Porous disc holders

Figure 21.4. Cross-sectional view of typical film-holders used in the thin-film balance (a) Scheludko cell (b) porous disc holder (from ref (11))...

To prevent imbibition of the non-wetting phase, which would destroy the continuous liquid connection needed to apply equation (21.4), the maximum imposed capillary pressure must not exceed the entry pressure for the porous-disc film holder. This pressure can be estimated by Pew = 2a/rpore, where r ore is the pore radius of the porous material used. Therefore, discs with smaller pores are required for higher capillary pressures. Standard porous discs can be found with nominal pore diameters from 50 to 1 pm. Thus, when using a porous frit with pore diameters 1 pm, a typical surfactant solution (a = 30 mN/m) gives Pcmax foam films. Due to low interfacial tensions, this value can be much lower for emulsion films. Special porous... 

Disc Filters Traditional types of large disc filter holders, if used at all for production purposes, are usually about 300 nun in diameter. The direction of fluid flow is from above the filter to below. The membrane is sandwiched between metal inlet and outlet plates equipped with the sanitary connections necessary to operate the filter. Because of the fragility of disc-type membranes, there must always be a support plate directly beneath the membrane. Support plates must be porous, often photoetched, chemically inert, and have minimal effects on flow rate. This often means an uneasy compronuse. Prevention of flow restriction requires extensive void space this acts against the plate s mechanical strength. Usually there is also a loose mesh drain plate beneath the support plate and resting on the outlet plate (Fig. 2). When serially stacked disc filters are used, each membrane requires its own support plate but there will only be one drain plate. 

To measure equilibrium disjoining pressure isotherms, single films are formed in a hole drilled through a porous glass disc, which has been fused to the end of a capillary tube and saturated with the surfactant solution under investigation. The film holder is enclosed in a hermetically sealed chamber with the free end of the capillary tube exposed to a constant reference pressure. 

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