In the preceding chapter structural models of foam rheology have been presented. The dominant variables affecting rheology are the volume fraction of the bubble (dispersed) phase, the sizes of the bubbles, and the interfacial tension. These models are relatively recent in origin and they highlight the problem with most of the data in the literature on foam rheology. In most studies the volume fraction of gas and interfacial tension are reported, but not the foam bubble sizes. Much of the experimental data on foam rheology actually reflect changes in bubble size under different bubble generation conditions. Studies report how rheology changes with flow rate or volume fraction of gas, but if bubble sizes change at the same time because the foam is created by flow through a packed bed or an orifice, the reported correlations are spurious. The most important point to remember is that at high volume fractions foams are structured materials; the rheology and structure are intimately coupled.
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