Porous-plate method

Fig. 3.40. Total disjoining pressure n vs. equivalent film thickness porous plate method aqueous...

One possible explanation of the observed dependence might be that the film rupture in om systems occurs by passing below the barrier II (Fig. 13c). Indeed, Bergeron [45] showed with large planar foam films (studied by the porous plate method) that in some systems II aI corresponded to an actual maximum of the calculated curve IlAs(fj), whereas in other systems ns was well below the maximum of the calculated nAs(fj) cmves (for a possible explanation see Ref. 45). Such a possibility is offered by different theoretical models of film rupture, in which the formation of imstable spots in large liquid films by various mechanisms is considered [40,45-47]. However, all these models are developed for large planar films and cannot be applied directly to om system without a careful analysis of the role of film curvature in the film rupture process. Fmther experimental and theoretical work is under way to reveal the actual mechanism of film rupture, to develop an adequate model of this process, and to explain the observed linear dependence of Has versus 1// eef-... 

Another method, which is especiafly suitable for low melting point solids or solids which decompose at low temperatures, is to place the material on a porous plate or pad of drying paper, and to cover the latter with another sheet of Alter paper perforated with a number of holes or with a large clock glass or sheet of glass supported upon corks. The air drying is continued until the solvent has been completely eliminated. 

Small quantities of solids may be spread upon unglazed porcelain plates. The chief disadvantage of this method is the comparatively high cost of the porous plates, since they cannot be conveniently cleaned nor can the same area be used for different substances. However, a plate may be broken and used for small amounts of material. 

Thiatriazole-5-thiol and its salts are readily obtained from the water-soluble alkali or alkaline earth azides with CS2 at 40 °C <64AHC(3)263>. However, these salts should be handled with extreme care as violent explosions have been reported when the alkali salts are spread on a porous plate or rubbed with a spatula. The slightly soluble heavy-metal salts are very sensitive to shock even under water. An improved method for the preparation and storage of sodium thiatriazole-5-thiolate has been reported. The free acid is obtained by addition of concentrated hydrochloric acid to a chilled solution of the sodium salt but can also be prepared from hydrazoic acid and carbon disulfide (Scheme 38). 

Gallagher et al.50 have reported a method that is somewhat similar to MSP. As with MSP, the GDM is initially saturated with water and placed on a porous plate that is also water-saturated. Instead of changing the GDM saturation by drying, as in MSP, the capillary pressure is controlled directly by applying suction to the porous plate. Capillary equilibrium is established between the GDM and the plate as water flows from the GDM into the plate and vice versa. 

Preparation of self-supporting ang thin films, in which the clay particles are oriented normal to the c axis, is one of the more useful drying methods. A simple apparatus in which such films can be prepared is illustrated in Figure 4. The sample suspension is transferred through the ball valve and placed onto the drying surface, which may be either a porous plate, porous filter overlying the porous plate, or a non-porous window resting on the porous plate. 

A simple and largely applied method for foam formation is dispersion of gas through porous plates (filters) placed at the lower parts of foam generation apparatus [5-10], This method is employed in flotation, in gas adsorption and dust collection in set-up with turbulent gas emulsion, and in the equipment for foam separation. The dispersity of a foam thus obtained depends on filter pore size or capillary diameter, hydrophility of the material used in the dispersion device construction, physicochemical properties of the foaming solution (surface tension, viscosity, surfactant concentration, etc.) and conditions of the dispersion process. 

There are various equipment for foam generation that employ the method of gas blowing through porous plates or capillaries [6,7,11], One of the first equipment of this type is the aspiration foam generator of Tyutyunnikov and Kasyanova [30]. A similar device that works with smaller volume of foaming solution is proposed by Japanese researchers [31]. 

The capillary pressure pa in a foam can be significantly increased by the Foam Pressure Drop Technique employed to produce dry foams [32-35]. The principle of this method is that the foam is brought into contact with a porous plate (usually sintered glass filter) under which a reduced pressure p 0 is created, and this pressure difference Ap = p0 - p 0 should not exceed the capillary pressure in filter pores 2acosd / rn (where 6 is the contact... 

The device, presented in Fig. 2.25 (without electrodes 1 and 6) has been used in [136] for the determination of K by the so-called stationary bubble method . A foam film forms on the porous plate acquiring the shape of a hemisphere. The radius of curvature R is practically equal to the radius of the perimeter at the base of the hemispherical bubble. Because of the gas passing from the bubble through the foam film into the atmosphere, R decreases with time t. The values of r are measured and K is calculated from... 

Approximate calculations for a gravitational foam drainage have shown [5] that at a 0.1 mm, a =30 mN/m and film thickness h = 0.1-0.2 pm (this state is usually reached 5-7 min after foam formation), the amount of liquid in Plateau borders exceeds more than 10 times that in films. This applies for a foam layer situated 4-5 cm above the porous plate. For low dispersity foams (a = 1 mm or more) the condition h r a is fulfilled at this height even at h 1 pm, i.e. practically at the moment of foam formation by the barbotage method. 

High expansion ratio foam can be produced by stream generators or by any other modes and then dried by creating reduced pressure from one side of the porous plate, the other side being in contact with the foam (see Section 1.2). This method of foam drying proves to be the only one used in industry [9-12]. The process of foam centrifugation in order to reach a... 

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