Contact Between Three Phases Wetting

For a critical value of the ratio R/D R/D = 1.509), the two solutions become identical. For RjD 1.509, solutions no longer exist and the film bursts. 

Wetting refers to the study of how a liquid deposited on a solid (or liquid) substrate spreads out. The phenomenon is pertinent to numerous industrial areas, a few of which are listed below  

It also plays a role in the life sciences. A few notable examples follow  

In the following section, we begin by characterizing two types of wetting  

Next we will describe criteria useful for predicting whether or not a liquid will wet a particular substrate. We will show how a simple mono-layer deposited on the substrate can reverse the behavior of the interface, i.e., change it from wetting to non-wetting and vice versa. We will pay particular attention to those liquids and solids that have been used to implement well-controlled systems and we will describe the most common surface treatments used in the physical chemistry of wetting. The modifications involved will be discussed in chapter 2. 

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Wetting Is an umbrella term describing all phenomena involving contacts between three phases, of which at least two are fluid. At that line of contact three interfaces meet, each with its own excess Helmholtz energy and interfacial tension. 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

A heterogeneous tubular reactor that incorporates three phases where gas and liquid reactants are contacted with the solid catalyst particles, is classified as a trickle-bed reactor. The liquid is usually allowed to flow down over the bed of catalyst, while the gas flows either up or down through the void spaces between the wetted pellets. Co-current downflow of the gas is generally preferred because it allows for better distribution of liquid over the catalyst bed and higher liquid flow rates are possible without flooding. 

The wetting of a polymer surface is characterized by the interfacial tension between the liquid and the surface. The contact angle at the three-phase line is a good measure of the wettability of the surface. Figure 25.5 depicts the force balance... 

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... 

In Chapter 2 we discussed a number of studies with three-phase catalytic membrane reactors. In these reactors the catalyst is impregnated within the membrane, which serves as a contactor between the gas phase (B) and liquid phase reactants (A), and the catalyst that resides within the membrane pores. When gas/liquid reactions occur in conventional (packed, -trickle or fluidized-bed) multiphase catalytic reactors the solid catalyst is wetted by a liquid film as a result, the gas, before reaching the catalyst particle surface or pore, has to diffuse through the liquid layer, which acts as an additional mass transfer resistance between the gas and the solid. In the case of a catalytic membrane reactor, as shown schematically in Fig. 5.16, the active membrane pores are filled simultaneously with the liquid and gas reactants, ensuring an effective contact between the three phases (gas/ liquid, and catalyst). One of the earliest studies of this type of reactor was reported by Akyurtlu et al [5.58], who developed a semi-analytical model coupling analytical results with a numerical solution for this type of reactor. Harold and coworkers (Harold and Ng... 

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... 

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... 

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