Contact angle concept

Special emphasis is given on the contact angle concept. Contact angles are often used simply as empirical parameters to quantify the wettability in technical solid-liquid systems. However, it is of great... 

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. 

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.

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 addition to the methods discussed here and in Section 6.2, there are a few other methods for measuring surface tension that are classified as dynamic methods as they involve the flow of the liquids involved (e.g., methods based on the dimensions of an oscillating liquid jet or of the ripples on a liquid film). As one might expect, the dynamic methods have their advantages as well as disadvantages. For example, the oscillating jet technique is ill-suited for air-liquid interfaces, but has been found quite useful in the case of surfactant solutions. A discussion of these methods, however, will require advanced fluid dynamics concepts that are beyond our scope here. As our primary objective in this chapter is simply to provide a basic introduction to surface tension and contact angle phenomena, we shall not consider dynamic methods here. Brief discussions of these methods and a comparison of the data obtained from different techniques are available elsewhere (e.g., see Adamson 1990 and references therein). 

The ACCA and APCA on an ideal solid surface are identical by definition, and are referred to as the "ideal contact angle (ICA)". As will be explained below, it is the value of the ICA that is required for the characterization of the wettability of a solid substrate in terms of its surface tension. Also, all predictions of wetting behavior start with the ICAas their basis. Therefore, even though ideal surfaces are rarely encountered in practice, the concept is of fundamental importance. 

At present, many authors I20-I26 follow another concept From the plot of pj p [M /t) versus the surface tension of the liquids, the geometric factor K is calculated for those liquids that should wet the solid completely. By inserting this K value and [t]/p- ] M2/t for these liquids into Eq. (13). their contact angles 0 are calculated and used for the interpretation of the solid-vapor surface tension of the porous material. This procedure is dubious, because it can be expected that the contact angles, calculated from the Washburn equation, are affected by roughness and porosity. If we apply this procedure to the PTFE powder for hexadecane, a contact tingle 0 = 88 would be obtained. However, it is well known that the contact angle of hexadecane on a flat and smooth... 

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. 

A novel method for detennination of the particle contact angle at the fluid/fluid interface based on the excluded area concept revealed some serious difliculties connected with the exact quantitative particle deposition at the interface and with changes in the particulate contact angle upon binary monolayer compression. The comprehensive theoretical consideration of the contact angle behavior made for such system allowed considerable improvement of the proposed method that was then successfully proven by experimental data. 

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