Pseudoplastic systems

Cross, M. M. J. Colloid Sci. 20 (1965) 417. Rheology of non-Newtonian fluids a new flow equation for pseudoplastic systems. 

Figure 3 shows plots of p versus shear rate at three different temperatures for the same latex (20% w/w latex A) at full coverage with PVA. These curves are typical of a pseudoplastic system showing a reduction of n with increasing shear rate, 7 p reaches a limiting value at If > 50 s l. It is also clear from fig. 3 that at 7 < 10 s-- -, n increases rapidly with reduction in 7. Comparison with nQ values obtained from the creep curves would indicate the p should increase very steeply with reduction of 7, in the low shear rate region (p is the limit of p as Y+0). °... 

This model was introduced by Neville and Hunter (13,14) for the case of sterically stabilized dispersions which have undergone reversible flocculation. It is assumed that the major contribution to the excess energy dissipation in such pseudoplastic systems comes from the need to provide energy from the shear field to separate contacting particles. Under these conditions, the extrapolated yield value is given by the expression (13,32,33),... 

The extrapolated yield stress gives 0.06 Pa and a plastic viscosity of 3.88 mPas. We can use this to estimate the force between the particles, which gives 425kBT/a, in fair agreement with the value determined using pair potential curves. Here the Casson model has been used to partially linearise a pseudoplastic system rather than a system with a true yield stress. 

Cross, M.M., Rheology of Non-Newtonian Fluids A New Flow Equation for Pseudoplastic Systems, /. Colloid ScL, 20, 417 (1965)... 

Figure 7.33 A plot of shear stress x against shear rate D for plastic and pseudoplastic suspensions. As )/ = r/D, the slope of the line represents the viscosity at each rate of shear in both the plastic and pseudoplastic systems the viscosity at level A is greater than that at level B.

The flow curve. Fig. 2, is a plot of Equation 3 showing experimental results obtained at the 1186°C isotherm. Fig. 3 is a log-log plot of apparent viscosity as a function of shear rate at the same temperature. The apparent viscosity decreases with increasing shear rate, which is characteristic for pseudoplastic systems (n ). The logarithm of the viscosity at unit shear rate, log Pq, is calculated from Equation 3 ... 

Figure 5.7 Dependence of shear stress on shear strain rate for Newtonian, dilatant and pseudoplastic systems.

The drop in viscosity with increasing shear rate was described well [110] for all systems investigated by means of the Cross equation [Eq. (16)] that was derived for the flow of pseudoplastic systems [111],... 

Crochet MJ, Dupret F, Verleye V (1994) Injection molding. In Advani SG (ed) Flow and rheology in polymer composites manufacturing. Elsevier, Amsterdam, pp 442-444 Cross MM (1965) Rheology of non-Newtonian fluids a new flow equation for pseudoplastic systems. J Colloid Sci 20 417-437... 

Konjak mannan dissolves in water to give highly viscous dispersions, which are pseudoplastic systems. Heating with a small... 

When that stress is exceeded, the shear rate grows. Further stress leads finally to linear (Newtonian) behaviour. Examples of plastic systems are chocolate, butter, cheese, various spreads and ice cream. In pseudoplastic systems the observed viscosity decreases with an increase in shear stress. An example of a pseudoplastic system is pudding. Dilatant systems resist deformation more than in proportion to the apphed force. The shear rate is growing much faster than that of Newtonian fluids and viscosity increases with an increase in shear stress. At low apphed forces, the system behaves as a Newtonian fluid. Examples of dilatants systems are honey with added dextran and a slurry of wet beach sand. Thixotropic systems become more fluid (they have lower viscosity) with increasing time of an apphed force. If the apphed force ceases to operate, the original viscosity of the system is restored due to a reversible transformation of the sol gel type. Examples of thixotropic systems are mayonnaise, ketchup, whipped and hardened fats, butter and processed cheeses. Rheopectic systems exhibit behaviour opposite to that of thixotropic systems. Their viscosity increases with increasing time of apphed force. An example is whipped egg white. 

Fig. 3.47 Shear stress and viscosity versus shear rate for a pseudoplastic system.

Fig. 4.14 Stress-shear rate and viscosity-shear rate relationship for a pseudoplastic system.

Cross, M.M. (1965). Rheology of non-Newtonian flow equation for pseudoplastic systems. Journal of Colloid Science, Vol.20, pp. 417-437 de Waele, A. (1923). Oil Color, Chemical Association Journal, Vol.6, pp. 23-88 Edwards, M.F., Jadallah, M.S.M. and Smith, R. (1985). Head losses in pipe fittings at low Reynolds numbers. Chemical Engineering Research and Design, Vol.63, (January 1985) pp. 43-50... 

For a Newtonian system the viscosity at levels A and B is the same (by definition). For the pseudoplastic system the viscosity at A is greater than that at B. In a plastic system with a yield value between A and B, no settling would occur. Samyn s own discussion of these cases is given in the legend to the figure. 

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