Colloids shear thinning

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. 

Use anionic polymers such as polyacrylic acids cross-linked with allyl ethers of pentaerythritol or sucrose as thickeners, if a gel structure and pseudoplastic (shear-thinning) properties are desirable. Consider adding colloidal alumina to further increase the viscosity at pH 13 [ 15]. 

M. M. Cross, Rheology of Non-Newtonian Fluids a New Flow Equation for Pseudoplastic Systems, J. Colloids Sci., 20, 417 137 (1965) also M. M. Cross, Relation Between Viscoe-lasiticity and Shear-thinning Behaviour in Liquids, Rheological Acta, 18, 609-614 (1979). 

Cantrell K. J., Kaplan D. I., and Gilmore T. J. (1997) Injection of colloidal Ee particles in sand with shear-thinning fluids. J. Environ. Eng. 123, 786—791. 

With increasing shear, Pe — the relative viscosity of suspensions — usually decreases (see Fig. 6.19). This shear thinning effect is quite moderate in colloidally stable suspensions, which actually can behave as nearly Newtonian up to... 

As discussed in Sect. 4, in the fluid, MCT-ITT flnds a linear or Newtonian regime in the limit y 0, where it recovers the standard MCT approximation for Newtonian viscosity rio of a viscoelastic fluid [2, 38]. Hence a yrio holds for Pe 1, as shown in Fig. 13, where Pe calculated with the structural relaxation time T is included. As discussed, the growth of T (asymptotically) dominates all transport coefficients of the colloidal suspension and causes a proportional increase in the viscosity j]. For Pe > 1, the non-linear viscosity shear thins, and a increases sublin-early with y. The stress vs strain rate plot in Fig. 13 clearly exhibits a broad crossover between the linear Newtonian and a much weaker (asymptotically) y-independent variation of the stress. In the fluid, the flow curve takes a S-shape in double logarithmic representation, while in the glass it is bent upward only. 

Keywords Elastohydrodynamic interactions Glass transition Linear viscoelasticity Nonlinear rheology Polymer-colloid materials Shear-thinning Wall slip... 

Temperature and Viscosity. The operating temperature can have a beneficial effect on flux primarily as a result of a decrease in viscosity.f There is an additional benefit for shear thinning viscoelastic fluids, where the viscosity reduces with an increase in shear (i.e., cross-flow velocity). Typical examples are clarification of fermentation broths and concentration of protein solutions. l l It must be noted that for most fermentation and biotechnology related applications, temperature control is necessary for microbial survival and/or for product stability (e.g., antibiotics, enzymes, proteins and other colloidal materials). 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (C2.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation ( 2.6.21 ean be used to define an apparent viseosity, rj at a given shear rate. If r pp deereases with increasing shear rate, the dispersion is ealled shear thinning (pseudoplastic) if it increases, this is known as shear thiekening (dilatant). The latter behaviour is typieal of eoneentrated suspensions. If a finite shear stress has to be applied before the suspension begins to flow, this is known as the yield stress. The apparent viscosity may also change as a funetion of time, upon applieation of a fixed shear rate, related to the formation or breakup of partiele networks. Thixotropie dispersions show a deerease in r pp with time, whereas an increase with time is called rheopexy. 

In the last section we introduced the concept of two asymptotic viscosity limits for shear thinning colloidal suspensions as a function of shear rate. One is the high shear limit which corresponds to high values of the Peclet number where viscous forces dominate over Brownian and interparticle surface forces. Generally this limit is attained with non-colloidal size particles since to achieve large Peclet numbers by increase in shear rate alone requires very large values for colloidal size particles. In this limit, non-Newtonian effects are negligible for colloidal as well as non-colloidal particles. 

Truly bimodal suspensions of colloidal and noncolloidal particles are of considerable practical interest. For such suspensions at low shear rates, the viscosity is high so that, for example, during storage, settling is reduced. On the other hand, because the mixture is shear thinning, at higher shear rates when the suspension is pumped the viscosity decreases, thereby enabling the mixture to be pumped at a lower pressure drop. 

The bimodal model has also been applied to polydisperse suspensions (Probstein et al. 1994), which in practice generally have particle sizes ranging from the submicrometer to hundreds of micrometers. In order to apply the bimodal model to a suspension with a continuous size distribution, a rational procedure is required for the separation of the distribution into fine and coarse fractions. Such a procedure has not been developed so that an inverse method had to be used wherein the separating size was selected which resulted in the best agreement with the measured viscosity. Again, however, the relatively small fraction of colloidal size particles was identified as the principal agent that acts independently of the rest of the system and characterizes the shear thinning nature of the suspension viscosity. 

Most emulsions of interest show shear-thinning rheological behaviour, and many show other non-Newtonian features such as elasticity, yield stress or time dependent effects. All of these can be explained qualitatively, and sometimes quantitatively, by a relatively simple set of parameters (Table 14.1) which takes into accQunt droplet size, d, droplet phase volume conservative (colloid interaction) forces, hydrodynamic forces and interfacial properties [Pg.295]

Gum Arabic acts as protective colloid and excellent emulsifier and the molecular aggregation can cause both shear thinning and time-dependent thickening behavior at low shear" Gum Arabic has the ability to create a strong protective film around oil droplets" and is compatible with the most other plant hydrocolloids, proteins, carbohydrates, and modified starches. The viscosity of a solution of a mixture of gum arabic and gum tragacanth tends to be lower than that of either constituent solution. 

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (ry), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an inter-particle or inter-molecular network, which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case, flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes a fluid. Examples include oil well drilling muds, greases, lipstick, toothpaste and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear-thinning behaviour as shear rate increases. 

In concentrated suspensions, the settling velocity of a sphere is less than the terminal falling velocity of a single particle. For coarse (non-colloidal) particles in mildly shear-thinning liquids (1 > n > 0.8) [Chhabra et al., 1992], the expression proposed by Richardson and Zaki [1954] for Newtonian fluids applies at values of Re(= up to about 2 ... 

Figure 9.5. Degree of shear thinning of silicon nitride suspensions at different solids content. (From L. Bergstrom, Colloids Suif., A, 133, 151-155 (1998) with permission from Elsevier Science)...

Chaffey, C.E. Wagstaff, I. "Shear thinning and thickening rheology". II. Volume fraction and size of disperses par-ticules. J. Colloid Interface Sci. 59 (1977) 63-75 /... 

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