Vane fixture

Figure 5.6.2 shows die vane fixture designed for samples with a yield stress. It can be rotated in a large container in the same way as the disk discussed above, but more recendy it has been used with a close-fitting cylindrical cup (Barnes and Camali, 1990, with review of previous work). A vane inserted into a fluid will create... 

The use of a rotating vane has become very popular as a simple to use technique that allows slip to be overcome (33,34). Alderman et al (35) used the vane method to determine the yield stress, yield strain and shear modulus of bentonite gels. In the latter work it is interesting to note that a typical toique/time plot exhibits a maximum torque (related to yield stress of the sample) after which the torque is observed to decrease with time. The fall in torque beyond the maximum point was described loosely as being a transition from a gel-like to a fluid-like behavior. However, it may also be caused by the development of a slip surface within the bulk material. Indeed, by the use of the marker line technique, Plucinski et al (15) found that in parallel plate fixtures and in slow steady shear motion, the onset of slip in mayonnaises coincided with the onset of decrease in torque (Fig. 8). These authors found slip to be present for... 

The advantage of the vane technique is that material is allowed in between the prongs of the vane and in effect shears the material upon itself (rather than inducing shearing between a mechanical fixture and the material, as would be seen if a standard cup-and-bob or Couette rheometer were used). This eliminates the potential for slippage between a mechanical fixture and the material. A typical response is given in Figure 4.5. 

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