the flotation of powdered silica. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

In the case of interactions between inclusions in lipid bilayers (Figure 5.19) the elasticity of the bilayer interior must also be taken into account. The calculated energy of capillary interaction between integral membrane proteins turns out to be of the order of several Hence, this interaction can be a possible explanation of the observed aggregation of membrane proteins. The lateral capillary forces have been calculated also for the case of particles captured in a spherical (rather than planar) thin liquid film or vesicle. ... 

Gibbs (16) noted an elasticity associated with a liquid film if the surface tension varies with the area of the surface for a thin liquid film of area s, the Gibbs elasticity is given by... 

For the transport of heavy ions to a solid surface coated with an adherent water film, like aluminium oxide, the visco-elastic properties of electric field forces and the concentration of heavy ions may be important for the rate of adsorption. For this reason we need information not only on relaxations restricted to a surface of an extended liquid, but also on the adherent water layer at the adsorbents. The last issue may be a bridge to the thermodynamics and flow properties of thin liquid films have been studied by some excellent research groups. 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

The role of surface viscosity and elasticity on the motion of a solid particle trapped in a thin film, at an interface, or at a membrane of a spherical vesicle has been recently investigated in Refs. [856,857]. The theoretical results [856,857] have been applied to process the experimental data for the drag coefficient of polystyrene latex particles moving throughout the membrane of a giant lipid vesicle [858-864]. Thus, the interfacial viscosity of membranes has been determined. The motion of particles with different shapes trapped in thin liquid films and at Langmuir monolayers is studied intensively both theoretically and experimentally because of biological and medical applications [865-887]. 

Rheological properties of foams (elasticity, plasticity, and viscosity) play an important role in foam production, transportation, and applications. In the absence of external stress, the bubbles in foams are symmetrical and the tensions of the formed foam films are balanced inside the foam and close to the walls of the vessel [929], At low external shear stresses, the bubbles deform and the deformations of the thin liquid films between them create elastic shear stresses. At a sufficiently large applied shear stress, the foam begins to flow. This stress is called the yield stress, Tq- Then, Equation 4.326 has to be replaced with the Bingham plastic model [930] ... 

Figure 8 Mechanical analog of the SFA experimental setup used for measurements of friction forces Fo. A block of mass m which is confining a thin liquid film of hexadecane between atomically smooth mica is pulled laterally at velocity v. The lateral forces, F, are measured with an elastic...

While we have presented results only for unstable thin liquid films, it should not be ignored that similar instability processes occur also in (visco)-elastic " or even solid films. " We also did not discuss related problems of the consequences of density gradients or the influence of additives. As an alternative to long-range vdW forces, destabilization may also be induced by an electric field. These concepts can also be extended to bilayer systems. " For a review of technological applications of dewetting, see chapter 14. 

In a wet foam, nearly spherical bubbles press against each other, creating small circular facets separated by thin liquid films, with both elastic and viscous interactions. Elastic forces in a wet foam are associated with the potential energy stored... 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Vn is the normal velocity of the surface caused by the atoms redistribution and ps is the surface density of atoms. The surface chemical potential is typically determined by the film elastic energy and surface energy, and as such it is a function of the him local thickness as well as its slope, curvature, and may be higher spahal derivahves (see below). For very thin hlms (a few atomic layers) wetting interactions between the him and the substrate can also become important. These interachons are somewhat similar to wetting interactions between a liquid him and a solid substrate. They are responsible for the presence of an ultra-thin wetting layer of the him material between the islands resulting from the him instabihty and depend on the him thickness and its slope. Naturally, this dependence decays rapidly with the increase of the him thickness. 

Surfactants improve foam formation by inducing film elasticity, which is helpful when the film is stretched as a gas bubble emerges from a liquid solution to become part of the foam matrix. As the film stretches, differentia surfactant adsorption at the interface leads to surface tension gradients and healing of the film (so it thins with approximately uniform thickness). The optimum surfactant concentration for foam formation (although not foam stability) is around the CMC. 

Lavrentovich, O.D., Pergamenshchik, V.M. Patterns in thin liquid crystal films and the divergence ( surfacelike ) elasticity. In Kumar, S. (ed.) Liquid Crystals in the Nineties and Beyond S, pp. 251-298. World Scientific, Singapore (1995)... 

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