Step changes in shear rate

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... 

A closely related phenomenon is thixotropy. This is the property of some polymer solutions to show a time dependent reduction in viscosity when sheared. The longer the fluid is subjected to the shear stress, the lower its viscosity becomes. So a thixotropic fluid is one which takes a finite time to attain its equilibrium viscosity in shear after being subjected to a step change in shear rate. In... 

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. 

Tliis is an extreme example of slip, where the structure actually breaks down in the sample in a very narrow zone at the interface of shear and the rotor breaks free of the surface. This can happen at high shear where the sample has slumped down from viscosity break down or from fast step changes in position [stress relaxation tests]. The latter is the same principle that allows an Oreo cookie to be split by a rapid twisting of the top cookie relative to the filling. When shear fracture occurs in a sample, it is usually accompanied by a dramatic fall in torque or an increase in shear rate. 

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. 

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-... 

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. 

In this test, a metal sample is rotated in the solution. A rotating cylinder is used to simphfy fluid dynamics equations so that corrosion rate can be correlated with shear stress or mass transfer, which in turn can be related to velocity effects in piping and equipment. The same electn> chemical techniques used on static samples are applicable to the rotating cylinder electrode. By coupling the samples to electrochemical measirring equipment, one can measure qualitatively the effects of stepped velocity changes in one experiment. 

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