Interfacial contact wetting equilibria

In systems where A G - A G < 0, the solid is not completely wetted by the liquid but a sessile drop of L is formed on S. Such a drop on a flat, smooth, and horizontally positioned solid surface is depicted in Rgure 8.1. The shape of the drop or, more precisely, the contact angle 0 with the solid is determined by the balance of interfacial tensions. In equilibrium... 

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.

The wetting (contact) angle method is used for solid surfaces. If a gas bubble sticks to a metal surface, then the individual interfacial tensions are distributed as shown in Fig. 4.11. It holds at equilibrium that... 

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 sketch in Fig. 10 shows the equilibrium of forces with an obtuse contact angle in the oil phase (6o). In this case the wetting tension, j, of the aqueous phase is positive, which means that the adhering oil droplet is pushed together by the aqueous phase. With the increase in j the tendency of an oil droplet to be cut off and removed from a solid substrate increases. Because of this, the impeding force for the removal of oil is the interfacial tension oil/water (Yq )> which should be minimized. By minimization of the interfacial tension, moreover, the requirements for emulsification and stabilization of soil in the washing and cleaning liquid will be improved. 

Wetting of a solid by a liquid is normally described in terms of the equilibrium contact angle 6 and the appropriate interfacial tensions as shown in Fig. 3.32. 

Wetting can be determined by contact angle measurements. It is governed by the Young equation, which relates the equilibrium contact angle 9 made by the wetting component on the substrate to the appropriate interfacial tensions ... 

The term ysv is the interfacial tension of the solid material in equilibrium with a fluid vapor yLV is the surface tension of the fluid material in equilibrium with its vapor and ySL is the interfacial tension between the solid and liquid materials. Complete, spontaneous wetting occurs when 9 = 0° or when the material spreads uniformly over a substrate to form a thin sheet. A contact angle of 0° occurs with pure water droplet on a clean, glass shde. Therefore, for complete spontaneous wetting, cos 9 > 1.0 or when... 

One of the first and most extensively applied models for the liquid-solid interfacial free energy is that due to Skapski. His approach begins with the assumption that a liquid will wet its crystal, so the contact angle 6 in the three-phase equilibrium between crystal, liquid, and vapor is 0. (This corresponds in Fig. 1 to the replacement of the substrate with the solid, the solid with the liquid, and the liquid with the vapor.) The analog of Eq. (2.15) then has 0 = 0 and cos 0 = 1, so that the result is... 

Instead of a film the condensate can also exist in the form of droplets, as shown in Fig. 4.2. This type of condensation is called drop condensation. Whether film or drop condensation prevails depends on whether the wall is completely or incompletely wetted. The decisive factor for this are the forces acting on a liquid droplet, which are illustrated in Fig. 4.3. aLG is the interfacial tension (SI units N/m) of the liquid (index L) against its own vapour (index G), (rSL is the tension of the solid wall (index S) against the liquid and is the interfacial tension of the wall with the vapour, so at equilibrium the contact angle f30 is formed according... 

The process of wetting involves replacing the solid/vapour interface (with interfacial tension ygy) with a solid/liquid interface (with interfacial tension yg ). Wetting can be described in equilibrium thermodynamics in terms of the contact angle 0 by Young s equation at the wetting line [5]. 

Because there can be degrees of wetting of particles at an interface, another quantity is needed. The contact angle, 6, in an oil—water—solid system is defined as the angle, measured through the aqueous phase, that is formed at the junction of the three phases. Whereas interfacial tension is defined for the boundary between two phases, the contact angle is defined for a three-phase junction. If the interfacial forces that act along the perimeter of the drop are represented by the interfacial tensions, then an equilibrium force balance can be written as... 

Surface tension. Assuming that a membrane stretches over each interface, the magnitudes of the interfacial tension between each pair of phases are the fluid-fluid interfacial tension Ogt, the wetting fluid-solid interfacial tension oft, and the nonwetting fluid-solid interfacial tension ogs. When in static equilibrium, the vectorial force balance at the line of contact (the law of Neumann triangle, Ref. 87) gives... 

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