Wetting thin-liquid films

The inability to obtain general analytical solution of the problem makes the linear instability analysis the most widely applied mathematical approach [487,537,538]. In Secs. Vni.B.l and VIII.B.2, we review the application of this approach to free and wetting thin liquid films. 

The theory of seaweed formation does not only apply to solidification processes but in fact to the completely different phenomenon of a wettingdewetting transition. To be precise, this applies to the so-called partial wetting scenario, where a thin liquid film may coexist with a dry surface on the same substrate. These equations are equivalent to the one-sided model of diffusional growth with an effective diffusion coefficient which depends on the viscosity and on the thermodynamical properties of the thin film. 

Fig. 30. Different types of the momentary morphologies which are typically observed during wetting (a-c) and dewetting (d-f) events on a solid flat substrate, a droplet, b spherical cap with a precursor film.c thin film (eventually with a multilayer structure), d thin liquid film, e ruptured film with rims at the dewetting front,f droplets...

A foam consists of a high volume fraction of gas dispersed in a liquid where the liquid forms a continuous phase. Wet foams with a high water content, e.g. immediately after the formation, can have more or less spherical bubbles. As a consequence of a drainage process of the foam lamellae, the wet foam loses water with time. Due to the resulting high volume fraction of gas, the bubbles are no longer spherical but they are deformed into a polyhedral shape. The polyhedra are separated from each other by thin liquid films. The intersection lines of the lamella are termed plateau borders (see Figure 3.28). 

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. 

A third aspect is the close analogy with wetting phenomena. One Important issue is whether or not a thin liquid film on a surface is stable or whether it spontaneously (that is, even without a second wall nearby) disproportionates into a droplet in contact with a very thin (usually (sub)-monolayer) film. In wetting language this disproportionation (usually in the other direction, as... 

For foam separation processes, adsorption takes place in solution, the essential basis exists for solute separation by foaming. Foam consists of gas bubbles separated by thin liquid films. The liquid films are often formed by the mutual approach of two already existing liquid surfaces (e.g., two bubbles below the surface). Foam structures may vary between two extreme situations. The first is wet foam, which consists of nearly spherical bubbles separated by rather thick liquid films. The second is dry foam, which may develop from the first type as a result of drainage (i.e., foam drainage). 

FIGURE 5.17 Flotation (a, c, e) and immersion (b, d, f) lateral capillary forces between two particles attached to fluid interface (a) and (b) are two similar particles (c) is a light and a heavy particle (d) is a hydrophibc and a hydrophobic particle (e) is small floating particles that do not deform the interface (f) is small particles captured in a thin liquid film deforming the interfaces due to the wetting effects. 

A reversed approach using a volatile Si-source (tetraethoxysilane, TEOS) was applied to prepare MFI membranes by wetting a porous alumina substrate with TPAOH-solution and exposing it to vapors of TEOS at elevated temperatures.[72] In this case the TEOS must hydrolyze and gel in situ in the thin liquid film on the porous substrate. 

Wetting phenomena are not only restricted to extended solid/liquid bulk phases. Droplets of a liquid in contact with a solid surface for instance can transform into a thin liquid film, called wetting film. Its stability on a solid surface, for example on polished quartz, is determined by all components acting in thin liquid films (cf. Appendix 2D) long-range forces, van der Waals forces, and double-layer forces. 

Unlike the wet foam calculation where primary viscous stresses are localized within Plateau border regions and derived from interfacial viscous properties, the total viscous stress for a dry foam (i.e., dispersed-phase volume fraction approaching 1) is distributed throughout the thin liquid films. The leading-order dilatational viscosity of a dry foam composed of a spatially periodic array of tetrakaidecahedron (39) bubbles was given by Edwards and Wasan (40) as follows ... 

Foam A dispersion of gas bubbles in a liquid or solid in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae). Sometimes distinctions are drawn between concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral (Dry Foam and Polyederschaum), and low concentration foams in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical (Gas Emulsions, Gas Dispersions, Wet Foams, and Kugelschaum). Bulk foams may also be distinguished from lamellar foams. See also Aerated Emulsion, Foam Texture, Froth. 

Fig. 1 Rim formation in the course of de wetting of a thin liquid film. At the front position F of the rim, the contact angle assumes Its dynamic value and at the rear position R it takes the value .

If the thermodynamic properties of the thin liquid films formed between particles and the subsequent chemical reactions are ignored, the phenomena that oeeur when water is added to leady oxide can be explained simply as follows (a) initially, PbO particles are hydrated and their surface is covered by absorbed water (b) at the sites of eontaet between the particles a wedge of liquid is formed (c) this wedge forms water rings on the surface of partially wetted particles in the contact regions (d) these rings exert eohesion forees which hold the separate particles together in a rather loose system (Fig. 6.22) [27]. 

The apparatus used for studying thin liquid films is schematically depicted in Fig. 6. This device, commonly known as a thin-film balance, allows drainage patterns of single foam, emulsion, or wetting films to be recorded. The film is formed in a specially constructed cell that is placed on the state of an inverted microscope. The reflected light from the film is split into two parts, one directed to a CCD camera and another to a fiber-optic probe tip located in the microscope eyepiece. The radius of the tip is only about 20... 

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