Porous media thin films

The basic mechanism of foam degradation in porous medium is film coalescence. It depends on film thickness and capillary pressure. In the process of advancement the film thickness changes considerably thickens in the narrow parts (pore throats) and thins in the wider parts (pore bodies). Visual observations of such a stretching-squeezing mechanism are reported by Huh et al. [178]. Therefore, the film thickness would depend on the liquid/gas ratio, the rate of movement and the ratio of pore-body to pore-throat. When the critical capillary (disjoining) pressure is reached, the film will rupture. 

One prominent example of rods with a soft interaction is Gay-Berne particles. Recently, elastic properties were calculated [89,90]. Using the classical Car-Parrinello scheme, the interactions between charged rods have been considered [91]. Concerning phase transitions, the sohd-fluid equihbria for hard dumbbells that interact additionally with a quadrupolar force was considered [92], as was the nematic-isotropic transition in a fluid of dipolar hard spherocylinders [93]. The influence of an additional attraction on the phase behavior of hard spherocylinders was considered by Bolhuis et al. [94]. The gelation transition typical for clays was found in a system of infinitely thin disks carrying point quadrupoles [95,96]. In confined hquid-crystalline films tilted molecular layers form near each wall [97]. Chakrabarti has found simulation evidence of critical behavior of the isotropic-nematic phase transition in a porous medium [98]. 

Since Thin Film Spreading Agents do not produce ultralow interfacial tensions, capillary forces can trap oil in pore bodies even though the oil has been displaced from the surface of the porous medium. Therefore, recovery of incremental oil is dependent on the formation of an oil bank. Muggee, F. D. U.S. Patent 3 396 792, 1968. 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

A third, related limit on the capillary pressure is created by the existence of an upper critical capillary pressure above which the life times of thin films become exceedingly short. Values of this critical capillary number were measured by Khistov and co-workers for single films and bulk foams (72). The importance of this phenomenon for dispersions in porous media was confirmed by Khatib and colleagues (41). Figure 5 shows the latter authors plot of the capillary pressures required for capillary entry by the nonwetting fluid and for lamella stability versus permeability of the porous medium. 

Many different types of filters are available commercially. They can be broadly classified into two types with, however, some overlap. Fibrous filters are composed of mats of fi bets that may be ntade of cellulose, quartz, glass, polymeric materials, or metals. Porous membrane filters are usually composed of thin films of polymeric materials 0.05 to 0.2 mm thick sufficiently porous for air to How through under pressure. Pore size is controlled in the manufacturing process and ranges from 0.02 to 10 /rm, A significant fraction of the panicles may be caught on the upstream surface of the filter, but some particles may also penetrate and be caught inside the pores of the medium as well. 

Schramm [257] considers it a big problem that foams are sensitive to the contact with oil under porous medium in oil recovery. While proposing a several foam breaking mechanisms under reservoir conditions, the author believes the emulsification process of oil in water is the most important step. In emulsification, the contact area of pseudo-emulsion films increases with the oil contents. In case the pseudo-emulsion films are stable, the foam stability and thus the process efficiency increases. Thinning of pseudo-emulsion films leads to its rupture when gas is continuously injected into the media, flooded with a surfactant solution at residual oil... 

In some circumstances a fluid can be totally wetting to a porous medium. In such cases a thin film of wetting fluid covers the solid. If bulk amounts of wetting fluid are present they connect to thin film through transition regions in which the film thickens the wetting fluid is then distributed not only in continuous and disconnected pendular states of bulk material, but also in thin film states. When the surface area of the solid is great, as it can be, for example because of clay minerals in sandstone oil reservoirs, the thin films can contain appreciable inventories of... 

For polymer solutions, a decrease in the solvent thermodynamic quality tends to decrease the polymer-solvent interactions and to increase the relative effect of the polymer-polymer interactions. This results in intermolecular association and subsequent macrophase separation. The term colloidally stable particles refers to particles that do not aggregate at a significant rate in a thermodynamically unfavourable medium. It is usually employed to describe colloidal systems that do not phase separate on the macroscopic level during the time of an experiment. Typical polymeric colloidally stable particles range in size from 1 nm to 1 xm and adopt various shapes, such as fibres, thin films, spheres, porous solids, gels etc. 

A very rough hydrophilic surface is somewhat similar to a thin porous medium. We can imagine that some liquid escapes from the drop and penetrates into the nooks and crannies of the solid. The volume of film captured in the recesses is generally negligible, and in the end the drop finds itself essentially on a wet substrate viewed as a patchwork of solid and liquid. 

The primary optimization parameter of porous electrodes is the ideal electrochem-ically active surface area per unit volume sa- rough approximation, the value ECSA is proportional to the amount of the electrocatalytically active material Pt, in the case of PEFC electrodes. It is inversely proportional to the feature size d, which could represent diameters of catalyst particles, of pores in a porous catalytic medium, or of rod-like structures (nanotubes or nanorods), onto which a thin film of catalyst is deposited. On the other hand, is also roughly proportional to the energy density... 

Conducting polymers can be prepared by chemical or electrochemical techniques. Electrochemical synthesis provides easier routes when compared with chemical synthesis and allows control over film formation, especially relevant if polymers are required as thin films deposited on the surface of metallic substrates. However, electrochemically synthesized polymers are usually more porous, a feature that requires consideration when a barrier effect is necessary. Another important aspect in the corrosion field is that the application of potential/current necessary to promote electropolymerization may accelerate dissolution (corrosion) of the metal. In some cases, an oxide pre-layer is deposited between the metal and the polymer to promote adhesion and hinder metal dissolution during the electropolymerization process (Tallman et al., 2002 Spinks et al., 2002). Alternatively, the application of layered coatings based on different conducting polymers can be a strategy to overcome the problem of metal dissolution. In the work of Lacroix et al. (2000), a layer of PPy was firstly deposited on zinc and mild steel in neutral conditions, followed by deposition of PANi in an acidic medium, because the direct deposition of PANi on those metallic substrates was not possible in an acidic medium, causing dissolution of the metal. 

Fig. 6. Cross-section through a fractal porous medium showing the configuration of the thin film phase in the pores. Only pores with sizes below the critical threshold level are entirely wetted. In the remaining pores the presence of the film phase is controlled by the magnitude of the adverse pressure (a) the porewails are entirely wetted (b) the film phase appears in corners only.

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