Bulk foam definition

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

One of the basic mechanical properties of the foam is its compressibility (elasticity). By definition the bulk modulus of elasticity Ev is expressed by... 

The thermophysical properties of a foam heat capacity and thermal conductivity, depend on the total liquid content and its distribution in the foam. The bubbles shape and distribution of liquid between borders, films and vertexes also affect thermal conductivity. By definition the bulk heat capacity CV (at p - const) equals... 

FIGURE 2.9 Measurement of disjoining pressure isotherms of air-liquid-air foam films, (a) Porous frit film holder located inside hermetically sealed cell subject to applied gas pressure, Pg. Bulk liquid pressure is Pl (= Pi + PhS hyd text for definition of terms)... 

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