Emulsions flow behaviour

The viscosity of the mixed oils is higher than that of the bagasse and the PR individually. This is due to the formation of con Iex three component emulsions (biooil, PR-derived hydrocarbons and water) with dispersed solid particles. As expected, the mixed oils exhibit non-Newtonian flow behaviour (herein not shown). The con lex emulsion obtained seems to be more stable than the one obtained by mixing die oils produced separately from bagasse and PR. The oils from bagasse, PR and the mixed oils were also observed by microscopy. The existence of three liquid emulsions was confirmed by microscopic analysis (Figure 4). 

Most emulsions, unless very dilute, display hoth plastic and pseudoplastic flow behaviour rather than simple Newtonian flow. The flow properties of fluid emulsions should have little influence on their biological behaviour, although the rheological characteristics of semisolid emulsions may affect their performance. The pourability, spreadability and syringeability of an emulsion will, however, be directly determined by its rheological properties. The high viscosity of w/o emulsions leads to problems with intramuscular administration of injectable formulations. Conversion to a multiple emulsion (w/o/w), in which the external oil phase is replaced by an aqueous phase, leads to a dramatic decrease in viscosity and consequent improved ease of injection. 

The emulsion behaviour in porous media is discussed in [235]. O/w emulsions with volume fractions of up to 50% show Newtonian behaviour, whereas those with more than 50% are non-Newtonian liquids, the apparent viscosity of which depends on the shear rate. The viscosities of such emulsions are more than 20 times that of water and sometimes can be even comparable with that of oil. When the emulsion is moving, a temporary permeability reduction of the reservoir may occur due to the capture of small droplets by the surface of the porous medium. In this case, stable o/w emulsions may flow not as a continuous liquid, i.e. the emulsion flow largely depends on the nature of the porous medium. Therefore, it is necessary to know about the structure and physicochemical characteristics of the oil reservoir (porous medium) porosity, the mean pore diameter, the mean pore size and pore size distribution, chemical composition of the minerals ( acidic , basic , neutral ), the nature of the pore surface, first of all wettability, for a successful application of the emulsion flooding method. 

Rheology is the study of material flow behaviour. The fluids referred to in the range of mixing problems discussed in section 1.1 cover an enormous variation of rheological properties. The possibilities are extended further when two- and three-phase mixtures, especially stable ones which are products of mixing operations such as emulsions, foams and dispersions are included. There are many books available which deal with this topic in some detail All that is presented here is sufficient to allow the reader to understand better those parts of later chapters which refer to rheologically-complex fluids. 

Bower, C., Gallegos, C., Mackley, M.R., and Madiedo, J.M. (1999) The rheological and microstructural characterisation of the non-linear flow behaviour of concentrated oil-in-water emulsions. Rheal. Acta, 38 (2), 145-159. 

This is called the Sisko equation, and it is very good at describing the flow behaviour of most emulsions and suspensions in the practical everyday shear rate range of 0.1 to 1000 s-i. 

Generally coating compositions consist of quite a few different raw materials. Emulsion polymers as binder as well as pigments and extenders are the most crucial and basic constituents. They usually make up for the major part of a coating. However, they cannot be formulated into a paint that fulfils the required application properties. Additives have to be employed to adjust flow behaviour, ensure a proper dispersing of the pigments, improve film formation, control foam formation, prevent microbiological attack etc. (Bielemann, 2000). 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

The behavioural features of the fluidized bed have been modeled based on a modified representation of the Fryer-Potter model. The restrictive assumption of plug flow of the emulsion gas has been removed, and model equations developed based on complete mixing of the emulsion gas. This simplification, in addition to bringing the model closer to reality, has led to the conversion of a boundary value problem (Fryer-Potter model) to a simpler initial value problem. Except at very low bubble diameters, the predictions of the two models (based on terminal conversion) agree closely with each other. On the other hand, agreement between the average concentration profiles in the bed predicted by the two models is less satisfactory. While therefore the modified model proposed in this work has the advantage of simplicity and is perhaps closer to reality, further experimental work on industrial size equipment is necessary for a firmer opinion on the latter (nature of gas flow in the emulsion phase). 

It has been shown by Chavarie and Grace (15) that the decomposition of ozone in a fluidized-bed is best described by Kunii and Levenspiel s model (16) but that the Orcutt and Davidson models (17) gave the next best approximation for the overall behaviour and are easier to use and were chosen for the simulation. They suppose a uniform bubble size distribution with mass transfer accomplished by percolation and diffusion. The difference between the two models is the presumption of the type of gas flow in the emulsion phase piston flow, PF, for one model and a perfectly mixed, PM, emulsion phase for the other model. The two models give the following expressions at the surface of the fluidized bed for first-order reaction mechanism ... 

Interestingly, this slip behaviour of hard-sphere glasses is different in nature from that found earlier by Meeker et al. in jammed systems of emulsion droplets [60]. There, a non-linear elasto-hydrodynamic lubrication model, appropriate for deformable particles, could quantitatively account for their observations. It therefore appears that, while slip is ubiquitous for yield stress fluids flowing along smooth walls, the mechanism for its occurrence can be highly system dependent. 

Mobilisation of NAPL generally leads to the formation of an oil bank (see Chapter 10.3) in front of the surfactant solution. If the solubilisation capacity of the surfactant solution is too low, large amounts of emulsions will be formed, which can clog the pore space. As the flow in columns is forced, these experiments may not correctly reflect the behaviour of the multiphase system under free flowing conditions in a three-dimensional pore space. 

Dolz M., Hernandez M. J., Delegido J., Alfaro M. C., Munoz J. 2007. Influence of xanthan gum and locust bean gum upon flow and thixotropic behaviour of food emulsions containing modified starch. r. Food Ena.. 81,179-186. 

Gels and emulsions show plastic and pseudo-plastic behaviour. At rest, these systems are more viscous than when they flow. Creams and ointments should be easily spreadable, but they should not drip from the skin. Emulsions and suspensions should be as stable as possible and pouring should be easy. 

The key factors that influence the selection of the hydrocolloid thickener for this type of application are its stability in relation to the product formulation and process and the stability and rheology of the final product. Dressings for example are usually manufactured with a cold make-up process. They often have high levels of salt and acid, which influence the hydration and stability of the hydrocolloid. Stability in the final product in terms of oil emulsions and suspension of herbs and spices is critical. The flow properties are also important and the pseudoplastic behaviour of hydrocolloids is ideal. This provides suspension and stability due to high viscosity at low shear rates but the product remains pourable at higher shear rates. Sauces are usually prepared with a hot process and consideration of the heat stability of the hydrocolloid is important. 

Experimentally, Newton s law is generally followed only by gases and by simple liquids in laminar flow. It is only at a very low deformation rate and shear stress that solutions of macromolecules, molten polymers, emulsions and suspensions, display approximately Newtonian behaviour. 

In an ideal liquid, the rate of flow is proportional to the force inducing that flow. Mobile liquids approximate to this ideal behaviour and are known as Newtonian liquids. For most commercial emulsion polymers the rate of flow increases mote rapidly than the rise in inducing force. This is known as pseudoplasticity. A simple illustration of different rheological behaviour is shown in Figure 7-14. 

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