Concept of Contact Angle

Wetting is a fundamental interfacial phenomenon in which one fluid phase is displaced completely or partially by another fluid phase from the surface of a sohd or a liquid. The most usefiil parameter that may describe wetting is the contact angle of a liquid on the substrate and this is discussed below. 

Liquid remains as a discrete drop Complete wetting Incomplete wetting 

Wetting Line - Three-phase Line (Solid/Liquid/Vapour) 

In practical systems such as in spray applications, one has to displace one fluid (air) with another (liquid) as quickly and as efficiently as possible. Dynamic contact angle measurements (associated with moving wetting line) are more relevant in 

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Fig. 7. The concept of contact angle with a captive bubble in an aqueous medium, adhering to a hydrophobic sofld P is the three-phase contact point. Here, the vector passes through P and forms a tangent to the curved surface of the air bubble. The contact angle 0 is drawn into the Hquid.

For systems in which there is a three-phase contact line between the phases as a result of a solid phase, the concept of contact angle is introduced. For such systems, the Phase Rule remains the same (Li et al, 1989). For highly curved interfaces where the thickness of the heterogeneous region between the phases is not small compared to r, there are other considerations in the derivation of the Phase Rule (Li et al., 1989 Li, 1994). 

The concept of contact-angle hysteresis and metastable configurations can be illustrated, following the example of Eustathopoulos et al. [8] by... 

Such analysis is based on the theories presented in this chapter, the concept of the contact angle and the associated Young equation discussed in Chapter 4. The analysis of solid interfaces and its application in understanding wetting and adhesion will be illustrated in Chapter 6, after the concept of contact angle is presented in Chapter 4 and surfactants in Chapter 5. Theories for interfacial tension wiU be discussed in more detail in Chapter 15. 

The second approach is particularly usefid and builds on the theories of interfacial tensions (Chapter 3) and concepts of contact angle/Young equation (Chapter 4) presented previously. When these theories are applied to solid-liquid interfaces and combined with Young s... 

Accepting the concept of contact angle, its importance may be appreciated in many sectors, biological and industrial. Without resort to equations, it is intuitive (and correct) to realize that it is more difficult to remove an oil film from steel (contact angle ca. 0°) than a drop of mercury from glass (contact angle ca. 130°). Not only is adherence of the oil intrinsically better, but for a given... 

The basic theory of wetting is reviewed, covering the concepts of contact angle and polymer surface and interfacial tensions. The necessity of taking into account the interactions between interfaces in the three -phase region is stressed. 

The concept of contact angle gives a notion of wettability. Zisman [2] defined spreading when 0 =0° and wetting when 0 180°. In oflier words, partial wetting occurs in most of the cases we encounter. 

A quantity that is closely related to surface tension is the contact angle. The contact angle 0 is defined as the angle (measured in the liquid) that is formed at the junction of three phases, for example, at the solid-liquid-gas junction as shown in Figure 6.2b. Although the surface tension is a property of the two phases that form the interface, 0 requires that three phases be specified for its characterization, as mentioned above. The above definition of contact angle is, however, highly simplified, and we take a more in-depth look at the concept later in this chapter. 

In this report, these concepts are applied to real proteins to collagen, an important structural material in tendons, bones, teeth, and skin, and to gelatin, the denatured product of collagen that is so important industrially. These materials are complex because of their 18 different, component amino acid side chains in addition, they present experimental difficulties because of their water solubility— they cannot be washed (e.g., with an aqueous detergent) to assure surface cleanliness. Furthermore, they are often of unknown purity. They do have the common polyamide backbone, and it is possible to transform the molecular configuration. The data are indicative of the potential utility of contact angle measurements of important, natural materials. No claim is made for adequate attention to the complex biochemistry of these materials. 

Current theories to explain hysteresis of contact angles are primarily based on the concepts of surface roughness, surface heterogeneity, friction, and adsorption phenomena. Unintentional adsorption, or contamination—the result of inadequate experimental technique—is, however, the most frequent explanation. As all systems involving solids are subject to the reasons indicated above for hysteresis, we chose the system mercury-benzene-water, which should be affected only by adsorption phenomena, controllable under proper experimentation. An additional advantage is the fact that all interfacial tensions involved can be measured. 

The results of Johnson et al. put into some doubt the earlier concepts of surface relaxation or rearrangement effects on the dynamic contact angle. Indeed, their results suggest that in the absence of contact angle hysteresis there is no effect of velocity on... 

If the contact angle is a function of the speed of the contact line relative to the solid surface, one needs to consider viscous stress forces among other effects. In fact, the viscous stresses show singular behavior at the contact line [1]. The concept of contact line is a continuum concept. This concept breaks down near the surface and one requires molecular considerations in the vicinity of the contact line. 

If information about a solid surface free energy (SFE) is needed, contact angle measurements and ink tests are two of the most frequently used methods. Here we present a comparative study of contact angle measurements and ink tests on 13 different materials. We observed major differences in the SFE values obtained by these two techniques and explained the differences on the basis of basic theoretical concepts of both methods. We found that test inks fail to monitor the efficiency of atmospheric plasma treatments on low surface energy solids. Moreover, we determined the polar and dispersion contributions to the test inks total surface tension (ST) in order to provide a more detailed understanding of these methods to determine a solid SFE. 

Figure 4.1 Concept of the contact angle. Values below 90 (c) indicate partial wetting of the solid by the liquid (e.g. water on clean glass), whereas a value equal to (F indicates excellent (complete) wetting. Values above 90° (b) indicate partial non-wetting, e.g. water on solid mercury, which becomes "worse" as the contact angle increases, upto 180°. This maximum value of contact angle indicates complete non-wetting and is rather rare (many liquids on mercury or on some other metals approach this value). Measurements and Interpretations of contact angle data can be complicated by solid roughness (a)...

In earlier studies [5], a completely new concept of hysteresis of contact angle on smooth homogeneous substrates has been suggested. This mechanism will be discussed in Section 3.10. In the following, we give a qualitative description of this phenomenon. 

The critical surface tension concept has provided a useful means of summarizing wetting behavior and allowing predictions of an interpolative nature. A schematic summary of 7 values is given in Fig. X-10 [123]. In addition, actual contact angles for various systems can be estimated since )3 in Eq. X-38 usually has a value of about 0.03-0.04. 

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