Interfacial transition region

Figure 11. Dividing surface or interfacial transition region xm2 — xml < molecular radius, r...

Fig. 53. (a) Cyclic voltammogram during the formic acid oxidation on a Bi-modified Pt ring electrode scan rate 5mV s-1. The change in the oscillation form close to t = 73.7 s (i.e. U = 0.16 V) indicates a qualitative change in the dynamics, (b) Spatio-temporal profile of the interfacial potential in the transition region of the oscillation form in (a). For the experimental set-up, see Fig. 49b. (Reproduced from J. Lee, J. Christoph, P. Strasser, M. Eiswrith and G. Ertl, J. Chem. Phys. (2001) 115, 1485 by permission of the American Institute of Physics.)...

Monte Carlo and molecular dynamics calculations of the density profile of model system of benzene-water [70], 1,2-dichloroethane-water [71], and decane-water [72] interfaces show that the thickness of the transition region at the interface is molecu-larly sharp, typically within 0.5 nm, rather than diffuse (Fig. 4). A similar sharp density profile has been reported also at several liquid-vapor interfaces [73, 74]. The sharpness of interfaces thus seems to be a general characteristic of the boundary between two stable phases and it is likely that the presence of supporting electrolytes would not significantly alter the thickness of the transition region at an ITIES. The interfacial mixed solvent layer [54, 55], if any, would probably have a thickness comparable with this thin inner layer. 

The VOF approach allows one to model various interfacial phenomena for example, wall adhesion and surface (or interfacial) tension can be modeled rigorously using this approach. Brackbill et al. (1992) developed a continuous surface force (CSF) model to describe interfacial surface tension. CSF model replaces surface force by a smoothly varying volumetric force acting on all the fluid elements in the interface transition region. For two-phase flows (dispersed or secondary phase is denoted by subscript 2), surface force, Fsf can be written (Brackbill et al., 1992) ... 

The second fairly modern group of methods introduces (of numerical reasons) a 3D continuous surface force (CSF) or a 3D continuum surface stress (CSS) acting locally within the whole transition region constituting a meso-scale interface. Notice that since we are primarily interested in the interfacial forces, the latter group of techniques were used approximating the surface effects without actually reconstructing the interface. 

The interfacial energy is contributed both by deviations from the equilibrium density levels in the transitional region and by the distortion energy localized there. The ID interaction kernel Q z) lumps intermolecular interaction between the layers 2 = const. It is computed by lateral integration using as an... 

Concrete is a composite typically composed of aggregate and cement paste. It is the connection between these phases - the interfacial transition zone (ITZ) that is most important.. The ITZ is the weakest link in the composite system with lowest mechanical properties of the three [1-4], The ITZ can affect the overall elastic module and the stress distributions in a concrete material. The ITZ is comparatively more porous than that of bulk cement paste, and often less well bonded to the aggregate [3]. This region can have a low formation of calcium-silicate-hydrate (C-S-H gel), a product of Portland cement hydration responsible for the good mechanical properties and durability [5]. 

Concrete is a multiple-phase material and the overall mechanical properties not only depend on each phase, but also upon the interface between them. Prior study shows that the interfacial transition zone (ITZ) which is characterized by the prevalence of calcium hydroxide and higher porosity, is the weakest region in a cementitious material. Interactions that take place there control many important properties such as strength, permeability and durability [1]. Methods have been... 

In typical microemulsion systems, a prominent composition region shows bicontinuous structures. With probes and quenchers confined to either oil or water, the domains in the bicontinuous region may be so large in all dimensions that normal exponential decays are observed. Only in the region with discrete droplets, and in a transition region where droplets cluster and merge, can the micellar type of quenching be expected. However, if the amphiphilic probes and quenchers are bound to the interfacial surfactant film in the bicontinuous microemulsions, one would expect 2-D behavior. 

The difference between the microstructure of cement paste and concrete should be taken into accoimt when the diffusion conditions are compared. The paste (cement matrix in concrete)— aggregate interfacial transition zone is of special importance. This transition zone is formed of the thin layer of cement paste, about 50 pm wide, which has different microstmcture than the bulk cement matrix in this concrete (see Sect. 6.2). The porosity of this interfacial transition zone is significantly higher the portlandite content is higher as well. In the case of high performance concrete with low w/c ratio the microstructural difference between the interfacial region and the matrix is negligible or none. 

In order to properly solve (17.5), sharp changes in the properties as well as pressure forces due to surface tension effects have to be resolved. In particular, surface tension results in a jump in pressure across a curved interface. The pressure jump is discontinuous and located only at the interface. This singularity creates difficulties when deriving a continuum formulation of the momentum equation. The interfacial conditions should be embedded in the field equations as source terms. Once the equations are discretized in a finite-thickness interfacial zone, the fiow properties are allowed to change smoothly. It is therefore necessary to create a continuum surface force (CSF) equal to the surface tension at the interface, or in a transitional region, and zero elsewhere. Therefore, the surface integral term in (17.5) could be rewritten into an appropriate volume integral... 

FIGURE 2.7. For a liquid-vapor system, the interfacial region will be smooth with a narrow transition region and smooth concentration profile. 

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