Measurement of relaxation times in extensional flow of weakly viscoelastic polymer solutions

Abstract

The characterization of the extensional rheology of polymeric solutions is important in several applications and industrial processes. Filament stretching and capillary breakup rheometers have been developed to characterize the extensional properties of polymeric solutions, mostly for high-viscosity fluids. However, for low concentration polymer solutions, the measurements are difficult using available devices, in terms of the minimum viscosity and relaxation times that can be measured accurately. In addition, when the slow retraction method is used, solvent evaporation can affect the measurements for volatile solvents. In this work, a new setup was tested for filament breakup experiments using the slow retraction method, high-speed imaging techniques, and an immiscible oil bath to reduce solvent evaporation and facilitate particle tracking in the thinning filament. Extensional relaxation times above around 100 μs were measured with the device for dilute and semi-dilute polymer solutions. Particle tracking velocimetry was also used to measure the velocity in the filament and the corresponding elongation rate, and to compare with the values obtained from the measured exponential decay of the filament diameter.

Rheological behavior of human blood in uniaxial extensional flow

“…The experimental setup used in the investigation is described in detail in Sousa et al [20]. The blood samples are placed between two cylindrical rods (2 mm in diameter), which are initially ~ 500 m apart.…”

“…Preliminary tests were performed using horizontal stretching of blood samples, as described in Sousa et al [20], but the results here presented were obtained using a vertical filament stretching as depicted in Fig. 1, in order to minimize the effects of sedimentation of blood cells in the region of interest where the measurements were performed, i.e.…”

“…The blood sample is placed in between the rods with an initial gap of about 500 m. More details of the experimental setup are presented in Sousa et al [20].…”

“…For that purpose, in this work we measure the extensional relaxation time of whole human blood using a capillary thinning device. A custom-built setup combining the slow retraction method and high-speed imaging techniques [20] is employed in order to generate a vertical stretching filament of blood. The temporal evolution of the minimum filament diameter is used, combined with an appropriate scaling law, to determine the extensional relaxation time.…”

Relaxation time of dilute polymer solutions: A microfluidic approach

“…Dinic et al [33] measured relaxation times below 1 ms by observing the capillary thinning and pinch-off dynamics of aqueous polymer solutions, generated by dripping the liquid directly onto a substrate. Sousa et al [34] studied the capillary thinning breakup of dilute polymer solutions in an immiscible oil bath (minimizing the fluid evaporation), and measured relaxation times down to 100 ls. Very recently, Haward [20] conducted a comprehensive review on microfluidic extensional rheometry using stagnation point flow (e.g., flow in a crossslot configuration), by employing the onset of the flow-induced birefringence in an extensional microfluidic device to extract the relaxation time of polymer solutions (see Sec.…”