Interrupted shear

There are several other transient flow experiments which can be instructive in identifying relaxation of orientation and structure. These experiments are summarized in Figure 3. In the interrupted shear experiment the fluid is initially sheared and then allowed to rest for a time The amount of recovery of... 

The interrupted shear test described in Figure 3 may be useful in identifying the time for the orientation to relax. In this test a LCP is sheared until steady state stresses are obtained. The flow is then stopped and the fluid allowed to rest for various lengths of time before flow is started up again. The stress growth behavior after various periods of relaxation is then compared with the initial response. Some representative data for the 60 mole % PHB/PET system is presented in Figure 17. After three minutes of rest time in the melt we see that the first peak is not recovered. However, the second peak is nearly fully recovered. In fact we have observed that the second peak recovers in about 6 seconds. 

An interesting variant on start-up and cessation of steady shear is interrupted shear, which is used to monitor relaxation processes that occur at very long times. The time at which data can no longer be obtained in a relaxation experiment is governed by the sensitivity of the stress or... 

The fitting parameter, t, was found to be much larger than the time required for relaxation after cessation of steady shear. Interrupted shear data have also been reported by Tsang and Dealy [50] for an HDPE and by Seay and Baird [51] for a branched metallocene polyethylene. Robertson et al. [52] reported that for entangled polybutadiene solutions t is more than ten times the disengagement time of the DE theory and that it increases with molecular weight in about the same way as the viscosity. It should be noted, however, that these two characteristic times, T(j and T, are defined in quite different ways and represent different weightings of the fluid s relaxation time spectrum. The fact that they are substantially different, therefore, does not necessarily imply that a new relaxation mechanism is involved. 

Besides describing the most important features of shear banding flow, our model can reproduce many of the details of the nonlinear rheological behavior of wormlike micelles such as inception of shear flow, instantaneous stress relaxation, and interrupted shear flow [14], Figure 7 depicts the experimental data (symbols) and model predictions (solid lines) for stress relaxation after cessation of flow for different shear strain levels. In this case. 

Let us consider the best case scenario, where the initial angle a = 0. During shear, the surface is reoriented and will eventually approach 90° hence, the mixing becomes very ineffective as the fluid interfaces are deformed. However, if during the flow, where the total shear, 7, is imposed upon the surfaces, the flow is interrupted in the middle so that the surface can be reoriented back to the initial position, we get... 

If the flow is interrupted twice, dividing the total shear into 3 sections we get... 

Figure 6.45 Increase in interfacial area as function of shear and number of interruptions.

Introduction of the expander, a high-shear extruder with an interrupted-flight screw, in the mid-1980s revolutionized oilseed extraction practices. Essentially all solvent-extracted cottonseed, and approximately 70 percent of domestic soybean tonnage now are processed... 

Figure 6.25 Shear stress as a function of shear strain for a polyamide 6 filled with 30% by weight glass fibers of different aspect ratios, where the fibers were initially oriented in the velocity-gradient direction. The shear was interrupted after about 7 strain units, and the stress was allowed to relax. The shecir was then restarted the shear stress quickly returned to the value it had before cessation of shearing, showing that fiber orientation only changes during shearing. (From Laun 1984b, reprinted with permission from Steinkopff Publishers.)...

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