Three-Phase Contact Line Wetting

Before we start a discussion on the thermodynamics of a contact between particles, it is worthwhile to briefly address the phenomena taking place at the three-phase contact line, and in particular, wetting and capillary forces acting within a liquid meniscus. We will also briefly summarize the principal methods of surface tension measurement. 

FIGURE 1.6 Young-Laplace representation of a three-phase contact line between solid, liquid, and gas phases. Cases of wetting and nonwetting liquids. 

FIGURE 1.7 The schematic illustration of the derivation of Young s equation by the variation method 5F, = 0. 

It is also worth emphasizing that the interatomic bonds are not fully compensated at the three-phase contact line. This results in a free energy excess and a linear tension, ae, acting along the perimeter of a three-phase boundary. This linear tension can be either positive or negative and does not exceed 10 dyn/cm. While the linear tension can in most cases be neglected, it plays an essential role in the case of very small droplets, particularly in nucleation. 

FIGURE 1.8 Interface between two liquids and a gas (a) linear tension, ae, (b) selective wetting 0 90°, hydrophilic (oleophobic) surface 0 90°, hydrophobic (oleophilic) surface (c). 

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The system energy of Fig. 12 can be estimated by using the above solution for the meniscus profile. As stated in the previous section, we consider the potential energy E, the energy of the liquid vapor interfacial area Elv, and the work done by the three-phase contact line wetting the cone surface E f/. The dry cone surface and the horizontal liquid surface (z = 0) are taken for the reference state of system energy. E and which represent the work necessary to form the axisymmetric meniscus shown in Fig. 12, are calculated from... 

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] ... 

At the three-phase contact line the surface tension exerts strong forces on the surface. For instance, if we consider a water drop on a polymer surface, typical contact angles are 90°. The surface tension pulls upwards on the solid surface. If we estimate the wetting line to have a width of 6 = 10 nm, the force F per unit length l can be related to the effective pressure exerted on the solid surface ... 

The work required to create a new three-phase contact line per unit length is called line tension. It is typically of the order of 0.1 nN. For tiny liquid drops the line tension can significantly influence the wetting behavior. 

Similar to (TMS -f O2) plasma-treated PP, many of the (TMS -f O2) plasma-treated polymers showed that the three-phase contact line hardly recedes on the wet surface. Specifically, (TMS + O2) plasma-treated PTFE, UHMWPE, PP, HDPE, PVDF, and nylon showed hardly any change in contact area during the receding process. This is due to the strong specific attractive forces formed at the surface underneath the bulk droplet, i.e., the interfacial tension (underneath the droplet) changed due to the interaction of water with the surface. 

Figure 26.10, and (d e) in Figure 26.11] until the force in the direction of the wet surface exceeds the adhesive tension of emersion, Te, at the polymer surface/water interface. After exceeding the adhesive tension, the three-phase contact line starts to move toward the prewetted surface, seen as (D E) in Figure 26.9 and (C D) in Figure 26.12. The adhesive tension in the receding process is usually less than that in the advancing process unless the surface was completely wetted by the first immersion (i.e., zero contact angle). When the complete wetting occurs on the first immersion, the first emersion line retraces the first immersion line, such is the case with O2 plasma-cleaned glass.

It should be noted that on the receding cycle the wet plate surface has previously interacted with water molecules for a different period of time depending on the immersion depth of the plate. Therefore, the bottom deeper immersed portions of the plate interact with the water molecules for a longer period than the shallow immersed portions closer to the top of the plate. This causes small but continuous changes in the meniscus shape even after the three-phase contact line starts to move in the advancing and receding processes. 

The interfacial behavior of block copolymers is of interest in several fields like stabilization of emulsions, foams, and wetting control [154]. Gerdes et al. [155] studied the wetting behavior of aqueous solutions of triblock copolymers on silica. The experimental approach was based on the use of a Wilhelmy force balance and direct images of contact angle. Their results show that the three-phase contact line advances in jumps over the surface when it is immersed at constant speed into the copolymer solution. Apparently the stick-slip spreading mechanism is the same as has been proposed for short chain cationic surfactants. 

Figure 7.33. Side-view of a partially wetting drop on a solid surface showing the macroscopic contact angle and the three interfacial tension vectors acting on the three-phase contact line (tcl)...

In many practical applications, the wetting liquid in question is a solution, e.g. an aqueous solution containing surface-active components. Then, the possibility of adsorption at all interfaces surrounding the three-phase contact-line (tcl) must be considered. According to the Gibbs isotherm for adsorption at the ij interface ... 

Wetting and Spreading, Fig. 1 Force balance at the three-phase contact line where the solid-liquid and vapor phases meet. The outward unit normals protruding from the solid and liquid at the contact line are shown. The angle 8 subtending these unit normal vectors or that subtending the interface between the drop and the solid substrate is known as the contact angle... 

Macroscopic droplets of oil (the less wetting phase) are contained within one or more interconnected pore bodies that are formed by solid phases of the porous medium. These immobilized portions of the oil phase, which can be referred to as ganglia or blobs, are surrounded with continuously connected reservoir brine (the more wetting phase). At the three-phase contact lines which are formed on the solid surfaces where the two immiscible liquids meet, the apparent contact angles reflect the relative affinities of the three phases for each other, surface textures, surface compositions, and solid surface saturation histories. 

A key factor in the consideration of the interfacial movement is the mechanism by which the three phase line moves. The easiest mechanism to envisage is when the displacing phase moves over a thin film of the displaced phase, i.e., the three phase line is only apparent, as in the definition of completely water wet introduced above. For a real three phase contact line, molecular pictures have been attempted (22). Still uncertain is whether the displacement of the three phase contact line results in a resistance to the flow. Measurements reported by Jacobs (21) have suggested that the presence of surfactants is associated with me-niscal resistance to flow, although the mechanism is obscure. 

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