Appearance and Foam Type

Between the time a liquid film is first formed, until it thins to an equilibrium black film, the thickness can be measured by techniques based on the interference of light. For monochromatic light, the intensity of light, IR, reflected at angle 8 from a fluid film compared to the maximum intensity yielded by constructive interference, I0, enables the determination of the film thickness, t, as  

1) Lipids are fatty acids or closely related compounds, such as palmitic, oleic, linoleic, lino-lenic, ricinoleic, and arachidic acids. 

For bulk foams, some researchers have used a variation of the turbidity Eq. (2.9) by fitting the intensity of light transmitted through a sample of foam (It) to an equation of the form  

One of the simplest representations for foam drainage, in terms of the drained volume of liquid, is given by  

The density of foams has been measured by such techniques as capacitance, resistance, and MRI imaging [109]. In calculating the density of a foam (pF) one can usually ignore the mass of gas involved so that  

Foam quality refers to the gas volume fraction in a foam. Expressed as a percentage, this fraction is sometimes referred to as Mitchell foam quality. In three-phase systems, other measures are used. For example, when foams are formulated to contain sohd particles as well, the slurry quality Qj, which gives the volume fraction of gas plus solid, can be used = (Vg -I- V s)/( g + + l)  

Foam texture refers to the bubble size distribution in a foam. For foams in porous media, it can be expressed in terms of the length scale of foam bubbles as compared with that for the spaces confining the foam. When the length scale of the confining space is comparable to or less than the length scale of the foam bubbles, the foam is sometimes termed lamellar foam, to distinguish it from the opposite case, termed bulk foam. 

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