Three-phase contacting

Fig. X-16. (a) Microscopic appearance of the three-phase contact region, (b) Wetting meniscus against a vertical plate showing the meniscus only, adsorbed film only, and joined profile. (From Ref. 226 with permission. Copyright 1980 American Chemical Society.)...

D. Platikanov and M. Nedyalkov, Contact Angles and Line Tension at Microscopic Three Phase Contacts, in Microscopic Aspects of Adhesion and Lubrication, J. M. Georges, ed., Elsevier, Amsterdam, 1982. 

In practice, it may be possible with care to float somewhat larger particles than those corresponding to the theoretical maximum. As illustrated in Fig. XIII-7, if the particle has an irregular shape, it will tend to float such that the three-phase contact occurs at an asperity since the particle would have to be depressed considerably for the line of contact to advance further. The resistance to rounding a sharp edge has been investigated by Mason and co-workers [62]. 

Fig. 7. The concept of contact angle with a captive bubble in an aqueous medium, adhering to a hydrophobic sofld P is the three-phase contact point. Here, the vector passes through P and forms a tangent to the curved surface of the air bubble. The contact angle 0 is drawn into the Hquid.

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 ... 

The constraint to be implemented at the three-phase contact line between the two fluids and a solid surface requires that the contact angle 0 (compare Figure 2.58) assumes a prescribed value. As discussed in Section 2.2.3, the contact angle might also be allowed to vary with the velocity of the contact line. Especially in microfluidic... 

FIG. 20-39 Normalized free-energy difference between distributed (II) and nondistributed (I) states of the solid particles versus three-phase contact angle (collection at the interface is not considered). A negative free-energy difference implies that the distributed state is preferred over the nondistributed state. Note especially the significant effect of n, the ratio of the liquid droplet to solid-particle radius. [From Jacques, Hovarongkura, and Henry, Am. Inst. Chem. Eng. J., 23(1), 160 (1979).]... 

In the first instance (1,3) two types of nickel are used on the side exposed to the gas, large pores are produced in the metal and adjacent to this structure, a network of smaller pores are produced to hold back the electrolyte. The reacting gases diffuse rapidly in the large pores and come in intimate contact with the electrolyte present in the small pores. For the electrochemical reaction po occur, a three phases contact is needed since a gaseous reactant produces a solvated reactior oro uct nd in this process an electron is given or withdrawn from a solid conducting substrate. 

Paunov VN. Novel method for determining the three-phase contact angle of colloid particles adsorbed at air-water and oil-water interfaces. Langmuir 2003 19 7970-7976. 

In summary, we have examined the role of structural disjoining pressure in the movement of a three phase contact fine. The movement of the contact fine is an integral process in the displacement of one fluid by another. Practical applications include the spreading of a fluid on a solid surface or the removal of a pollutant drop from a solid surface by the action of a surfactant solution. 

In a surprisingly large number of industrially important processes reactions are involved that require the simultaneous contacting of a gas, a liquid and solid particles 28 . Very often the solid is a catalyst and it is on the surface of the solid that the chemical reaction occurs. The need for three-phase contacting can be appreciated by considering, as an example, the hydro-desulphurisation of a residual petroleum fraction, i.e. of the liquid taken from the base of a crude oil distillation column. 

Experimental justification for specification of the angle at the point of three-phase contact comes from the results of Surek and Chalmers (139), which verify that a particular value of < )0 measured macroscopically can be associated with the crystal growth of a material in a specific crystallographic orientation and that < >o is roughly independent of growth rate. 

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] ... 

Here, a is the radius of curvature of the three-phase contact line. For a drop with circular contact area it is the contact radius. 

Dissolved substances often adsorb at the three-phase contact line. Advancing or receding of the liquid is hindered by the deposited substances. 

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 ... 

Figure 7.10 Microscopic view of the three-phase contact region for a repulsive force between the solid-liquid and liquid-gas interface (A) and an attractive force between the two interfaces...

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. 

Contact angle hysteresis can be caused by surface roughness, heterogeneity, dissolved substances, and structural changes of the solid at the three-phase contact line. 

The equation results from the following consideration. At equilibrium the three-phase contact line does not move. Hence, the sum of the forces acting in horizontal direction must be zero. The surface of liquid A pulls with a force (per unit length) of 7a cos 03 to the left. The surface of liquid B pulls to the right with a force 7b cos i and so does the liquid A-liquid B interface 7ab cos 2. 

---