Dispersions Newtonian behavior

According to the structure of this equation the quantity cp indicates the influence of the filler on yield stress, and t r on Newtonian (more exactly, quasi-Newtonian due to yield stress) viscosity. Both these dependences Y(cp) andr r(cp) were discussed above. Non-Newtonian behavior of the dispersion medium in (10) is reflected through characteristic time of relaxation X, i.e. in the absence of a filler the flow curve of a melt is described by the formula ... 

The pivotal role that superplasticizers play in the formulation of self-leveling mortars is due to the dramatic effects they produce on flow behavior. Such effects are believed to be derived by the adsorption of the admixture on the surfaces of cement grains, thereby providing surfaces of a similar or zero charge which are mutually repulsive. They thus fully disperse cement particles, freeing more water for lubrication and reducing interparticle attraction. Both yield stress and plastic viscosity are decreased and the decrease is greater for yield stress it may be completely eliminated if sufficient admixture is added so that Newtonian behavior is observed (Fig. 7.25) [75, 76]. 

Spherical latex particles with a reasonably well-defined number of charges per particle can be synthesized and used to study the non-Newtonian behavior of charged dispersions and related electroviscous phenomena (described in Chapter 4). The surface... 

We examine some major conditions under which Einstein s theory breaks down. For our purpose, (he two important reasons are (a) the effect of the concentration of the dispersion and (b) the effects of interparticle forces, particularly the electrostatic repulsive forces or polymer additives. This leads us next to the non-Newtonian behavior of dispersions. 

The electroviscous effects and the other effects discussed in Sections 4.7a-c lead to what is called non-Newtonian behavior in the flow of dispersions. In the next section, we begin with a brief review of the basic concepts concerning deviations from Newtonian flow behavior and then move on to consider how high particle concentrations and electroviscous effects affect the flow and viscosity. 

Let us first consider an inverted W/O emulsion made of 10% of 0.1 M NaCl large droplets dispersed in sorbitan monooleate (Span 80), a liquid surfactant which also acts as the dispersing continuous phase. At this low droplet volume fraction, the rheological properties of the premixed emulsion is essentially determined by the continuous medium. The rheological behavior of the oil phase can be described as follows it exhibits a Newtonian behavior with a viscosity of 1 Pa s up to 1000 s 1 and a pronounced shear thinning behavior above this threshold value. Between 1000 s 1 and 3000 s1, although the stress is approximately unchanged, the viscosity ratio is increased by a factor of 4. 

Emulsion Quality. The quality of an emulsion is defined as the volume fraction (or percent) of the dispersed phase in the emulsion. The quality of emulsions strongly affects their rheology. Several studies have been reported for the relationship of isothermal shear stress to shear rate for emulsions of different qualities. OAV emulsions having qualities less than 0.5 (or 50%) exhibit Newtonian behavior, and those having higher qualities exhibit non-Newtonian behavior (9, 16, 25),... 

For example, when a liquid is sheared between two plates parallel to the xy plane, we have y = dvjdz.) A typical plot of y vs. x is shown in Figure 5.52a. For low and high shear rates, we observe Newtonian behavior (r = const.), whereas in the intermediate region a transition from the lower shear rate viscosity, r o, to the higher shear rate viscosity, takes place. This is also visualized in Figure 5.52b, where the viscosity of the colloidal dispersion, q is plotted vs. the shear rate, y note that in the intermediate zone q has a minimum."" ... 

Dispersed Systems. Newtonian Behavior. Most film-forming liquids are dispersions. The rheology of these systems is simpler than the rheology of polymers and is treated first. 

The calculated particle diameters from Equation 12.2 may be considered a lower limit, that is, the Taylor limit, due to the assumption of Newtonian behavior of the system and vanishingly small concentration of the dispersed phase. Polymers exhibit non-Newtonian behavior, namely, the droplets elongate elastically before breaking. This behavior corresponds to an increase in interfacial tension, and therefore, particle size increases as predicted by Equation 12.1, over that predicted from Equation 12.2. (This is discussed below and can be seen in the last two columns of Table 12.3). 

The main physical and chemical properties of dispersed systems, which influence viscosity and give rise to the so-called non-Newtonian behavior, are discussed in Sec. III. U will be seen that the viscosity and rheological behavior of dispersed systems are the result of the interplay of many variables, and at least a qualitative knowledge of these effects is required to control, to some extent, their flow properties. 

Asphalts are also regarded as colloidal suspensions, in which the oily constituents are the dispersant and the asphaltene constituents, principally, the dispersoid. Oily constituents of an aromatic nature lead to sol-like dispersions of lower temperature susceptibility and non-Newtonian behavior that is, viscosity changes less with temperature but is affected by rate of shear for the velocity gradient. 

While it is conceded that synthetic bitumen obtained from the hydrogenation of coal is not the same as a petroleum asphalt, still there are similarities that may permit characterization in similar terms. The same element of immiscibility exists between oils and asphaltene constituents or tars (oxygenated materials). It has been noted that these raw synthetic bitumen contain volatile oils and hard, asphaltene-like constituents. The oils are predominantly aromatic, giving stable sol-like dispersions with high temperature susceptibility and exhibiting Newtonian behavior. 

The dispersion of clay platelets (exfoliation and intercalation level of the silicate layers) and surface area of silicate platelets have the potential to alter the rheological behavior of the nanocomposites. In-situ polymerized nano composites exhibit more exfoliated structure than the composites prepared by the melt blending technique. Irrespective of the processing parameter, the nanocomposites show shear thinning behavior at high shear rate (Figure 9.14), whereas the pristine polyamide exhibits Newtonian behavior (i.e., the viscosity remains almost the same). It has also been reported that the polymer nanocomposite possesses higher steady shear viscosity than pristine polyamide at low shear rates. 

In upflow bubble operation the consumption of the gas phase by reaction must also be considered in the model if the reactor operates under lower pressure (<20 bar) and if the reactor length is of technical dimensions (L>2 m) additionally gas phase dispersion (radial and axial) may have an influence on conversion [65]. As this reactor type is also used in waste water treatment as well as in fermentation processes, the possible non-Newtonian behavior of the liquid phase as well as the coalescence behavior of the system must be taken into account. Finally, it should be remembered that - comparable to fluidized bed reactors - results from laboratory reactors with small column diameter and/or particle sizes smaller than 0.2 cm usually cannot be regarded as representative for technical upflow units, because capillary force as well as lare scale circulation in the liquid phase may be significantly different. 

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