Thin wetting

J. L. Harden, D. Andelman. Thermal fluctuations of thin wetting films on disordered solids. Langmuir 5 2547-2551, 1992. 

Distillation is a countercurrent process which maintains liquid-vapor equilibrium, usually in a vertical column containing packing, thin wetted walls, or bubble-cap... 

A. M. Cazabat, N. Fraysse, and F. Heslot, Thin wetting films, Colloids Surf. 52, 1-8 (1991). 

Interaction between double layers, one of the building bricks of colloid stability, is an important theme planned for Volume IV. It has a large number of spin-offs, in, for instance ion exchange, thin wetting films, free films, membranes, association colloids, vesicles, polyelectroljdes, emulsions and rheology. The dramatic influence of electroljrtes on these phenomena finds its origin in the changes in the double layer, discussed in this chapter. 

Two issues cloud this picture, however. First, for strongly wetting materials, thin wetting films cover all surfaces. These films provide connectivity in the strictest sense, but do not allow for measurable flow. Second, Dullien et al. showed that if sphere surfaces are roughened, wetting fluid continues to drain even after the rings become geometrically disconnected. 

Thin wetting films. The Scheludko s technique of thin wetting films was adapted to study the behavior of proteins at various interfaces and in particular or contact lenses. 

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. 

Figure 15. Different stages of coarsening of the initial periodic structure, yielding the formation of localized dots divided by a thin wetting layer - numerical solution of eq.(43) with the wetting potential (29) ho = 1.5 nm, w = 8.2 x 10 Jm , = 1.0, woi = 0.1),...

Most common in practice is a trickling flow that occurs at relatively low gas and liquid flow rates. Under these conditions in trickle-bed reactors (TBRs) complete liquid films around the particles would break up into partial films, rivulets and droplets and a so-called partial wetting trickling regime exist. It is easier for hydrogen to enter the no wetted or very thin wetted part of partially wetted catalyst pellets because the mass-transfer resistance between a gas and the particle sur-... 

Ribbing evenly spaced lines down the web Capillary number too high/coating too thin/wet coverage too low/Reynolds number too low/speed too fast/surface tension too high/gap too small. 

The wetting exhibited by some trisiloxane surfactants is so extensive and rapid that it has been referred to as superspreading [3]. Typically, a small drop (50 pL) of a diluted aqueous solution (0.1 % w/w) of such a trisiloxane spreads out on a hydrophobic surface such as a polypropylene sheet into a thin, wetting film approx. 80 mm in diameter within tens of seconds. This is about 20 times the area wetted by a 1 % w/w solution of a conventional organic surfactant such as a nonylphenol ethoxylate. 

When droplet-based multiphase flow is considered, there are two possible configurations. In the first, a drop forms a distinct contact line with the walls of the channel. In the other, a drop is separated from the boundaries by a thin wetting film. Because of the affinity of fluids and surfaces, hydrophilic fluids prefer to flow along hydrophilic channel surfaces, and hydrophobic fluids like to flow along hydrophobic surfaces. As hydrophilic liquids are imbibed into hydrophilic channels by capillary force, hydrophobic fluids flow into hydrophobic channels much more easily. 

Each droplet is spherical because it strives to minimize its surface area. In the same way, the droplets should aggregate to reduce the overall surface and form a single large sphere, going from Fig. 10.2(a) to (b). The energies involved in this coalescence are known from experiments like the Wilhelmy plate shown in Fig. 10.2(c). A thin wetted plate, width L, dips into the water and is sucked in by... 

If a thin wet film of emulsion adhesive is applied between two porous surfaces and these three components subjected to an adequate degree of compression, the initial state of the adhesive is one of a water phase containing discrete and stable polymer particles. Wetting of the substrate surfaces then takes place followed by the commencement of penetration of the water phase into the substrates (see Wetting and spreading). As this occurs, the solid content of the adhesive film rises and it becomes more viscous and tacky. Capillary forces in the substrate then continue to draw water (and, dependent on substrate porosity, possibly some polymer solids) from the adhesive until its solid content rises to the point where it coalesces into a continuous film. 

Distillation is a countercurrent process in which equihbrium is maintained between Uquid and vapor, normally in a vertical column containing packing, thin wetted walls, or bubble cap plates. The single-stage separation factor for distillation is given by the vapor pressure isotope effect. Because mixtures of isotopic molecules show little or no nonideality in either liquid or vapor phase (Jancso et al. 1993), liquid vapor separation factors should be virtually independent of isotope enrichment (plate number), and the analysis of the distillation process is... 

FIGURE 7.11. Thickness fluctuations of a very thin wetting film. 

If 5 > 0, the liquid wets the contact totally The interface always remains lubricated in the presence of a thin wetting film that is stable. Such a situation has been thoroughly studied, notably by A. Roberts. ... 

Formation of thin wetting layers on the windows hampers accurate measurement of the reflecthity R((o) and absorption K co) near the critical region. It is therefore necessary to determine the optical gap from measurements only at temperatures and densities for which significant... 

For further experiments, cetyl alcohol (CA), a surfactant insoluble in water whose adsorption on water surface significantly decreases the coefficient of condensation [14], was used. However, the problem arose of how to apply the surfactant on the meniscus surface in a narrow capillary. First, CA was adsorbed onto the inner capillary surface from its solution in ethanol. The capillary was filled with 0.5-1% solution, which was then displaced, retaining on the capillary surface a thin wetting film. After that the capillary was dried, filled with water, and sealed. Observation of evaporation was initiated after the capillary was broken inside the region filled with water, so that at the beginning there was no CA on the meniscus. 

S. I. Karakashev, K. W. Stckelhuber, R. Tsekov, C. M. Phan, and G. Heinrich, Tribology of thin wetting films between bubble and moving solid surface, Adv. Colloid Interface Sci., 210, 39-46 (2014). 

The spontaneous spreading of thin microdroplets on solid substrates is slow and can be conveniently monitored using spatially resolved ellipsometry. On the other hand, extended thin wetting films can be spin-coated on solids or deposited on liquid substrates and thereafter studied under a polarized microscope for nanometric thicknesses or a Brewster angle microscope for molecular ones. X-ray reflectometry provides complementary information. 

In principle, the analysis of the droplet profile near the triple line provides quantitative information on the shape of the "effective interface potential" or the excess free energy of a thin wetting film due to intermolecular interactions [see also chapter 3). In turn, the so-called "line tension"—or the energy density of the triple line—can be estimated by integration of the effective interface potential. Alternatively, the line tension can be inferred from the size dependence of the droplet contact angle according to the "modified Young equation" ... 

---