Flocculated suspensions viscosity

Because concentrated flocculated suspensions generally have high apparent viscosities at the shear rates existing in pipelines, they are frequently transported under laminar flow conditions. Pressure drops are then readily calculated from their rheology, as described in Chapter 3. When the flow is turbulent, the pressure drop is difficult to predict accurately and will generally be somewhat less than that calculated assuming Newtonian behaviour. As the Reynolds number becomes greater, the effects of non-Newtonian behaviour become... 

Plastic fluids are Newtonian or pseudoplastic liquids that exhibit a yield value (Fig. 3a and b, curves C). At rest they behave like a solid due to their interparticle association. The external force has to overcome these attractive forces between the particles and disrupt the structure. Beyond this point, the material changes its behavior from that of a solid to that of a liquid. The viscosity can then either be a constant (ideal Bingham liquid) or a function of the shear rate. In the latter case, the viscosity can initially decrease and then become a constant (real Bingham liquid) or continuously decrease, as in the case of a pseudoplastic liquid (Casson liquid). Plastic flow is often observed in flocculated suspensions. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

Adsorption of nonionic and anionic polyacrylamides on kaolinite clay is studied together with various flocculation properties (settling rate, sediment volume, supernatant clarity and suspension viscosity) under controlled conditions of pH, ionic strength and agitation. Adsorption and flocculation data obtained simultaneously for selected systems were correlated to obtain information on the dependence of flocculation on the surface coverage. Interestingly, optimum polymer concentration and type vary depending upon the flocculation response that is monitored. This is discussed in terms of the different properties of the floes and the floe network that control different flocculation responses. Flocculation itself is examined as the cumulative result of many subprocesses that can depend differently on system properties. 

Because most shear-thinning fluids, particularly polymer solutions and flocculated suspensions, have high apparent viscosities, even relatively coarse particles may have velocities in the creeping-flow of Stokes law regime. Chhabra(35,36) has proposed that both theoretical and experimental results for the drag force F on an isolated spherical particle of diameter d moving at a velocity u may be expressed as a modified form of Stokes law ... 

Smith,T.L Bruce, C.A. (1979) Intrinsic viscosities and other rheological properties of flocculated suspensions of non magnetic and magnetic ferric oxides. J. Colloid Interface Sci. 72 13-25... 

Flocculated suspensions often have high viscosities at low shear strain rates, which give the impression of the existence of a yield stress. The nearly Newtonian flow observed is due to a dramatic reduction in the effective number of particles as a result of the particle association process. 

A Brookfield viscometer with a helipath attachment (Stoughton, MA) is a useful rheological instrument for measuring the settling behavior and structure of pharmaceutical suspensions and for characterizing the properties and stability of flocculated suspensions. The viscometer should be properly calibrated to measure the apparent viscosity of the suspension at equilibrium at a given temperature to establish suspension reproducibility. Apparent viscosity, like pH, is an exponential term, and therefore the log-apparent viscosity is an appropriate way of reporting the results. 

Consideration must be given to establishing good physical stability of a suspension. If the particles settle and eventually produce a cake at the bottom of the container, they must redisperse readily to achieve dosage uniformity. Viscosity-enhancing agents can be used to keep the particles suspended. Preparation of flocculated suspensions is not recommended because the larger floes may irritate the eye. 

Figure 26. Steady shear viscosity for a strongly flocculated suspension of 2.5% fumed silica in methyl laurate measured with different rheometries...

The rheological properties of concentrated suspensions often depend not only on the shear rate but also on the time. When the viscosity decreases with time under shear but recovers to its original value after flow ceases, the behavior is known as thixotropic (Fig. 4.36). This type of behavior is more often observed in flocculated suspensions and colloidal gels. When the suspension is sheared, the floes are broken down leading a distribution of floe sizes. Often the regeneration of the floes is slow which causes the resistance to flow to decrease. The opposite behavior, when the viscosity increases with shear rate and is also time dependent, is known as rheopectic. 

Hudson et al. [31] studied the time dependent effect of suspensions of pigment particles. Experimental work was undertaken to relate steady, transient and time dependent oscillatory flow in a unified manner. Hudson et al [31] also developed a theory to explain the suspension behavior in terms of the structure formed as a result of flocculation. The viscosity rjf of a homogeneous susp>ension of floccules containing N] floccules was given by... 

Most concentrated structured liquids shown strong viscoelastic effects at small deformations, and their measurement is very useful as a physical probe of the microstructure. However at large deformations such as steady-state flow, the manifestation of viscoelastic effects—even from those systems that show a large linear effects—can be quite different. Polymer melts show strong non-linear viscoelastic effects (see chap. 14), as do concentrated polymer solutions of linear coils, but other liquids ranging from a highly branched polymer such as Carbopol, through to flocculated suspensions, show no overt elastic effects such as normal forces, extrudate swell or an increase in extensional viscosity with extension rate [1]. 

However, this is not possible (and often not necessary) for disperse systems such as suspensions or emulsions, where the elastic forces in steady state are much smaller than the equivalent values in oscillatory flow. In some cases such as a flocculated suspension, the very elastic structure at low stresses gives way to a low viscosity, inelastic liquid at high stresses. 

From this equation we also see the sensitivity of viscosity to interparticle force, for the exponent 3/D - 1 will vary from 0 when D is 3 (for a small attractive force), to 0.5 for a D of 2 (for a large force). This would mean that for p = 100, the effective phase volume would vary by a factor of 10 for D going from 3 to 2. However we also see another important fact from this equation—the effective phase volume of the suspension now depends on the size of the floes, via the value of p, the number of particles per floe. For example if D = 2 (for most real flocculated suspensions D usually varies in the range 2 0.25), the effective phase volume decreases by a factor of 10 when the number of particles in a large floe is reduced from 10,000 to 100 under the action of shearing. 

FIGURE 8.88 (A) Schematic of viscosity versus volume fraction of solids in the suspension. (B) schematic of a flocculated suspension. 

In low sohds muds, vinyl acetate—maleic anhydride copolymers were once used to extend or enhance the viscosity of bentonite suspensions (141). This function is largely performed by polyacrylamides. The vinyl acetate—maleic anhydride copolymers can also have a flocculating effect on drill sohds. Concentrations generally are quite low (0.14—0.57 kg/m (0.05—0.2 Ib/bbl)). 

For suspensions primarily stabilized by a polymeric material, it is important to carefully consider the optimal pH value of the product since certain polymer properties, especially the rheological behavior, can strongly depend on the pH of the system. For example, the viscosity of hydrophilic colloids, such as xanthan gums and colloidal microcrystalline cellulose, is known to be somewhat pH- dependent. Most disperse systems are stable over a pH range of 4-10 but may flocculate under extreme pH conditions. Therefore, each dispersion should be examined for pH stability over an adequate storage period. Any... 

Viscosities of concentrated suspensions of carbon black in a white mineral oil (Fisher "paraffin" oil of 125/135 Saybolt viscosity) were measured with a Brookfield viscometer as a function of OLOA-1200 content. Figure 13 shows the viscosities of dispersions with 30 w%, 35 w% and 70 w% carbon black. In all cases the viscosity fell rapidly as the 0L0A-1200 content increased from 0 to 1%, then fell more gradually and levelled off as the 0L0A-1200 content approached 2%. In many respects the reduction in viscosity with increasing OLOA-1200 content parallels the conductivity measurements both phenomena are sensing the buildup of the steric barrier, and this steric barrier weakens, softens, and lubricates the interparticle contacts. As evidenced in foregoing sections, the particles are still flocculated but can be easily stirred and separated mechanically. The onset of electrostatic repulsion at OLOA-1200 contents in excess of 2.5% did not affect viscosities. 

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