The Role of Structure in Nonnewtonian Behavior

These descriptions are in accord with most observed behavior of these fluids and thus offer a mental picture of what may be going on within the fluid. However, they are by no means rigorous descriptions of the microscopic internal behavior of such fluids, and they may be modified by further studies of nonnewtonian fluids. 

Much of the past and present research in nonnewtonian fluids has consisted of measuring their stress-rate-of-strain curves (such as Fig. 1.5) and trying to find mathematical descriptions of these curves. The study of the flow behavior of materials is called rheology (from Greek words meaning the study of flow ), and diagrams like Fig. 1.5 are often called rheograms. 

As shown in Sec. 1.5, the basic definition of viscosity is in terms of the sliding-plate experiment shown in Fig. 1.4. For newtonian fluids it was shown in Sec. 6.3 (Example 6.2) that the viscosity could be determined easily by a capillary-tube viscometer. It can be shown both theoretically and experimentally that the viscosity determined by such a viscometer for a newtonian liquid is exactly the same as the viscosity one would determine on a sliding-plate viscometer. Since capillary-tube viscometers are cheap and simple to operate, they are widely used in industry for newtonian fluids. 

For nonnewtonial fluids which are not time-dependent or viscoelastic, it is possible to convert capillary-tube viscometer measurements to the equivalent sliding-plate measurements, but this involves some mathematical manipulations. For time-dependent (e.g., thixotropic) fluids, this does not seem to be 

In such a device a motor-driven cylindrical cup is rotated at a constant speed. The fluid being tested is in the thin, annular region between the cup and the bob. The shear stress generated by the fluid on the wall of the bob tends to turn the bob, but this turning motion is resisted tiy the torsion wire which supports the bob. The bob takes up a position wherejthe torque exerted by the torsion wire is equal and opposite to the torque supplied by the fluid shear on its surface from its position, as indicated by a pointer and scale and the calibration of the torsion wire, one can readily compute the shear stress at the wall. 

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