Interfacial wetting

Figure 3.12 Two liquids A (detergent) and B (oily soil) on a solid surface (a) separated and (b) in contact, yA and yB = wetting tensions, yAB = interfacial tension, R = interfacial wetting tension [3].

Microemuisions exist for values of the parameter y [in and see Eqs. (33) and (34)] less than 1 and greater than —1, with more negative values associated with more structure. As can be seen, the correlation function is an exponentially decaying oscillatory function of the separation r. On the other hand, for values of y > 1, the Fourier transform is simply a sum of two monotonically decaying exponential functions, and the liquid is unstructured. It is this difference in bulk behavior that proves crucial to the interfacial wetting behavior. 

Once more, we emphasize that our treatment has simplified the topic enormously, and only the most salient features were mentioned. Complications dne to the pressure dependence in blends and due to the gas-liquid transitions in polymer solvent-systems may lead to new effects. For example, for the interfaces between gaseous CO2 bubbles in a C16H34 matrix one obtains interfacial wetting of fluid CO2, which reduces the interfacial tension drastically and hence facilitates nucleation (118). In fact, an imderstanding of nucleation in polymer blends (119) and solutions (118) is just emerging. 

With respect to the quality of surfactants, a basic method is the measurement of interfacial wetting, as this factor has a decisive influence on structure formation and structural stability in creams. 

Most biodegradable composites being tested employ a thermoplastic matrix. There are inherent difficulties in interfacial wetting between linear thermoplastics and reinforcing fibres which limit optimum stress transfer between the fibre and matrix. This also deleteriously affects hydrolytic stability [63]. While the use of solid polymer without fibre has been considered [24, 71-73],... 

The enhancement of adhesion provided by asymmetric organotitanates and zirconates in reinforced composites appears to be a consequence of the interaction of multiple mechanisms, including ligand-specific interfacial wetting enhancement, primary chemical bond formation between the substrate particulate and resin matrix, and, in many instances, matrix repolymerization and reinforcement surface modification. The particular mechanism which is dominant in a specific application appears to... 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

Thus, to encourage wetting, 7sl and 7lv should be made as small as possible. This is done in practice by adding a surfactant to the liquid phase. The surfactant adsorbs to both the liquid-solid and liquid-vapor interfaces, lowering those interfacial tensions. Nonvolatile surfactants do not affect 7sv appreciably (see, however. Section X-7). It might be thought that it would be sufficient merely to lower ytv and that a rather small variety of additives would suffice to meet all needs. Actually it is equally if not more important that the surfactant lower 7sL> and each solid will make its own demands. 

Templeton obtained data of the following type for the rate of displacement of water in a 30-/im capillary by oil (n-cetane) (the capillary having previously been wet by water). The capillary was 10 cm long, and the driving pressure was 45 cm of water. When the meniscus was 2 cm from the oil end of the capillary, the velocity of motion of the meniscus was 3.6 x 10 cm/sec, and when the meniscus was 8 cm from the oil end, its velocity was 1 x 10 cm/sec. Water wet the capillary, and the water-oil interfacial tension was 30 dyn/cm. Calculate the apparent viscosities of the oil and the water. Assuming that both come out to be 0.9 of the actual bulk viscosities, calculate the thickness of the stagnant annular film of liquid in the capillary. 

By virtue of their simple stnicture, some properties of continuum models can be solved analytically in a mean field approxunation. The phase behaviour interfacial properties and the wetting properties have been explored. The effect of fluctuations is hrvestigated in Monte Carlo simulations as well as non-equilibrium phenomena (e.g., phase separation kinetics). Extensions of this one-order-parameter model are described in the review by Gompper and Schick [76]. A very interesting feature of tiiese models is that effective quantities of the interface—like the interfacial tension and the bending moduli—can be expressed as a fiinctional of the order parameter profiles across an interface [78]. These quantities can then be used as input for an even more coarse-grained description. 

A large number of studies concerned witli tliiol-tenninated molecules has been directed at tire preparation of tailored organic surfaces, since tlieir importance has been steadily increasing in various applications. Films of o> functionalized alkanetliiols have facilitated fundamental studies of interfacial phenomena, such as adhesion [190, 191], corrosion protection [192], electrochemistry [193], wetting [194], protein adsorjDtion [195, 196] or molecular recognition [197, 198, 199, 200 and 201] to mention only a few. 

In liquid-phase sintering, densification and microstmcture development can be assessed on the basis of the liquid contact or wetting angle, ( ), fonned as a result of the interfacial energy balance at the solid-liquid-vapour intersection as defined by the Young equation ... 

Figure C2.11.8. An illustration of the equilibrium contact (i.e. wetting) angle, ( ), fonned by the balance of interfacial energies for or a liquid (sessile) drop on a flat solid surface.

Other correlations based partially on theoretical considerations but made to fit existing data also exist (71—75). A number of researchers have also attempted to separate from a by measuring the latter, sometimes in terms of the wetted area (76—78). Finally, a number of correlations for the mass transfer coefficient itself exist. These ate based on a mote fundamental theory of mass transfer in packed columns (79—82). Although certain predictions were verified by experimental evidence, these models often cannot serve as design basis because the equations contain the interfacial area as an independent variable. 

Fig. 3. Two-dimensional schematic illustrating the distribution of Hquid between the Plateau borders and the films separating three adjacent gas bubbles. The radius of curvature r of the interface at the Plateau border depends on the Hquid content and the competition between surface tension and interfacial forces, (a) Flat films and highly curved borders occur for dry foams with strong interfacial forces, (b) Nearly spherical bubbles occur for wet foams where...

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