Question

1. The following data were collected at 20 C with a co-axial rheometer:

                           ____________________________________________

                                Shear rate (s-1)                       Shear stress (Pa)

                           ____________________________________________

                                            

                                           3.3                                          17.8

                                           9.8                                          22.5

                                          29.4                                          30.9

                                          88.0                                          49.6

                                        264.0                                          91.5

                           ____________________________________________

Transform the data as needed to fit the Bingham, power-law and Casson models. Plot these data on linear graph paper and determine the relevant models for all three models with Excel. Which model fits the data best? Why?

        

2. The following data was collected using a co-axial rheometer at 20 C.

         _____________________________________________________________

Spindle speed                  Meter reading (S)

      (rpm)                                                                           

                              2% PGA in water      2% PGA in skim milk

___________________________________________________________

             583.2                            155.8                        398.9

             291.6                            106.1                        295.6

             194.4                              76.7                        245.9

               97.2                              47.5                        197.1

               64.8                              18.3                        161.4

               32.4                              17.7                        138.9

               21.6                              12.6                        123.9

               10.8                              10.3                        98.6

                7.2                                9.2                        92.9

                3.6                                8.1                        77.0 __________________________________________________________________

         Note: the shear rate factor 0.907 (s-1/rpm) and shear stress factor is 4.185(Pa/meter unit).

Determine the most accurate measure of the yield stress of this data using the Casson, power-law plastic and Bingham models. Assume Newtonian shear rates. Which model fits best?

Answer 1