Angle wetting

The wettability of a liquid on a substrate is an important property in a number of ceramic processes. With a decreased wetting angle, i.e. an increased wettability, densification is enhanced in liquid phase sintering and the bonding in brazing or soldering is improved. 

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In liquid-phase sintering, densification and microstmcture development can be assessed on the basis of the liquid contact or wetting angle, ( ), fonned as a result of the interfacial energy balance at the solid-liquid-vapour intersection as defined by the Young equation ... 

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.

When the nucleus is a liquid, the angle 6 is called tire wetting angle. It can be seen that the critical radius in heterogeneous nucleation is given by the same equation as tlrat for homogeneous nucleation, but the radius now refers... 

Figure 1.10 The formation of a spherical-cap nucleus of radius r on a substrate upon which the nucleus has a wetting angle 6...

The calculation above is vahd for a spherical nucleus forming in bulk solution or on an electrode surface completely wetted by the hquid electrolyte, where the wetting angle a — 0 (Fig. 14.8a). The work of nucleation decreases markedly when wetting is incomplete (Fig. 14.8fc), since the electrode-electrolyte contact area is smaller. The work also decreases when asperities, microcracks, and the like are present on the surface. Thus, Eq. (14.33) states merely the highest possible value of work... 

FIGURE 14.9 Volumes of the departing gas bubbles as a function of wetting angle (the solid line is the calcnlated function). (From Kabanov and Frumkin, 1933). 

Table V. Wetting Angle at the Contact with Water ( fa) and Formamide (- jO and Critical Surface Tension (1 ) at 20°C...

Sample Wetting Angle (Degree) Critical Tension Surface (mN/m) ... 

The quantity ys, is a function of the electrode potential, the quantity y,g is independent of the potential and ysg should not depend on the potential, provided that the surface is dry below the bubble. As there is always a trace of moisture below the bubble in this arrangement, the value of ysg changes slightly with potential, but far less than ysl. The further the electrode potential is from the potential of the electrocapillary maximum, the more ys, decreases, i.e. cos 6 increases and the wetting angle 6 decreases. Obviously,... 

Fig. 4.11 Interfacial tensions between a solid, liquid and gaseous phase, ygs, y,s and ygl. 6 denotes the wetting angle...

Submerged Wetting Angle", Contact Lens Manufacturers Assoc., Chicago, adopted February, 1981. 

The similar mechanism determines the consequence of clearing of the cavities during the desorption of wetting liquid (DES) [3], and the main deviations are variations of wetting angle and desorption from a film in already cleaned cavities. 

Another possible approach to burn-out prediction is to study film breakdown due to hydrodynamic effects. Presumably, if thin spots occur in a film for any reason, the film becomes hotter, the surface tension is reduced, and increased vaporization tends to cause a break in the liquid layer. Although studies of surface tension and wetting angle in thin-film flow have been made, no successful correlation of burn-out in these terms has yet been offered. 

The partial wetting of a melt on a low-index plane of its solid has already been predicted - and observed on Cd", Ga , Ge, NaCT and KCl . However, as mentioned earlier, indications of anisotropic wetting have been limited to Ge and icel In the case of Pb, the wetting angle is shown to increase as the surface free energy, or the atomic density of the surface plane, decreases. Germanium was quoted as having a similar behaviouf. 

In the equation, generally if the magnitude of the right-hand side exceeds one, the wetting angle would be complex with crystallite (or particle) spreading... 

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