Wetting porous surfaces

MAISEL and SHERWOOD(46) also carried out experiments in a wind tunnel in which water was evaporated from a wet porous surface preceded by a dry surface of length Lq. Thus, a velocity boundary layer had become established in the air before it came into... 

FIGURE 9.21. (a) Meniscus alongside a wet porous material (b) drop resting on a wet porous surface. 

Adhesives for paper tubes, paperboard, cormgated paperboard, and laminated fiber board are made from dispersions of clays suspended with fully hydrolyzed poly(vinyl alcohol). Addition of boric acid improves wet tack and reduces penetration into porous surfaces (312,313). The tackified grades have higher solution viscosity than unmodified PVA and must be maintained at pH 4.6—4.9 for optimum wet adhesion. 

Wetting is an absolute condition for detergency. However, wetting plays an important role in other applications as well. A special case is the penetration of fluids in porous material. That may be a bundle of fibers in the dying process or the stone matrix in enhanced oil recovery. One of the steps of lubrication is wetting of surfaces by lubricant liquids. Because other conditions must also be considered, the use of phosphorus-containing surfactants is beneficial. 

The mercury penetration approach is based on the fact that liquid mercury has a very high surface tension and the observation that mercury does not wet most catalyst surfaces. This situation holds true for oxide catalysts and supported metal catalysts that make up by far the overwhelming majority of the porous commercial materials of interest. Since mercury does not wet such surfaces, the pressure required to force mercury into the pores will depend on the pore radius. This provides a basis for measuring pore size distributions through measurements of the... 

Wet a sheet of filter paper a little bit larger than the gel with 5% glycerol in water. Place this sheet on the porous surface of the apparatus, lay the gel onto the paper, and cover it with wetted cellophane or another type of plastic foil. 

A convenient method to produce porous surfaces is the anodic oxidation of aluminum plates. Such microstructured aluminum platelets have been coated by wet impregnation with Pt-, V- and Zr-precursors [35], and tested under catalytic methane combustion conditions. The conversion rate of oxygen followed directly the platinum content in the catalysts. These data were well reproducible even after five different runs. 

In the case of catalytic methane combustion, aluminum was chosen as an appropriate material for the catalyst wafers since anodic oxidation of aluminum can be used to obtain porous surfaces. Such micro structured aluminum platelets were coated by wet impregnation with Pt, V and Zr precursors [50],... 

The chief source of acidity in paper is the alum used in the papermaking process. Paper (largely cellulose) i6 a very hydrophilic substance, and its surface has a high specific energy. Thus, water readily wets paper surfaces. The very porous structure of paper makes it act like a sponge in the presence of liquids. Hence, various chemicals have been developed to make paper reasonably water... 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

Primers of low viscosity, usually dispersions or solutions, may be applied in thin layers to the substrates and generally are allowed to dry before the adhesive proper is spread. Such primers are intended to wet the surfaces thoroughly, and should be compatible with the relevant adhesive. In some instances they offer the further advantage of sealing porous substrates and so making possible more consistent bonds. 

Figure 11 Heterogenous solid compartments in soils and sediments. NAPL signifies nonaqueous phase hquid and SOM represents soil organic matter. A—absorption into amorphous organic matter or NAPL, B—absorption into soot or black carbon, C—adsorption onto water-wet organic surfaces, D—adsorption to non-porous mineral surfaces, and E—adsorption into microvoids or microporous minerals (reproduced by permission of American Chemical Society...

Besides the traditional wet granulation, surface adsorbents, especially porous materials, are very helpful for maintaining liquid in a granulation. For example, calcium silicate, a fine porous powder, has been successfully used to adsorb an oily medicine, tocopheryl nicotinate. Due to the high capillarity of the pores inside calcium silicate, the adsorbent has an excellent liquid-holding ability. In the formulation preparation, after drug dissolved in ethanol was adsorbed on calcium silicate, hydroxypropylcellulose (HPC) was added to the mixture and granulated to improve flowability. 

As depicted for the cases in Figure 2, the phase that preferentially wets the pores can be determined from free energy considerations of the system. A phase will wet a porous surface if the free energy of the wetted surface is lower than die sum of the free energies for the original liquid surface and the original porous surface. A... 

Two common liquid membrane support materials, polytetrafluoroethylene and polypropylene, have critical surface tensions of 18 mN/m and 35 mN/m, respectively. Manufacturers often supply critical surface tensions for their porous films. Liquids with a surface tension, y, less than the critical surface tension will probably wet the surface. Therefore, hydrocarbons will wet polypropylene, but water (y = 72 mN/m) will not. Shafrin and Zisman (30) have summarized critical surface tension data for many materials and correlated the data such that critical surface tensions may be estimated from knowledge of the functional groups in the chemical structure of the surface. 

HEPA) filters to remove as much dust as possible. If necessary, wash (wet-vac) with copious amounts of alkaline detergent and water. Do not use bleach or detergents containing bleach as they may interact with agents to produce toxic decomposition products. Porous surfaces may be difficult to decontaminate. 

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