Interface unstable

Figure 11-15. a) Phase boundary b reflecting the miscibility gap between the phases (Fe,Mn)0 and (Fe,Mn)304 in the system Fe-Mn-O. Reaction path is plotted in terms of p0 =/(uMno)- Dotted lines (almost) parallel to b indicate the supersaturation for nucleation. b) Unstable interface b and two-phase region between (Fe,Mn)0 and (Fe,Mn)304) after (Fe,Mn)0 has been exposed to an oxygen potential gradient [Y. Ueshima, et al. (1989)] (see text). 

Figure 11-16. Unstable interface (cathode) in a solid state electrolysis cell A/AX/A (e.g., Ag/AgBr/Ag).

While the primary difficulty is estimating the interfacial area due to the unstable interface, a secondary problem is that freshly made, unstable surface gives higher transfer than older, more stable surface. 

Table 1 provides some values of y and C and shows that these quantities are small and become smaller with Increasing radius. While y and C are of the same order of magnitude, C is the larger of the two. Expression (21a) and (21c) for Af lead to comparable results while expression (21b) leads to smaller values. It is of interest to note that the transition to unstable interfaces can... 

For example, an unstable interface is observed when a more viscous fluid is displaced by a less viscous fluid in porous media (i.e., as M increases). However, capillary forces can offset the effects of viscosity differences, but as the speed of displacement increases (i.e., as Ca increases), instabilities are more likely. Likewise, as Bo increases, instabilities are more likely because of fluid density differences. 

A grout with a viscosity lower than that of water will always have an unstable interface with groundwater. Instead of a spherical interface, the grout will penetrate the groundwater with a number of intrusive fingers and... 

Figure 3.16 Unstable interfaces in an SLM containing microliquid necks. From Ref. [84] with permission. (Q 2008 Elsevier.

Unlike the other cases, in the aniline/water system the addition of Atlas G1300 does not have much effect on the interfacial tension, indicating the unlikelihood of the system having an unstable interface, as shown in the Schlieren images (Figure 2, case Ic). However, even the small changes in interfacial tension values shown for SDS and DTAB are sufficient for the onset of interfacial tension in comparison with the clean aniline/water system. 

The effect surfactants have on ternary liquid-liquid systems unstable interfaces is of either reducing or suppressing completely any interfacial convection present. In turn, this causes the mass transfer rates to be reduced. 

As before, this equation applies to both stable and unstable interfaces. 

When the interfadal tension goes to zero, the blend becomes miscible. Large interfadal tensions lead to unstable interfaces, especially when the viscosity is low. 

Fig. 15.19 (a) Dissolution mode of unstable interface (0111) into the more stable (0001 )-Z plane and (OllO)-Y plane, (b) The dominant temperature gradient for determining the advance rates of these planes. (Reproduced by the permission of Elsevier Ltd.)... 

This concept is illustrated in Fig. 8.11 for a poly(ethylene terephthalate) substrate and a mild steel (ferric oxide) substrate with, in both cases, water as the hostile environment. Values of y and yl of the various adhesives may be measured, as described in Chapter 2, or extracted from the literature (see Table 2.3) for example, considering a styrene-butadiene rubbery adhesive the values are 27.8 and 1.3 mJ/m, respectively, and for a typical epoxy adhesive they are 41.2 and 5.0 mJ/m, respectively. Hence, it is evident that these (and most other) adhesives will form an environmentally water-stable interface with the poly(ethylene terephthalate) substrate but an unstable interface with mild steel. Indeed, the data confirm that if only secondary molecular forces are acting across the interface then water will virtually always desorb organic adhesives, which typically have low surface free energies of less than about 60 mJ/m, from a metal oxide surface. Hence, for such interfaces, stronger intrinsic adhesion forces must be forged which are more resistant to rupture by water. 

Ion-Selective electrodes with liquid electro-active phases require special construction techniques. The rather unstable interface between two immiscible liquids must be stabilized, so that hydrostatic and hydrodynamic fluctuations in the sample solution do not exert too large an influence on the spacial charge distribution near the interface. There are a number of ways in which this can be accomplished. 

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