Suspensions flow properties

A suspension is a dispersion of solid particles in a liquid. A colloidal suspension is a sol. Colloidal properties become significant when the size of the parhcles is of the order of a few micrometer or less. In suspensions of large particles, for example, of some 10 pm or higher, hydrod5mamic interactions dominate the suspension flow properties emd particle packing behaviour. In colloidal suspensions interaction forces between the particles as well as hydro-dynamic interactions play a role in determining the flow and particle packing properties. 

Detailed treatments of the rheology of various dispersed systems are available (71—73), as are reviews of the viscous and elastic behavior of dispersions (74,75), of the flow properties of concentrated suspensions (75—82), and of viscoelastic properties (83—85). References are also available that deal with blood red ceU suspensions (69,70,86). 

Suspensions of fine sohds may have pseudoplastic or plastic-flow properties. When they are in laminar flow in a stirred vessel, motion in remote parts of the vessel where shear rates are low may become negligible or cease completely. To compensate for this behavior of slurries, large-diameter impellers or paddles are used, with (D /Df) > 0.6, where Df is the tank diameter. In some cases, for example, with some anchors, > 0.95 Df. Two or more paddles may be used in deep tanks to avoid stagnant regions in slurries. 

The rheological behavior of storage XGs was characterized by steady and dynamic shear rheometry [104,266]. Tamarind seed XG [266] showed a marked dependence of zero-shear viscosity on concentration in the semi-dilute region, which was similar to that of other stiff neutral polysaccharides, and ascribed to hyper-entanglements. In a later paper [292], the flow properties of XGs from different plant species, namely, suspension-cultured tobacco cells, apple pomace, and tamarind seed, were compared. The three XGs differed in composition and structural features (as mentioned in the former section) and... 

The flow properties of suspensions are complex. The apparent viscosity at a given shear rate increases with increasing solids concentration and rises extremely rapidly when the volume fraction of solids reaches about 50 per cent. The flow properties also depend on the particle size distribution and the particle shape, as well as the flow properties of the suspending liquid. 

The transfer coefficients can be found from correlations relating the flow properties, the geometry of the system and the physical properties of the fluid. Typical correlation equations for ciystals growing in a suspension may be found in the literature (9-101. 

The rheological properties of a suspension depend upon factors such as the size, shape and concentration of the particles, the stability of the suspension and the viscosity of the medium. Flow properties can be modified by altering the surface charge... 

Given the vast number of possible matrix-reinforcement combinations in composites and the relative inability of current theories to describe the viscosity of even the most compositionally simple suspensions and solutions, it is fruitless to attempt to describe the momentum transport properties of composite precursors in a general manner. There are, however, two topics that can be addressed here in an introductory fashion flow properties of matrix/reinforcement mixtures and flow of matrix precursor materials through the reinforcement. In both cases, we will concentrate on the flow of molten polymeric materials or precursors, since the vast majority of high-performance composites are polymer-based. Fnrthermore, the principles here are general, and they apply to the flnid-based processing of most metal-, ceramic-, and polymer-matrix composites. 

It appears that one can develop a qualitative understanding of the simple flow properties at moderate concentration without going beyond concepts which are already inherent either in the dilute solution theory of polymers or in the properties of particulate suspensions. The dependence of viscosity on c[i ] is believed to reflect a particle-like or equivalent sphere (127) hydrodynamics in solutions of low to moderate concentration. In particular, the experimental facts do not force the consideration of effects which might arise from the permanent connectedness of the polymer backbones. Situations conducive to the entangling of molecules may be present, e.g., overlap of the coils, but either entanglement contributions are small, or else they are overwhelmed by the c[f ] interactions. 

The nature of the polymerization reactor also depends upon the desired form of the product (pellet, powder, bead, etc.). For example, extruder reactors (Stuber and Tirrell, 1985) are best suited to producing pellets, sheets, and coatings. The beads that may be directly useful in processing are best produced by the suspension polymerization process. The round beads, however, may not have suitable bulk-flow properties and are dangerous if spilled. Alternate shapes and the appropriate methods of production are, therefore, often employed. 

Smith WE, Zukoski CF. (2004) Flow properties of hard structured particle suspensions. J Rheol 48 1375-1388. 

Rao, M. A. 1975. Measurement of flow properties of food suspensions with a mixer. J. Texture Stud. 6 533-539. 

Rao, M. A. 1987. Predicting the flow properties of food suspensions of plant origin. Mathematical models help clarify the relationship between composition and theological behavior. Food Technol. 41(3) 85-88. 

Suspensions or dispersions of particles in a liquid medium are ubiquitous. Blood, paint, ink, and cement are examples that hint at the diversity and technological importance of suspensions. Suspensions include drilling muds, foodstuffs, pharmaceuticals, ointments and cremes, and abrasive cleansers and are precursors of many manufactured goods, such as composites and ceramics. Control of the structure and flow properties of such suspensions is often vital to the commercial success of the product or of its manufacture. For example, in consumer products, such as toothpaste, the rheology of the suspension can often determine consumer satisfaction. In ceramic processing, dense suspensions are sometimes molded (Lange 1989) and then dried and sintered or fired into optical components, porcelin insulators, turbine blades, fuel cells, and bricks (Rice 1990 Simon 1993). Crucial to the success of the processing is the ability to transform a liquid, moldable suspension into a solid-like one that retains its shape when removed from the mold. These examples could be multiplied many times over. 

As mentioned earlier, suspensions of particulate rods or fibers are almost always non-Brownian. Such fiber suspensions are important precursors to composite materials that use fiber inclusions as mechanical reinforcement agents or as modifiers of thermal, electrical, or dielectrical properties. A common example is that of glass-fiber-reinforced composites, in which the matrix is a thermoplastic or a thermosetting polymer (Darlington et al. 1977). Fiber suspensions are also important in the pulp and paper industry. These materials are often molded, cast, or coated in the liquid suspension state, and the flow properties of the suspension are therefore relevant to the final composite properties. Especially important is the distribution of fiber orientations, which controls transport properties in the composite. There have been many experimental and theoretical studies of the flow properties of fibrous suspensions, which have been reviewed by Ganani and Powell (1985) and by Zimsak et al. (1994). 

The flow properties of disordered micellar phases are now reasonably well understood. For spherical micelles the viscosity can be estimated from modified hard-sphere-suspension theories, while for disordered semidilute cylindrical micelles the Cates theory of entangled living polymers provides at least a good starting point, and in some cases nearly quantitative prediction of rheological properties. 

Particle size control may also be desirable to facilitate solids handling during formulation. Solids produced by uncontrolled crystallization or precipitation processes can have a broad size distribution that can result in poor flow properties or tendency to segregate. The particle size of APIs can also impact efficiency of blending with excipients, compressibility, flow/ suspension behavior, and compaction performance in downstream equipment. Small particle sizes may be important for controlling dose uniformity in the final formulation, especially for low doses. ... 

Corry WD, Jackson LJ, Seaman GV. Action of hydroxyethyl starch on the flow properties of human erythrocyte suspensions. Biorheology 1983 20(5) 705-17. 

Conventionally, polycarbophil is used as a thickening agent at very low concentrations (less than 1 %) to produce a wide range of viscosities and flow properties in topical lotions, creams, and gels, in oral suspensions, and in transdermal gel reservoirs. It is also used as an emulsifying agent in topical oil-in-water systems. 

The external shape of the crystal can be described in terms of its habit, which is affected by the rate of crystallisation and by the presence of impurities, particularly surfactants. The habit of a crystal is of pharmaceutical importance, since it affects the compression characteristics and flow properties of the dmg during tableting and also the ease with which the suspensions of insoluble dmgs will pass through syringe needles. 

A charged particle in suspension with its inner immobile Stern layer and outer diffuse Gouy (or Debye-ffiickel) layer presents a different problem from that arising with a smooth and small nonpolar sphere. In movement such particles experience electroviscous effects which have two sources (a) the resistance of the ion cloud to deformation, and (b) the repulsion between particles in close contact. When particles interact, for example to form pairs in the system, the new particle will have a different shape from the original and will have different flow properties. The coefficient 2.5 in Einstein s equation (7.30)... 

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