Casson viscosity

Product/Supplier Applications Viscosity, Casson Pas 2S°C Yield Value, Pa 25 >C Process Conditions... 

Brookfield 10 rpm unless indicated 1 10 rpm unless indicated time required to double viscosity Casson, 0.4-30 s unless indicated Haake Viscometer (30s ) N/A - Not Available. Source Compiled from suppliers technical data.sheets. Contact suppliers for specification values. 

Fig. 161a, h. Plastic viscosities (Casson modd) vs concentration of X7G solutions in h res( temperatures 70 °C( ), 100°C(H) and 120 °C (A) a linear scal b semi-logarithmic scale (constructed from data xesent-ed in Table 1 of [420])... 

Addition Casson plastic viscosity Casson yield value... 

The other models can be appHed to non-Newtonian materials where time-dependent effects are absent. This situation encompasses many technically important materials from polymer solutions to latices, pigment slurries, and polymer melts. At high shear rates most of these materials tend to a Newtonian viscosity limit. At low shear rates they tend either to a yield point or to a low shear Newtonian limiting viscosity. At intermediate shear rates, the power law or the Casson model is a useful approximation. 

The power law model can be extended by including the yield value r — Tq = / 7 , which is called the Herschel-BulMey model, or by adding the Newtonian limiting viscosity,. The latter is done in the Sisko model, 77 +. These two models, along with the Newtonian, Bingham, and Casson... 

The square root of viscosity is plotted against the reciprocal of the square root of shear rate (Fig. 3). The square of the slope is Tq, the yield stress the square of the intercept is, the viscosity at infinite shear rate. No material actually experiences an infinite shear rate, but is a good representation of the condition where all rheological stmcture has been broken down. The Casson yield stress Tq is somewhat different from the yield stress discussed earlier in that there may or may not be an intercept on the shear stress—shear rate curve for the material. If there is an intercept, then the Casson yield stress is quite close to that value. If there is no intercept, but the material is shear thinning, a Casson plot gives a value for Tq that is indicative of the degree of shear thinning. 

Fig. 3. Examples of Casson plots (viscosity) vs (shear rate). The Casson equation is 77 = 77), + Tq 7...

Chocolate does not behave as a tme Hquid owing to the presence of cocoa particles and the viscosity control of chocolate is quite compHcated. This non-Newtonian behavior has been described (28). When the square root of the rate of shear is plotted against the square root of shear stress for chocolate, a straight line is produced. With this Casson relationship method (29) two values are obtained, Casson viscosity and Casson yield value, which describe the flow of chocolate. The chocolate industry was slow in adopting the Casson relationship but this method now prevails over the simpler MacMichael viscometer. Instmments such as the Carri-Med Rheometer and the Brookfield and Haake Viscometers are now replacing the MacMichael. 

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. 

A Casson fluid is Theologically identified by two parameters yield value and plastic viscosity. The plastic viscosity relates to the asymmetry of the flow particles and the yield value is connected with the forces of attraction between particles. The... 

The term /pj is the plastic viscosity and aB> 0c and [Pg.215]

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. 

A more convenient extrapolation technique is to approximate the experimental data with a viscosity model. The Power Law, shown in Eq. 6, is the most commonly used two-parameter model. The Bingham model, shown in Eq. 7, postulates a linear relationship between x and y but can lead to overprediction of the yield stress. Extrapolation of the nonlinear Casson model (1954), shown in Eq. 8, is straightforward from a linear plot of x°5 vs y05. Application of the Herschel-Bulkley model (1926), shown in Eq. 9, is more tedious and less certain although systematic procedures for determining the yield value and the other model parameters are available (11) ... 

Newtonian viscosity density shear stress Casson parameters Herschel-Bulkley parameters... 

For a food whose flow behavior follows the Casson model, a straight line results when the square root of shear rate, (y), is plotted against the square root of shear stress, (cr) , with slope Kc and intercept Kqc (Figure 2-2). The Casson yield stress is calculated as the square of the intercept, ctoc = (Kocf and the Casson plastic viscosity as the square of the slope, r]ca = The data in Figure 2-2 are of Steiner (1958) on a chocolate sample. The International Office of Cocoa and Chocolate has adopted the Casson model as the official method for interpretation of flow data on chocolates. However, it was suggested that the vane yield stress would be a more reliable measure of the yield stress of chocolate and cocoa products (Servais et al., 2004). 

The Casson plastic viscosity can be used as the infinite shear viscosity, t]oo, (Metz et al., 1979) of dispersions by considering the limiting viscosity at infinite shear rate ... 

Figure 2-2 Plot of versus for a Food that Follows the Casson Model. The Square of the intercept is the yield stress and that of slope is the casson plastic viscosity.

Table 3-2 Casson Yield Stress,

Table 4-9 Volume Fraction (cQ), Casson Yield Stress (ooc. Pa, /n = 0.25 and m = 0.50) and Infinite Shear Viscosity (r]co, mPa s), Viscosity of Supernatant (ijs, mPa s), and Relative Viscosity ( jr) of Tapioca Starch Dispersions...

Table 5-H Casson Model Parameters Yield Stress (croc) nd Plastic Viscosity (rjoo) of Cocoa Mass as a Function of Temperature (Fang et ai., 1996)...

Fig. 7 Viscosity as a function of the shear strain rate for a shear thinning (a), shear thickening (b), and a Casson body (c).

---