Rheological Properties of Foams

The numerous previous studies of the flow of foam in porous media and of its application for. improving the displacement of oil from such media, have almost always been conducted under ambient conditions of temperature and pressure there have been very few reports of laboratory studies under reservoir conditions. Although many interfacial properties are known to be temperature dependant, little attention has been paid to the influence of temperature upon the properties of foam. Furthermore, the rheological properties of foams, and their effectiveness for the displacement of oil are strongly dependant upon foam quality, which is in turn... 

In a later investigation, Kraynik and Hansen [62] demonstrated that the shear rate and liquid film viscosity greatly affect the rheological properties of foams. They studied the effect on foam properties and structure with variation of capillary number, Ca, which is the ratio of viscous to surface tension forces in the liquid films, and is given by -... 

While the Bingham plastic model is an adequate approximate description of foam rheology, it is by no means exact, especially at low strain rates. More detailed models attempl to relate the rheological properties of foams to the structure and behavior of the bubbles. 

This quality is often used when determining rheological properties of foams when proppant is present (6) (Figure 2). 

Rheological properties of foams (elasticity, plasticity, and viscosity) play an important role in foam production, transportation, and applications. In the absence of external stress, the bubbles in foams are symmetrical and the tensions of the formed foam films are balanced inside the foam and close to the walls of the vessel [929], At low external shear stresses, the bubbles deform and the deformations of the thin liquid films between them create elastic shear stresses. At a sufficiently large applied shear stress, the foam begins to flow. This stress is called the yield stress, Tq- Then, Equation 4.326 has to be replaced with the Bingham plastic model [930] ... 

Reliable measurement of rheological properties of foams is not an easy task. For example, in a Couette viscosimeter the foam is not homogeneously deformed part of it remains in elastic solid, while another part flows. Flow in pipe, for example, may consist of a plug flow in some places and fluid type in other parts. 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of soHds, Hquids, and gases. Like an ordinary soHd, foams have a finite shear modulus and respond elastically to a small shear stress. However, if the appHed stress is increased beyond the yield stress, the foam flows like a viscous Hquid. In addition, because they contain a large volume fraction of gas, foams are quite compressible, like gases. Thus foams defy classification as soHd, Hquid, or vapor, and their mechanical response to external forces can be very complex. 

Journal of Polymer Science Polymer Physics Edition 39, No.18, 15th Sept.2001, p.2159-67 RHEOLOGICAL PROPERTIES AND FOAM PROCESSABILITY FOR BLENDS OF LINEAR AND CROSSLINKED POLYETHYLENES Yamaguchi M Susuki K-I Tosoh Corp. 

In much of the work on rheology, foams and HIPEs have been considered as analogous. The expressions derived are applicable to both systems, only the actual values are different. Consequently, workers in this area choose to study either emulsions or foams (or both) and so, in this section, the rheological properties of HIPEs and high gas-fraction (or dry ) foams will be discussed jointly. 

Another important rheological property of dry foams and highly concentrated emulsions is G, the shear modulus. Princen and Kiss [57] demonstrated that this property was dependent on < >, the volume fraction of the system. Previously, Stamenovic et al. [58] and, much earlier, Derjaguin and coworker [59], had derived an expression for the shear modulus of foams of volume fraction very close to unity. The value was found to depend on the surface tension of the liquid phase (in foams), for the particular case of (Jja 1. However, Princen demonstrated that the values of G obtained were overestimated by a factor of two. This error was attributed to the model used by Stamenovic and coworker, which failed to maintain the equilibrium condition that three films always meet at angles of 120° during deformation. 

Thus, it is highly evident that wall-slip must be taken into account when investigating the rheological properties of HIPEs and foams [87], Failure to do so will result in false and irreproducible results. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of solids, liquids, and gases, and their mechanical response to external forces can he very complex. 

Irrespective of the method used, the quality of a syntactic foam depends substantially on the rheological properties of the initial mixture with the microspheres68 70). The investigation of the rheology of syntactic compositions by Petrilaenkova et al. 71)... 

Nam GJ, Yoo JH, Lee JW (2005) Effect of long-chain branches of polypropylene on rheological properties and foam-extrusion performances. J Appl Polym Sci 96 1793-1800... 

The categorizations just presented are helpful in describing the rheological properties of many emulsions, foams, and suspensions. It should be remembered, however, that some dispersions are not well described by any single category. Some dispersions exhibit Newtonian behaviour at low shear rates, shear thinning at moderate shear rates,... 

Malhotra, A.K. Wasan, D.T. Interfacial Rheological Properties of Adsorbed Surfactant Films with Applications to Emulsion and Foam Stability in Thin Liquid Films, Ivanov, I.B. (Ed.), Dekker New York, 1988, pp. 829-890. 

The results of the experiments discussed and the conclusions drawn indicate that foam films play an extremely important role in foam stability. However, there is a number of other factors, such as rheological, that also affect foam stability, though in most cases the stability can be explained when the properties of foam films are taken into account. In this sense the correlation foam film/foam should be regarded as a general trend. 

Princen and Kiss [13] have studied the rheological properties of emulsions in which the effects of drainage, coalescence and diffusion transfer is much less expressed than in foams. A concentric-cylinder viscometer was used. The slip was estimated by rheograms in the t vs. 0) lx form (t is the stress measured on the inner cylinder wall, co is the angular velocity of the outer cylinder). The Xq values obtained are conform well with those from Eq. (8.24) but Xo (y) function is not a Bingham one, i.e. does not obey Eq. (8.11) at rj = const. 

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