Angle of wettability

The influence of three important physical parameters on the effective area of the packing are discussed below. Th are die wettability of the packing material expressed through the angle of wettability, the liquid surfece tension,... 

The forces responsible for die holding of the liquid phase on die surface of the packing are the intermolecular adhesion forces. Their efi can be measured tiirough the angle of wettability presented in Fig, 12. 

The angle of wettability varied from zero (fully wetted) to 90° (not wetted matmial). 

Let us assume that the surface is covered with small sharp channels, Fig. 13, with width and distance between them quite less than the dimension of the liquid drop. When the channels are crossed, the liquid phase, fellen on the surfece, spreads in the channels thus wetting the whole suifru%. As it is easy to see from Fig. 13 (a), the real value of the angle of wettability, in relation to the horizontal plane, is less than zero, i.e., the material is fijUy wetted. There are different ways to obtain small crossing channel structure on the poking surface. 

Wettability Design of treatment/recovery Angle of contact related to... 

The contact angles of water and adhesive resin on wood are higher in the case of freshly harvested wood compared to stored chips. This means that the surface of particles from this fresh wood is more hydrophobic. This influences the wetting and the penetration negatively and with this the gluability. Reason for this lower wettability of freshly harvested wood is a higher content of some wood chemical components, or wood extractives, as has been determined by water extraction. This result, however, must not be confused with the better wettability of a freshly cut surface, independently if it is freshly harvested or stored wood. 

While the above refers mainly to the static limit, new effects come into play when a moving contact line, i.e. spreading, is considered. It has been observed experimentally that the contact angle of a moving contact line 0, the dynamic contact angle, deviates from the corresponding static value 0. As an example, for a completely wettable surface (i.e. 6(, = 0), a relationship of the form... 

Wettability measurements show that most soil constituents are water wettable or hydrophilic,28 although calcium carbonates [calcite, CaC03, and dolomite, CaMg(C03)2] are slightly hydrophobic for example, the contact angle of water and heptane is 100 to 105°. Therefore, carbonaceous reservoirs are usually oil-wet. 

For the DMFC, Zhang et al. [127] used the sessile drop method to study the wettabilities of liquid methanol solutions on the surface of the anode DLs and MPLs. They were able to observe that the contact angles of the materials were the smallest with low PTFE content. In addition, the effect of Nafion ionomer content on the MPL (to increase hydrophilicity see Section 4.3.2) was also shown through the contact angle measurements (i.e., smaller contact angles compared to MPLs with PTFE). 

Active control of wettability is a subject of current research activity and is beyond our scope here. However, the first step in this process is a study of interfacial tension and contact angle, their basis in thermodynamics, and methods to measure these properties. This is the objective of this chapter. 

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