Rheometry and Viscoelastic Fluid Flow

The relaxation time in rheology, and particularly in rotational rheometry, is a measure of the rate at which the viscoelastic fluid changes in response to the change in flow due to the oscillatory movements of the fluid. Typically, an apparent relaxation time is defined as the time for the disturbances to decrease by a factor of 1/e, that is, 0.368. 

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. 

---