Shearing histories

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Results from measurements of time-dependent effects depend on the sample history and experimental conditions and should be considered approximate. For example, the state of an unsheared or undisturbed sample is a function of its previous shear history and the length of time since it underwent shear. The area of a thixotropic loop depends on the shear range covered, the rate of shear acceleration, and the length of time at the highest shear rate. However, measurements of time-dependent behavior can be usehil in evaluating and comparing a number of industrial products and in solving flow problems. 

Figure 7 Effect of shearing history on swelling ratio for HP LDPE sample (A) Brabender worked at 190°C (A) solvent treated sample. Source Ref. 39.

Many fluids, including some that are encountered very widely both industrially and domestically, exhibit non-Newtonian behaviour and their apparent viscosities may depend on the rate at which they are sheared and on their previous shear history. At any position and time in the fluid, the apparent viscosity pa which is defined as the ratio of the shear stress to the shear rate at that point is given by ... 

Viscosity versus shear history iow temp mixes iow RPM-dump at 230°F... 

Viscosity versus shear history low temp mixes low RPM-dump at 330°F... 

The rheology of hydroxypropylguar is greatly complicated by the cross-linking reactions with titanium ions. A study to better understand the rheology of the reaction of hydroxypropylguar with titanium chelates and how the rheology depends on the residence time, shear history, and chemical... 

Harris, P.C. and Penny, G.S. "Influence of Temperature and Shear History on Fracturing Fluid Efficiency," SPE paper 14258, 1985 SPE Annual Technical Conference and Exhibition, Las Vegas, September 22 25. 

Craigie, L.J. "A New Method for Determining the Rheology of Crosslinked Fracturing Fluids using Shear History Simulation," SPE/DOE paper 11635, 1983 SPE/DOE Symposium on Low Permeability, Denver, March 14-16. 

Instruments are controlled by information contained 1n the experimental setup file. For each type of instrument (shear history simulator, rotational viscometer, reciprocating capillary viscometer), the hardware 1s controlled so that the parameters of shear rate, temperature and time comply with the desired test conditions. This involves controlling devices such as pumps, bath heaters, valves and variable-speed motors. The setup and control parameters are recorded in the experiment file along with the resulting measured data. If necessary, the experiment can easily be repeated. 

The shear history simulators operate at a single shear rate during an experiment and do not run shear ramps. For these Instruments, apparent viscosity at a single shear rate Is determined by the relationship of differential pressure (AP), capillary length (L) and radius (r), and volumetric flow rates (Q), as follows. 

In this experiment, a Tubing Shear History Simulator was coupled with a Reciprocating Capillary Viscometer to simulate the above conditions. Results from the experiment are given in Tables I and II and Figure 3, and were retrieved directly from the project data base. Total Instrument use time for this experiment was 17 hr, of which 16.5 hr were completely unattended operation.. Data analysis, including plotting of figures, required less than five minutes. 

Equation 3.91 gives the shear stress at time t arising from the complete shear history over all earlier times t. ... 

Rokudai, M., Mihara, S., and Fujiki, T., Influence of Shearing History on the Rheo-iogical Properties and Processability of Branched Polymers. II. Optical Properties of Low-Density Polyethylene Blown Films, /. Appl. Polym. Set, 23, 3289 (1979)... 

Entanglements that are not trapped have transistory existence (and will be referred to as "temporary" entanglements). Since the dynamic moduli of these systems depend on the frequency, temperature and shear history of the system, we shall refer to such system as a gel-like or pseudo-gel in contrast to a true gel. A schematic diagram of the pseudo-gel structure is shown in Figure 1. 

Why is the shear history often important in the case of dispersions and polymer solutions ... 

Ellis AT, Ting RY, Napolink RH (1970) Some effects of storage and shear history on the friction reducing properties of dilute polymer solutions Prog Astronaut Aeronaut 70 Viscous flow flow drag reduction (AIAA) 532... 

The rheological behaviour of thermotropic polymers is complex and not yet well understood. It is undoubtedly complicated in some cases by smectic phase formation and by variation in crystallinity arising from differences in thermal history. Such variations in crystallinity may be associated either with the rates of the physical processes of formation or destruction of crystallites, or with chemical redistribution of repeating units to produce non-random sequences. Since both shear history and thermal history affect the measured values of viscosity, and frequently neither is adequately defined, comparison of results between workers and between polymers is at present hazardous. 

Prud homme, R. K., Constien, V., and Knoll, S. (1989). The effects of shear history on the rheology of hydroxypropyl guar gels. In Polymers in Aqueous Media, Glass, J. E. (Ed.), Advances in Chemistry 223, pp. 90-112. Am. Chem. Soc., Washington, DC. 

With the concept of nanoreactors one can take advantage of an additional mode control for the design of nanoparticles where both thermodynamic aspects as well as shear history enter the particle size and the inner structure of the latexes or hybrid particles. The polymerization in such nanoreactors takes place in a highly parallel fashion, i.e., the synthesis is performed in 1018-102° nanocompartments per liter that are separated from each other by a continuous phase. In miniemulsion polymerization, the principle of small nanoreactors is realized as demonstrated in Fig. 1. 

The viscosity-temperature relationship and its dependence on previous shear history... 

If)/ is independent of shear history, the material is said to be time independent. Such liquids can exhibit different behavior patterns, however, if, as is frequently the case with polymers, )/ varies with shear rate. A material whose viscosity is independent of shear rate, e.g., water, is a Newtonian fluid. Figure 11-26 illustrates shear-thickening, Newtonian and shear-thinning rj-y relations. Most polymer melts and solutions are shear-thinning. (Low-molecular-weight polymers and dilute solutions often exhibit Newtonian characteristics.) Wet sand is a familiar example of a shear-thickening substance. It feels hard if you run on it, but you can sink down while standing still. 

Chow, M. K. and Zukoski, C. F. 1995a. Gap size and shear history dependencies in shear thickening of a suspension ordered at rest. J. Rheol. 39 15-32,... 

When these, or other dimensionally correct, relaxation terms are added to the right sides of Eqs. (9-43) and (9-44), distinctive scaling relationships can be derived, which can be expressed as follows. Let a t, [y(t)]) be the stress tensor at time t during a shearing flow with shear history [y (f)]. Now choose a new shear history [y (t)] = c[y(cf)] in which the shear rate at each instant r is a constant c times the shear rate in the old history at time ct. Then, the stress a in the new shear history is given by... 

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