Dimple rupture

B. The direct identification of such microscopic fracture modes as transgranular cleavage, radial fracture, intergranular separation or ductile dimpled rupture, was shown to be considerably more dependable by SEM than by OM... 

Fracture modes of the forged steels were dominated by the dimpled rupture process in unexposed, hydrogen-exposed and tritium-exposed steels and welds. Heavily sensitized steels had a similar fracture appearance as tritium-exposed-and-aged steels... 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Figure 2.26 Replicas prepared from fracture surfaces and viewed in TEM. (a) River patterns of a brittle failure of a steel fastener (b) fine, equiaxed ductile dimples on the surface of a tensile fracture of high-strength steel. The small carbide particles where local rupture and void formation initiated are apparent. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

It was first believed that the dimple in foam films decreases and even disappears at small film thickness. Later, experimental investigations of NaDoS aqueous films proved that the rate of thinning is practically equal in both thin and thick film domains, i.e. the difference by thickness between the thinnest and the thickest domains does not decrease up to the critical thickness of rupture. This leads to an increase in the non-uniformity by thickness (Fig. 3.4). 

FIGURE 5.36 Main stages of formation and evolution of a thin liquid film between two bubbles or drops (a) mutual approach of slightly deformed surfaces (b) at a given separation, the curvature at the center inverts its sign and a dimple arises (c) the dimple disappears, and eventually an almost plane-parallel film forms (d) due to thermal fluctuations or other disturbances the film either ruptures or transforms into a thinner Newton black film (e), which expands until reaching the final equilibrium state (f). 

In the case of predominant van der Waals attfaction, instead of a dimple, a reverse bell-shaped deformation, called a pimple, appears and the film quickly ruptures. 4 5,472,48i,484 jjjg thickness, hp, at which the pimple appears can be calculated from the relationship ... 

The common nonionic surfactants are often soluble in both water and oil phases. In the practice of emulsion preparation, the surfactant (the emulsifier) is initially dissolved in one of the liquid phases and then the emulsion is prepared by homogenization. In such a case, the initial distribution of the surfactant between the two phases of the emulsion is not in equilibrium therefore, surfactant diffusion fluxes appear across the surfaces of the emulsion droplets. The process of surfactant redistribution usually lasts from many hours to several days, until finally equilibrium distribution is established. The diffusion fluxes across the interfaces, directed either from the continuous phase toward the droplets or the reverse, are found to stabilize both thin films and emulsions. In particular, even films, which are thermodynamically unstable, may exist several days because of the diffusion surfactant transfer however, they rupture immediately after the diffusive equilibrium has been established. Experimentally, this effect manifests itself in phenomena called cyclic dimpling and osmotic swelling. These two phenomena, as well as the equilibration of two phases across a film,568.569 3j.g described and interpreted below. 

After the whole film area is occupied by the Newton black film, the equilibrium between the film and the meniscus is violated and the NBF expands imtil reaching its final equilibrium radius, corresponding to an equilibrium contact angle 0 (see Fig. 33f). Sometimes, no plane-parallel film forms, but, instead, the dimple persists until rupture or formation of NBF occurs. For more details, see Part IV.C of Ref. 267. 

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