Contact Angle, Adhesion and Cohesion

In Chapter 4.2 we introduced the interfacial (surface) tension (equivalent to surface or interfacial energy) as the minimum work required to create a differential increment in surface area. The interfacial energy, equally applicable to solids and liquids, was referred to as the interfacial Gibbs free energy (at constant temperature, pressure and composition) (n refers to the composition other than the surfactant under consideration). 

Components of interfacial tension (energy) for the equilibrium of a liquid drop on a smooth surface in contact with air (or the vapor) phase. The liquid (in most instances) will not wet the surface but remains as a drop having a definite angle of contact between the liquid and solid phase. 

The Contact Angle. Three phases are in contact when a drop of liquid (e.g., water) is placed on a perfectly smooth solid surface and all three phases are allowed to come to equilibrium. 

The change in surface free energy, AGS, accompanying a small displacement of the liquid such that the change in area of solid covered, AA, is 

This equation, called the Young Equation, is in accord with the concept that the various surface forces can be represented by surface tensions acting in the direction of the surfaces. Eq. (A.4.3) results from equating the horizontal components of these tensions. 

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APPENDIX 9.2 CONTACT ANGLE, ADHESION AND COHESION, THE OIL-WATER INTERFACE... 

Appendix 9.2 Contact Angle, Adhesion and Cohesion, Oil-Water Interface 609... 

Wu SJ, "Surface and Interfacial Tensions of Polymers, Oligomers, Plasticizers and Organic Pigments" In Brandrup J, Immergut EH and Grulke EA (Eds) "Polymer Handbook", Part VI, p 521, Wiley, New York, 4th Ed, 1999. Zisman WA, In "Contact Angle, Wettability and Adhesion", Adv Chem Ser 43, Am Chem Soc 1964, pp 1-51 in Weiss P (Ed) "Adhesion and Cohesion", Elsevier, Amsterdam, 1962, pp 176-208. 

Surface Tension, Capillarity, and Contact Angle, Fig. 1 Adhesive and cohesive forces on molecules of a liquid close to an interface and away from it... 

The work of cohesion, the material with itself, is equal to 2y" and has a contact angle of 0°. In general for a simple fluid the work of adhesion and cohesion are equal and for 9 < 90°, the liquid is wetting the solid and when 9 > 90° the liquid... 

Wetting is the ability of a liquid drop to keep contact with a solid surface. The degree of wetting is qualified by a contact angle (9 ), at which the liquid-vapor interface meets the solid-liquid interface, as shown in Figure 1. The contact angle is governed by the balance between adhesive and cohesive forces and it provides an inverse measure of wettability. 

If, when a liquid drop is placed on a smooth surface, the forces of adhesion between the solid and the liquid are greater than the forces of cohesion of the liquid, then the liquid will spread and will perfectly wet the surface spontaneously. If the forces reach an intermediate balance determined by the interfacial energies ylv, ysj and ysv, then the liquid drop will form a definite contact angle (0) with the solid surface (Figure 4.12). 

This equation has been known for over a century it was given by Young2 (without proof ) and by Dupre 3 it can be deduced also from Laplace s theory of Capillarity, or indeed from any theory of the cohesive forces, since it can be obtained from consideration of energies only. Until recent years it has been little noticed, which is unfortunate, as the meaning of the contact angles is much clarified when the work of adhesion is introduced, and the surface tensions of the solid surfaces, which are not measurable, are eliminated. Most authors are now, however, expressing their results in terms of the work of adhesion or of closely related expressions. 

Measurements on molten metals. The maximum bubble pressure method has proved one of the most satisfactory, but sessile drops, and drop-volumes have also been used with success.2 The principal difficulty lies in the proneness of metals to form skins of oxides, or other compounds, on their surfaces and these are sure to reduce the surface tension. Unless work is conducted in a very high vacuum, a freshly formed surface is almost a necessity if the sessile bubble method is used, the course of formation of a surface layer may, if great precautions are taken, be traced by the alteration in surface tension. Another difficulty lies in the high contact angles formed by liquid metals with almost all non-metallic surfaces, which are due to the very high cohesion of metals compared with their adhesion to other substances. 

Equation (1.46) shows that the contact angle results from the competition of two types of forces cohesion forces responsible for crLv (= Wc/2) and adhesion forces responsible for W . Depending on the strength of S/L and L/L interactions, different contact angles can be obtained (Table 1.1). 

The size of the contact angle depends on the magnitude of the liquid-solid adhesive force compared with that of the liquid-liquid cohesive force. Specifically, Young s equation (also called the Young and Dupre equation) indicates that... 

Fig. 8.1 Schematic diagram of the contact angle formed between a glass wall and a liquid where the adhesive force on a molecule near the air water interface and wall is and the cohesive force with the bulk,/e. The vector sum of...

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