Polyacrylamide shear thickening

Figure 12 The shear (dotted lines) and extensional (solid lines) flow curves for aqueous solutions of polyacrylamide (tension thickening) and callogen( tension thinning) Rheometrics promotional literature.

Fig. 5.37—Shear thickening during flow of 500-pptn polyacrylamide in 3% NaCI solution through sandstone. ...

Prediction of Polymer Mobility in the Shear-Thicketting Region. At high frontal-advance rates, polyacrylamides exhibit an unusual flow behavior in porous rocks. The flowing fluid appears to become more viscous as the flow rate increases. This behavior is called shear thickening. Fig. 5.37 shows the development of shear-thickening behavior (i.e., the sharp increase in the resistance factor) as flow rate increases for a 500-ppm solution of partially hydrolyzed polyacrylamide flowing in a sandstone. 42... 

Shear thickening is caused by the viscoelastic nature of polyacrylamide. Polyacrylamide has a flexible coil conformation in solution. When the flexible polyacrylamide molecule flows from pore to pore, it deforms (i.e., stretches) to adjust to the flow field. If the average flow time from one constriction to the next is large relative to the time required for the polymer molecule to relax and assume the random coil configuration, the polymer remains shear thinning. The... 

Beyond a critical rate of flow in cores the viscoelasticity of polyacrylamide solutions is reflected by shear thickening behaviour. Practical information is provided on the critical flow rate and magnitude of shear thickening and their dependency on core permeability, temperature, molecular weight and concentration of commercial polymers. To describe the onset of shear thickening, critical Deborah numbers were calculated using fluid relaxation times obtained by oscillation rheometry. Using three core-flow parameters, viz. the critical flow rate and two power-law exponents, the viscoelastic effects are quantified. 

For the flow of polyacrylamide solutions in consolidated and unconsolidated sandstones, the onset of shear thickening in terms of a critical flow rate shifts towards higher values with increasing permeability, temperature and decreasing molecular weight and concentration of the polymer. 

The extensional thickening of polymer solutions is one form of viscoelastic behavior. This ability to support a tensile stress can also be demonstrated in a tubeless syphon with dilute aqueous solutions of polsrmers such as polyacrylamide or polyethylene oxide. If you suck up solution with a medicine dropper attached to a water aspirator and then lift the dropper out of the solution, the solution will still be sucked up. In shear, viscoelastic fluids develop normal stresses, which causes rod climbing on a rotating shaft, as opposed to the vortex and depressed surfaces that form with Newtonian liquids. Polsrmer solutions and semiliquid poljnners exhibit other viscoelastic behaviors, where, on short time scales, they behave as elastic solids. Silly putty, a childrens toy, can be formed into a ball and will slowly turn into a puddle if left on a flat surface. But if dropped to the floor it boimces. 

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