Shear rate, step changes

The best experiments to properly measure thixotropy are those where the sample to be tested is sheared at a given shear rate until equilibrium is obtained, then as quickly as possible the shear rate is changed to another value. The typical response to such a step-wise change from one steady-state condition to another is, in terms of the viscosity, often characterised by the so-called stretched exponential model ... 

A rotational viscometer connected to a recorder is used. After the sample is loaded and allowed to come to mechanical and thermal equiUbtium, the viscometer is turned on and the rotational speed is increased in steps, starting from the lowest speed. The resultant shear stress is recorded with time. On each speed change the shear stress reaches a maximum value and then decreases exponentially toward an equiUbrium level. The peak shear stress, which is obtained by extrapolating the curve to zero time, and the equiUbrium shear stress are indicative of the viscosity—shear behavior of unsheared and sheared material, respectively. The stress-decay curves are indicative of the time-dependent behavior. A rate constant for the relaxation process can be deterrnined at each shear rate. In addition, zero-time and equiUbrium shear stress values can be used to constmct a hysteresis loop that is similar to that shown in Figure 5, but unlike that plot, is independent of acceleration and time of shear. 

Step 7 Finally, you replace etaO by eta in Eq. (9.33) in order to solve the problem for a non-Newtonian fluid. Click = with a circular arrow, or choose Solve/Restart. Plot the function c(v), as shown in Figure 9.11. If the pressure drop is changed, the shear rate changes and this causes the viscosity to change. Thus, the velocity changes its shape. 

An alternative method for studying thixotropy is to apply a step change test, whereby the suspension is suddenly subjected to a constant high shear rate and... 

The dynamic filtration theory of Outmans (127) requires experimental terms such as particle-particle stresses, particle friction factors, and thickness of a shear zone within the filter cake that would be difficult to determine. However, the qualitative picture of dynamic filtration presented by Outmans, namely, irreversible adhesion of solid particles up to a certain thickness that is determined by the shear stress (or shear rate) at the surface of the cake, accords with the experiments of Fordham and co-workers (129,135). Once a filter cake has formed under dynamic conditions, it is difficult to remove it by subsequent changes in yc or vm. Figure 44 shows the effect of changes in the flow rate on cumulative filtrate volume. The limiting filtration rate obtained when the initial flow rate of the drilling fluid was 1.8 m3/h remained unaltered when the flow rate of the drilling fluid was increased to 7.0 m3/h in a step-... 

Experimental verification of Eqs 7.94 indicated that the scaling relationships are valid, but the shape of experimental transient stress curves, after step-change of shear rate, did not agree with Doi-Ohta s theory [Takahashi et al., 1994]. Similar conclusions were reported for PA-66 blends with 25 wt% PET [Guenther and Baird, 1996]. For steady shear flow the agreement was poor, even when the strain-rate dependence of the component viscosities was incorporated. Similarly, the... 

The viscosity is then measured at ambient temperature at increasing shear rates of 85, 170, 255, 340, and 425 s The shear rate scans are produced in the rotational viscometer by increasing the rotation rate, and the scans are produced in the reciprocating capillary by changing the flow rate in discrete steps. After the initial shear scan, the sample is heated at about 2.7 C/min while being sheared at a constant rate of 170 s The sample temperature is about 80 °C in 25 min time zero is at the start of the ambient temperature scan. Shear scans are taken at 25, 40, 55, and 70 min with the shear rate maintained at 170 s between scans. Power-law parameters n and K are calculated from the shear stress measurements obtained from the scans. The viscosity at 170 s is calculated from these power-law parameters and reported. 

A step change of viscosity at critical conditions to infinity (3) is a convenient model for investigating the flow of a curing liquid. The simplest model employs the most important physical property of the process. In a number of papers [83-87], the dependence of the induction period t, the time during which a reactive substance retains the ability to flow, on the rate of shear y is studied. 

TRANSIENTS IN THE STRUCTURE AND STRESS OF ENTANGLED POLYMERS SUBJECTED TO STEP CHANGES IN SHEAR RATE... 

The procedure illustrated above to compute transient entanglement stresses can be easily generalized to flow programs consisting of sequential step changes in shear rate. Equation 3 can now be solved for each of the time intervals when the shear rate is maintained constant to give ... 

We have successfully carried out the first phase of extending our original kinetic network model for calculating steady-state properties (3,4) to apply to transient experiments involving step changes in shear rate. The model is seen to possess the ability to describe these stress transients. In addition, a number of new rheological tests have been proposed as potential means to... 

Figure 18.52 Characterisation of polymer flow behaviour with TMS rheometer showing (A) viscosity at very low shear rates, the stress measured is a function of viscosity, (B) transient flow-step changes from low to high shear rates generate a peak stress value which is a product of the thixotropic and structural features of the sample, (C) elasticity - high shear rates produce results that represent elastic behaviour. Source Negretti Automation, Aylesbury, UK)...

A critical step in the data analysis procedure is the correction of the calculated apparent viscosity for the influences of pressure and viscous heating. The key element in this procedure is the approximation that the fractional change in the viscosity of the polymer solution due to pressure and viscous heating effects is equal to the corresponding fractional change in the viscosity of a hypothetical Newtonian fluid which exhibits the same rheological properties as the polymer solution at low-shear rates where it approaches Newtonian behavior. The hypothetical Newtonian fluid would have a constant viscosity at all shear rates equal to the viscosity of the polymer solution at low-shear rates, and the influence of temperature and pressure on the viscosity of this Newtonian fluid would be identical to the influence of these variables on the low-shear rate viscosity of the polymer solution. 

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