Viscosity residual

The viscoelastic behavior of concentrated (20% w/w)aqueous polystryene latex dispersions (particle radius 92nm), in the presence of physically adsorbed poly(vinyl alcohol), has been investigated as a function of surface coverage by the polymer using creep measurements. From the creep curves both the instantaneous shear modulus, G0, and residual viscosity, nQ, were calculated. 

Goodeve and Whitfield (1837-39) studied the equilibrium between the spontaneous building of an internal structure and its breakdown. The apparent viscosity of a thixotropic system was measured at different rates of shear. If r)r represent the apparent viscosity and extrapolated residual viscosity, respectively, then we have,... 

The influence of addition of sodium bentonite (a commonly used antisettling system) on the rheological behaviour of a pesticide suspension concentrate (250 g dm ) has been investigated. Steady state shear stress-shear rate curves were carried out to obtain the yield value and viscosity as a function of shear rate. The shear modulus was also measured using a pulse shearometer, and the residual viscosity was obtained in afew cases from creep measurements. The rheological parameters Tg (Bingham yield value),... 

Gg (instantaneous modulus), Hg (residual viscosity) and G (shear modulus) all showed a rapid increase above 30g dm bentonite. This was attributed to the formation of a gel network structure in the continuous medium and the strength of such a gel increased with increase in bentonite concentration. The results could be qualitatively described in terms of the elastic floe model of Hunter and co-workers. Moreover, the settling characteristics of the structured suspensions were found to be consistent with the predictions from the rheological measurements. This demonstrates the value of rheological studies in predicting the longterm physical stability of suspension concentrates. 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

The increase in gel strength with increase in bentonite concentration above the gel point is consistent with the increase in yield value and modulus. On the other hand, the limited creep measurements carried out on the present suspension showed a high residual viscosity Oq of the order of 9000 Nm s when the bentonite concentration was 45g dm. As recently pointed out by Buscall et al (27) the settling rate in concentrated suspensions depends on 0. With a model system of polystyrene latex (of radius 1.55 vim and density 1.05 g cm ) which was thickened with ethyl hydroxy ethyl cellulose, a zero shear viscosity of lONm was considered to be sufficient to reduce settling of the suspension with = 0.05. The present pesticide system thickened with bentonite gave values that are fairly high and therefore no settling was observed. 

Using the latter two relations we may also approximate the residual dissipation rate, e.g., in terms of the residual viscosity determined by other LES closures, in case this parameter is needed for reactor modeling purposes like in population balance kernels and in species mixing model parameterizations provided that the overall closure models remain consistent [21]. 

The above-described thickeners produce a three-dimensional gel network by the overlap of the polymer coils of H EC or the double helices of xanthan gum. Apart from their effect in reducing creaming and sedimentation by producing a high residual viscosity (at low shear rates), these polymers will also prevent the... 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

As mentioned above, thickeners reduce creaming or sedimentation by increasing the residual viscosity tj(o), which must be measured at stresses compared to... 

Figure 21.1 Variation of creaming or sedimentation rate with residual viscosity.

As mentioned above, thickeners reduce creaming or sedimentation by increasing the residual viscosity //(o), which must be measured at stresses compared to those exerted by the droplets or particles (mostly less than 0.1 Pa). At such low stresses, / (o) increases very rapidly with increase in thickener concentration ... 

It can be seen that the limiting residual viscosity increases rapidly with increase in EHEC concentration. A plot of sedimentation rate for 1.55 pm PS latex particles versus rj o) is shown in Eigure 21.7 which shows an excellent correlation [2], in which case a value of rj o) > 10 Pa-s is sufficient for reducing the rate of sedimentation to 0. 

Figure 21.9 Variation of residual viscosity with Arlatone V-100 concentration at various...

The viscosity of hydrogen is more conventional in its behavior. In this case, liquid-phase viscosity data have been extrapolated [i -32] from close to atmospheric conditions to a critical-point condition in the coordinates of residual viscosity vs. density. Both papers report nearly identical results ( 30 x 10 g/cm-sec) for the critical-point viscosity. Brebach and Thodos indicate that this value should be increased to bring it more in line with their reduced correlations for other diatomic gases and give a final value of rjc =37.6 x 10 g/cm-sec. Values of viscosity for hydrogen from both sources, over the liquid range for which no data exist, lie very close to the hydrogen curve of Fig. 3. The data from these sources, however, extend to values for the critical viscosity which appear to be somewhat low (see Table IV). No comparable studies for deuterium and tritium have previously been made. 

Another method for studying flocculation is that of constant stress (creep) measurements that was described before. This allows one to obtain the residual viscosity q(o) and critical stress a . The values of q(o) and may be used to assess the flocculation of the suspension on storage. If flocculation occurs on storage (without... 

Figure 8.41 Variation of (a) residual viscosity, rjo and (b) residual compliance, Jq with approximate mixed emulsifier concentraton for liquid paraffin-in-water emulsions stabilized by cetosteary 1 alcohol and the alkyltrimethylammonium bromides dodecy 1-C12, tetradecyl-Ci4, hexadecyl-C,6 and octadecyl-Cjs From Barry and Eccleston [99] with permission.

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