Porous fracture surface

The bulk of the cement is extremely porous as the fractured surface of a specimen shows (Figure 6.3c). The pores are 0-5 pm in diameter and more abundant in the depth of the cement. The porosity arises from excess unbound water which separates out as globules in the cement and is trapped by the rapid setting. Subsequent diffusion of these globules leaves the cement porous. This makes the cement permeable to dyes (Wisth, 1972). 

Fig. 14.19 Fracture surface of an anode-supported cell. From left to right, the porous Ni-YSZ anode, the dense 8YSZ electrolyte, and the porous LSM-YSZ cathode.

The appearance of the individual microcapsules is shown in Fig. 1. Most individual microcapsules are approximately spherical and show a surface made up of deposited plates of poly(DL-lactic acid) in which the drug is embedded. Many of the larger microcapsules are cemented together by further plates of poly(DL-lactic acid). The effect of compression on these microcapsules is shown in Fig. 2. At a compressive force of 2 kN (Fig. 2(a)) the electron micrograph of the tablet fracture surface shows that the microcapsules, while distorted, remain essentially intact and rounded, with a relatively open porous structure to the tablet as a whole. At 10 kN force (Fig. 2(b)) the microcapsules at the fracture are flattened, cracked and distorted so that the fracture surface shows a far less open, porous aspect. Both of these microcap tablets have a very different appearance from that produced by the simple mixture (Fig. 3), where the individual plates of poly(DL-lactic acid) are mixed with the drug crystals in an open structure from which release would be easily... 

Figure 2. Fracture surface SEM image of porous YSZ electrolyte [4]...

The approach for unsaturated conductivity outlined in previous sections was extended to modeling the unsaturated hydraulic conductivity of rough fracture surfaces (Or Tuller, 2000). Flow on rough fracture surfaces is an essential component required for deriving constitutive relationships for flow in unsaturated fractured porous media (Or Tuller, 2001). The detailed derivations are obtained by consideration of a dual porosity model (matrix - fracture) and the proportional contributions to flow from these different pore spaces. 

The results for flow on a single fracture surface are incorporated in the derivation of hydraulic properties of unsaturated fractured rock mass. Liquid retention and hydraulic conductivity in partially saturated fractured porous media are modeled in angular pores and slit-shaped spaces representing rock matrix and fractures, respectively. A bimodal distribution of pore sizes and apertures accounts for the two disparate pore scales and porosity. These considerations provide a framework for derivation of retention and hydraulic conductivity functions for fractured porous media (Or Tuller, 2001). 

Tokunaga, T.K., and J. Wan. 1997. Water film flow along fracture surfaces of porous rock. Water Resour. Res. 33 1287-1295. 

Figure 13. Appearance (left) and SEM of the fracture surface (right) of a porous silica glass prepared by the sol-gel method.

Questions have been raised whether only the rock nearest the fracture surface (1-5 cm) is porous. This would imply that solutes would not be able to penetrate more than a few cm (in rock under natural stress conditions. 

The rock matrix nearest to the fracture surfaces is much more porous... 

Figure 2. SEM micrograph of fracture surfaces of cordierite-bonded porous SiC ceramics (a) without C addition and (b) with 40 vol.% C, sintered at 1350 °C for 2 h, where the weight ratio of...

Fig. 5 Miciostructures of alumina filler loaded preceramic paper left - pulp fiber paper sintered al 1600 °C (porous) right - fracture surface of sintered alumina fibre paper.

Natural fractals such as clouds, polymers, aerogels, porous media, dendrites, colloidal aggregates, cracks, fractured surfaces of solids, etc., possess only statistical self-similarity, which, furthermore, takes place only in a restricted range of sizes in space [1,4,16]. It has heen shown experimentally for solid polymers [22] that this range is from several angstroms to several tens of angstroms. 

It may not be possible, even in principle, to ascribe a unique surface area to a surface. It has long been recognized from work on gas adsorption on porous solids that the surface area measured depends on the size of the probe molecule. A small probe can enter finer surface features and therefore may give a larger value. The surface area is, as Rideal [59] recognized in 1930, in a sense arbitrary, not absolute. More recently evidence has been produced suggesting that many engineering surfaces and many fracture surfaces are fractal in nature [60,61]. For a fractal surface, the area depends on the size of the tile used to... 

Laboratory and field experiments show die presence and interplay of such processes as intrafracture film flow along fracture surfaces, coalescence and divergence of multiple flow paths along fracture surteces, and intrafracture water dripping. The nonlinear dynamics of flow and transport processes in unsaturated fractured porous media arise from die dynamic feedback and conpetition between various nonlinear physical processes along widi the complex geometry of flow paths. The apparent randomness of (he flow field does not prohibit the system s determinism and is, in fact, described by deterministic chaotic models using deterministic differential or difference-differential equations. 

Figure 7. Fracture surface of as-sintered sample, at (a) x500 (b) xl.5k (c) x5k showing a porous...

Figure 8.23 Fracture surface of porous silicon nitride with large fibrous grain alignment (porosity 14%). The large fracture energy of this porous silicon nitride is attributable to sliding...

Figure 8.26 Fracture surfaces of porous silicon nitride with fine fibrous grain alignment fabricated through a sinter-forging technique. The porosity is 24%. Illustration courtesy of Naoki Kondo et al.

A second example concerns another common processing step - chemical passivation of porous silicon surfaces, in particles or patterned wafers. There are three dedicated reviews in the handbook that deal with this Oxidation of mesoporous silicon, Silicon-Carbon Bond Formation for Porous Silicon, and Photoluminescent Nanoparticle Derivatization for Porous Silicon. It can be important to perform these passivation treatments after the particle sizing or patterning process otherwise freshly fractured or patterned porous silicon surfaces will not be passivated. Note that for some applications one can also choose to derivatize the pore walls during anodization (Mattei and Valentini 2003) rather than the more common sequence to derivatize after, or even both during and after anodization to get specific surface chemistries and spatially selective functionalization (Valentini et al. 2007). 

Fuentes NO, Faybishenko BA. (2003) Study of the fracture surface characteristics that determine the main paths for liquid flow and contaminant transport in fractured-porous media. Proc VI Annual Meeting ofSETAC — LA, Buenos Aires, 20-23 October 2003, 179-180. 

FuentesNO, Faybishenko BA. (2003) Evaluation ofthe conditions imposed by the fracture surface geometry on water seepage through fracmred porous media. Proc XXX Annual Meeting ofAATN, Buenos Aires, Argentina, 029 1-6. 

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