Foam viscosity, determination

This work shows that high shear rates are required before viscous effects make a significant contribution to the shear stress at low rates of shear the effects are minimal. However, Princen claims that, experimentally, this does not apply. Shear stress was observed to increase at moderate rates of shear [64]. This difference was attributed to the use of the dubious model of all continuous phase liquid being present in the thin films between the cells, with Plateau borders of no, or negligible, liquid content [65]. The opposite is more realistic i.e. most of the liquid continuous phase is confined to the Plateau borders. Princen used this model to determine the viscous contribution to the overall foam or emulsion viscosity, for extensional strain up to the elastic limit. The results indicate that significant contributions to the effective viscosity were observed at moderate strain, and that the foam viscosity could be several orders of magnitude higher than the continuous phase viscosity. 

The stability of a foam is determined through the interplay of a number of factors that involve bulk solution, interfacial properties, and also external forces. We have summarized some of the effects on foam stability of gravity drainage, capillary suction, surface elasticity, viscosity, electric doublelayer repulsion, dispersion force attraction, and steric repulsion. Foams are such complex systems that Lucassen (56) has stated that any attempt to understand their properties in terms of a simple all-embracing theory is doomed to failure. Nevertheless, we have attempted to provide an introduction to the occurrence, properties, and importance of foams as they relate to the petroleum industry. More detailed aspects are taken up in the subsequent chapters of this book. 

In an early work [52], A. M. Kraynik proposed another formula for determining lcr. According to [52], densities and viscosities of foamed and unfoamed melt have to be known in order to calculate lcr. Determination of these parameters is by no means easy, which is why Eq. (17) appears better than the one proposed by A. M. Kraynik. 

Filling of liquid products is determined by their viscosity, surface tension, foam-producing, and compatibility with filling machine components. Liquids are often filled at a higher temperature to allow better flow. In most instances,... 

For the studies presented in this chapter, samples of peanut and cottonseed meal suspensions were evaluated for foam capacity, stability, and viscosity measurements as described by Cherry and coworkers (23, 24, 22). Vegetable protein suspensions at the appropriate concentration and pH were whipped in a Waring-type blender. After blending, the whipped products were transferred to a graduated cyclinder. Milliliters of foam were recorded immediately and at various time intervals to determine capacity and stability. A Brookfield viscometer and... 

To determine rheological parameters such as the yield stress and effective viscosity of a foam, commercial rheometers are available rotational and conlinuous-lfow-tubc viscometry are most commonly employed (See also Rheology). However, obtaining reproducible results independent of the sample geometry is a diflicull goal which arguably has not been achieved in most of the experiments reported in the scientific lileralure... 

The main factor determining the stability of such foams is the rate and extent of drainage from the thin liquid film. In general, this type of foam is relatively unstable. The stability may be enhanced by increasing the viscosity of the liquid by increasing the dry matter content or adding certain hydrocolloids. The foam stability may also be enhanced with hydrocolloids, in particular microcrystalline cellulose. 

Foams are always thermodynamically stable. The stability of liquid foams is largely determined by the repulsion between surfactants and the viscosity of the liquid. They decay by drainage driven by the negative Laplace pressure in the Plateau borders. 

Disclaimer As in all theoretical variable determinations, these equations presented for Du calculation are subject to field-test verification. Equations (4.14) and (4.16) are not presented as being infallible or able to predict accurately every case of particle size with a given medium viscosity. For example, a crude with a high asphaltene content should be field tested before a final design for construction is issued on the basis of these equations. Small asphaltene crude contents (less than 2%) were used in deriving Eq. (4.16). More tests are needed for foam-liquid separations. Readers and users of this criterion, can perhaps contribute more data, and I indeed solicit such contributions of better methods and data as you may discover. 

Rheological analyses give information about the viscosity and gel point. Such analyses have been coupled with thermogravimetry and have been used to determine a good way to prepare thermostable foams [32]. 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

A typical dependence of drainage onset on foam column height at foam expansion ratio n = 70 is given in Fig. 5.14. [6,22], For high foam columns (H > 16 cm) zb is small and does not practically depend on H. It is mainly determined by the hydrodynamic properties of the system (borders size and viscosity), i.e. of the microsyneresis rate. For small foam column heights t0 strongly depends on H and is determined by the rate of internal foam collapse. These dependences indicate that for a quantitative description of drainage detailed... 

When a cooking oil oxidizes, polymers are formed that cause the oil to foam. In addition, the viscosity of the oxidized oil increases, making the cooked food look oily as a result of retention of a higher amount of oil on the surface of the food. Many methods have been proposed for the determination of polymers in the oil (71-74). 

Investigating column performance starts with heat and material balance calculations, which generate liquid and vapor flows and properties. The fluid foaming characteristics and corrosivity are also determined. Typically, the calculations are done on a computer simulator, which generates liquid and vapor flows as well as physical properties, such as densities, viscosities, and surface tension. 

Characterisation of foams employs one of two techniques, either static or dynamic. In the static method, a foam is generated by sparging a gas into a liquid under controlled conditions and then stopped. The decay of the foam level is then monitored against time. The half-life of the foam is termed the foam lifetime. The dynamic method generates the foam continuously under standard conditions and the equilibrium volume measured. With care, a linear relationship between foam volume and gas velocity can be determined. The gradient of this linear response is the foam lifetime. It is not surprising that foam lifetime and viscosity show an identical dependence on temperature. 

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