Interconnected pores

Effective porosity (Ne) is of more importance and, along with permeabihty (the ability of a material to transmit fluids), determines the overall ability of the material to store and transmit fluids or vapors readily. Where porosity is a basic feature of sediments, permeability is dependent upon the effective porosity, the shape and size of the pores, pore interconnectiveness (throats), and properties of the fluid or vapor. Fluid properties include capillary force, viscosity, and pressure gradient. 

Any progress in understanding the effects of coke on the rates of the reactions beyond the empirical level of the type of equation (2) requires more information on the structure of the coke and of the catalyst. In other words is the effect of coke limited to the coverage of sites Or can it grow to a size blocking the pores What is the pore size distribution of the catalyst Are the pores interconnected ... 

Fig. 5 A schematic illustrating the necessity of an interconnected pore system for the formation of mesoporous carbon. When MCM-41 (A), which does not have interconnections, was used as template, the structure collapsed upon calcination. The use of large pore interconnected SBA-15 (B) led to a stable, highly ordered material. (Adapted from Ref. I)...

By using Vg, Sg, and p, Wheeler replaced the complex porous pellet with an assembly (having a porosity e) of cylindrical pores of radius a. To p e- diet Dg from the model the only other property necessary is the length of the diffusion path. If we assume that, on the average, the pore makes an angle of 45° with the coordinate r in the resultant direction of diffusion (for example, the radial direction in a spherical pellet), =. Jl r. Owing to pore interconnections and noncylindrical shape, this value of is not very satisfactory. Hence, it is customary to define in terms of an adjustable parameter, the tortuosity factor 5, as follows ... 

The behavior of foam in porous media is intimately related to the connectivity and geometry of the medium in which it resides. Porous media have several attributes that are important to foam flow. First, they are characterized by a size distribution of pore-bodies (sometimes called pores) interconnected through pore-throats of another size distribution. Body and throat size distributions are important as is their possible correlation to determine the distribution of body to throat size ratios. Foam generation and destruction mechanisms in porous media depend strongly on the body to throat size aspect ratio. 

In this case, the solid is permeated by a system of pores (interconnected or not), somewhat in the manner of a sponge. The net result is the creation of a lai e internal surface. The pore openings of such active solids should not be too narrow since they must allow the gases to penetrate into the interior (see [8]). 

Murphy WL, Dennis RG, Kileny JL et al (2002) Salt fusion an approach to improve pore interconnectivity within tissue engineering scaffolds. Tissue Eng 8 43-52... 

Figure 11.23 TEM images of macroporous metal films, (a) Top view of a Cu film with 325 nm diameter voids, (b) Top view of a Ag film with 353 nm diameter voids, (c) Higher magnification of a Ni film showing the smaller pores interconnecting the air holes, (d) Cu film (a) at lower magnification. (Reproduced with permission from K. M. Kulinowski et al., Adv. Mater. 2000,12, 833.)...

The inclusion of additives in the aqueous phase has several objectives. These additives can be used to control the pore and interconnect sizes but, most importantly, they can be used to chemically modify the polymer after polymerization. Surface as well as bulk modification of PHP cannot be achieved through post-polymerization impregnation especially when the pore/interconnect sizes are small and when the sample is thick. Since the additives are uniformly distributed in the aqueous phase droplets, at the postpolymerization modification stage, the additives are uniformly distributed within the pores. As the aqueous phase is the major emulsion phase, large quantities of desired substances can be incorporated within PHP at levels comparable to that of the polymer phase. 

In another variation, dense gas CO can be used to induce porosity in polymeric biomaterials. As dense gas CO generally has a low solubility in hydrophilic polymers, various attempts have been made to improve the ability of dense gas to diffuse into hydrogel precursor solutions and produce porosity in hydrogel matrices, such as COj-water emulsion templating or the use of cosolvent systems [69]. With this technique, it is difficult to control pore size and ensure pore interconnectivity. 

By using microgels with well-defined properties as building blocks, macroscale hydrogels with unique spatial properties, such as gradients in mechanical and/or biomolecular characteristics, may be built from the bottom up. Microsphere-based scaffolds additionally offer 3D pore interconnectivity and desirable pore size. For instance, chitosan microsphere scaffolds have been produced for cartilage and osteochondral tissue engineering [81]. 

---