Contact angle phenomena and wetting

Israelachvili, J. N., Intermodular and Surface Forces, 2d ed., Academic Press, London, 1991. (Undergraduate and graduate levels. Contains good discussions and illustrations of the relation among surface tension, wetting, and contact angle phenomena and van der Waals forces.)... 

The basic framework for the application of contact angles and wetting phenomena lies in the field of thermodynamics. However, in practical apphcations it is often difficult to make a direct correlation between observed phenomena and basic thermodynamic principles. Nevertheless, the fundamental validity of the analysis of contact angle data and wetting phenomena helps to instill confidence in its apphcation to nonideal situations. 

Why does a drop of pentane spread into a thin film when placed on a water surface, whereas a larger hydrocarbon such as dodecane breaks up into smaller droplets This is not an academic question, as should be evident from the importance of wetting and contact angle phenomena that we discussed in Chapter 6. Why is it that we can produce relatively stable bubbles with a soap solution but not with pure water Water droplets on an oily surface, dewdrops on a blade of grass, and soap bubbles or foams are so common in our daily life that they rarely engage our attention, but to a scientist they are a constant reminder of the ubiquitous van der Waals forces ... 

Before we can discuss the experimental techniques used to measure the surface tension, we need to introduce the so called contact angle 0. When we put a drop of liquid on a solid surface the edge usually forms a defined angle which depends only on the material properties of the liquid and the solid (Fig. 2.8). This is the contact angle. Here we only need to know what it is. In Chapter 8, contact angle phenomena are discussed in more detail. For a wetting surface we have 0 = 0. 

De Gennes (1985) analysed the static and dynamic problems of different wetting phenomena. In this book only the dynamics of spreading and the wetting transition are of interest. Wetting transition takes place at a characteristic temperature where the contact angle diminishes and a wetting film is formed. 

Obviously, contact angle measurements and their interpretation are not without their hidden pitfalls and blind alleys. However, because of the ease of making such measurements, the low cost of the necessary apparatus, and the potential utility of the concept, they should be seriously considered as a rapid diagnostic tool for any process in which wetting phenomena play a role. 

The practical aspects of dynamic contact angle phenomena, then, center around determining the maximum wetting rates that can be attained before entrainment or wetting failure occurs, and how a system can be modified to increase that maximum velocity, since in many, if not most, cases, speed is money. Liquid coating operations are obviously impacted by the dynamic restraints of a system. If one can increase the maximum speed of coating a substrate from, say, 200 to 400 m min productivity gains will be impressive. 

Figure 3.11 Capillary rise phenomena for (a) a wetting liquid (contact angle <90°) and (b) a nonwetting liquid (contact angle >90°).

AU theories, in combination with contact angle data and information from liquid interfaces, will be used later in the book (Chapter 6) for estimating the interfacial tensions of solid-interfaces (solid-liquid, soUd-soUd) and for characterizing solid surfaces and thus for understanding important phenomena such as wetting, lubrication and adhesion. Charter 15 offers a more detailed presentation of theories for estimating the interfacial tension as weU as some comparisons between them. 

Fig. 1. Physical phenomena governed by contact angle (a) extremely limited wetting and spreading, tendency to retract, does not penetrate (b) limited wetting and spreading, no tendency to penetrate (c) extensive wetting and spreading, strong tendency to penetrate.

The next two chapters are concerned with wetting and capillarity. Wetting phenomena are still poorly understood contact angles, for example, are simply an empirical parameter to quantify wettability. Chapter 6 reviews the use of scanning polarization force... 

Trapaga and Szekely 515 conducted a mathematical modeling study of the isothermal impingement of liquid droplets in spray processes using a commercial CFD code called FLOW-3D. Their model is similar to that of Harlow and Shannon 397 except that viscosity and surface tension were included and wetting was simulated with a contact angle of 10°. In a subsequent study, 371 heat transfer and solidification phenomena were also addressed. These studies provided detailed... 

Surface tension and contact angle, wetting phenomena, effects of the curvature of the surface on capillarity and phase equilibria, and porosimetry (Chapter 6)... 

As mentioned in Section 6.1a, surface tension and contact angle determine wetting phenomena we examine this in Section 6.6. We take a closer look at the definition of contact angle and some complications associated with it in Section 6.7. 

Young s equation is the basis for a quantitative description of wetting phenomena. If a drop of a liquid is placed on a solid surface there are two possibilities the liquid spreads on the surface completely (contact angle 0 = 0°) or a finite contact angle is established.1 In the second case a three-phase contact line — also called wetting line — is formed. At this line three phases are in contact the solid, the liquid, and the vapor (Fig. 7.1). Young s equation relates the contact angle to the interfacial tensions 75, 7l, and 7sl [222,223] ... 

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