Rheology of foams

Foams have unique rheological properties. The bulk flow of foam is very different from that of either Newtonian (laminar or turbulent) fluid or conventional two-phase fluids. The presence of a significant 

Finally, one more practical example, illustrating some of the principles of this chapter, is presented. 

In fermentation reactors, microorganisms produce enzymes (and other metabolic products) on a large scale. The enzymes are sold to companies producing, for example, detergent powders. Now and then, severe foaming appears in the stirred fermenters, which may cause loss of enzymes upon mechanical removal of the foam. 

During a particular foaming incident, a uniform foam with a lamella thickness of 100 pm (assumed here to be independent of time) is formed. 

Suggest and discuss a potential mechanism for foam formation in fermenters. 

Unfortunately, for three-dimensional foams, no analog of von Neumann s law has been discovered, and theory of coarsening is much less developed. A review of the work on this problem can be found in Glazier and Weaire (1992).------------------------------- 

Measurement of the viscoelastic properties of foams and dense emulsions is also complicated by slip at the rheometer surfaces (Yoshimura and Prud homme 1988). The liquid in an aqueous foam lubricates flat rheometer fixtures, reducing the strain imposed on the bulk foam. This lubrication is a desirable feature of some foam products such as shaving cream, but it complicates rheological studies. The use of roughened surfaces, such as sandpaper bonded to the rheometer fixtures, seems to be an effective countermeasure (Khan et al. 1988).  

Nowadays an increasing attention is given to the dynamics of the foams. This is not an easy subject, but the well-defined structure of the foam gives us hope to successfully understand the flow properties. The present stage of the physics, including the rheology of the fluid foams, is substantially described by the book of Weaire and Hutzler.i Our summary is based on this book and on more recent reviews. 

In general, the fluid soaps behave as Bingham fluids, and their shear strain follows the Bingham model  

The shear modulus and yield stress are strongly dependent on the liquid fraction of the foam. Both become smaller at larger liquid concentrations. 

Typical variation of the effective viscosity with the strain rate. 

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 Rheology of foam Rheometer Rheopexy Rheosyst Rheotron Rheovibron Rheumatic fever Rheumatoid arthritis... 

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. 

Foams have a wide variety of appHcations that exploit their different physical properties. The low density, or high volume fraction of gas, enable foams to float on top of other fluids and to fiU large volumes with relatively Httle fluid material. These features are of particular importance in their use for fire fighting. The very high internal surface area of foams makes them useful in many separation processes. The unique rheology of foams also results in a wide variety of uses, as a foam can behave as a soHd, while stiH being able to flow once its yield stress is exceeded. 

H.M. Princen Rheology of Foams and Highly Concentrated Emulsions I. Elastic Properties and Yield Stress of a Cyhndrical Model System. J. Colloid Interface Sci. 91, 160 (1983). 

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. 

The rheology of foams coupled with the proper applicators permits the creation of unique random coloring effects not easily achieved by conventional dyeing methods. Carpets are particularly well suited for the production of random coloring effects. 

Princen HM. Rheology of foams and highly concentrated emulsions. I. Elastic properties and yield stress of a cylindrical model system. J Colloid Interface Sci 1983 91 160-175. 

The problem of dynamic adsorption layers becomes very complicated in concentrated foams and emulsions and is probably of interest for rheology of foams and emulsions. Thus, a systematic study of the dynamic adsorption layer of an individual bubble is important not only by itself, but to a larger degree as investigation of fundamental processes in a variety of more complex situations sparring a range of fluid technologies. 

Princen HM, Kiss AD (1986) Rheology of foams and highly concentrated emulsions. III. Static shear modulus. J Colloid Interface Sci 112 427-437 Ramsaywak PC, Labb6 G, Siemann S et al. (2004) Molecular cloning, expression, purilication, and characterization of fructose 1,6-bisphosphate aldolase from Mycobacterium tuberculosis -a novel Class II A tetramer. Protein Expres Purif 37 220-228 Richard JP (1993) Mechanism for the formation of methylglyoxal from triosephosphates. Biochem Soc 121 549-553... 

Pri ncen, H.H. Kiss, A.D. Rheology of foams and highly concentrated emulsions. III. Static shear modulus. J. Colloid Interface Sci., 112 427. 1986. 

HM Princen, AD Kiss. Rheology of foam and highly concentrated emulsions. IV. Experimental study of the shear viscosity and yield stress of concentrated emulsions. J Colloid Interface Sci 128 177,1989. 

Princen, H.M. (1985) Rheology of foams and highly concentrated emulsions. II. Experimental study of the yield stress and wall effects for concentrated oil-in-water... 

The rheology of foam is complex, and viscosity must be measured under flowing conditions. The major things to know are that foams are viscous as well as shear thinning, even when the surfactants and liquid display Newtonian rheology. This means that foam will suspend solids when the flow stops, and the apparent viscosity decreases as flow increases (when it is being pumped). 

Foams that are relatively stable on experimentally accessible time scales can be considered a form of matter but defy classification as either solid, liquid, or vapor. They are solid-like in being able to support shear elastically they are liquid-like in being able to flow and deform into arbitrary shapes and they are vapor-like in being highly compressible. The rheology of foams is thus both complex and unique, and makes possible a variety of important applications. Many features of foam rheology can be understood in terms of its microscopic structure and its response to macroscopically imposed forces. 

Kraynik AM, Neilsen MK, Reinelt DA, Warren WE (1999) Foam Micromechanics Structure and Rheology of Foams, Emulsions and Cellular Solids. In Sadoc F, Rivier N (eds) Foams and Emulsions. NATO ASl Series. Kluwer, Dordrecht, p 259... 

Rheology of Foams—Flow in a Tube. Rheological behavior of foams has been measured in a viscometry apparatus (Fig. 5.102). 138.145 Typically, the foam constituents are mixed and passed through a foam generator, such as a packed bed. The foam is then displaced through a small-diameter tube, and flow rates, pressure drop, and temperature are measured. 

Princen HM, Kiss AD. 1986. Rheology of foams and highly concentrated emulsions 3. 

In this study, with the help of a tandem foam extrasion system, we present the first part of our investigation of the extrasion rheology of foams, both with and without cell nucleation and growth. The pressirre data obtained serve as the basis of evaluating existing cell growth models, and can facilitate die design. 

---