A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number ${\mathrm{Wi}}_{\mathrm{crit}}\approx 0.5$. Birefringence and bulk pressure drop measurements provide self-consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates ($26{\mathrm{s}}^{-1}\le \dot{\epsilon }\le 435{\mathrm{s}}^{-1}$) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model.

The dynamics of elastocapillary thinning in high molecular weight polymer solutions are reexamined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and the competition of elastic, capillary, and inertial forces leads to the formation of a periodic array of beads connected by axially uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and the ligaments. This iterated process results in the development of multiple generations of beads in agreement with the predictions of Chang, Demekin, and Kalaidin [“Iterated stretching of viscoelastic jets,” Phys. Fluids 11, 1717 (1999)] although experiments yield a different recursion relation between successive generations. At long times, finite molecular extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to a progressive coalescence before the rupture of the filament.

The non-Newtonian properties of blood are of great importance since they are closely related with incident cardiovascular diseases. A good understanding of the hemodynamics through the main vessels of the human circulatory system is thus fundamental in the detection and especially in the treatment of these diseases. Very often such studies take place in vitro for convenience and better flow control and these generally require blood analogue solutions that not only adequately mimic the viscoelastic properties of blood but also minimize undesirable optical distortions arising from vessel curvature that could interfere in flow visualizations or particle image velocimetry measurements. In this work, we present the viscoelastic moduli of whole human blood obtained by means of passive microrheology experiments. These results and existing shear and extensional rheological data for whole human blood in the literature enabled us to develop solutions with rheological behavior analogous to real whole blood and with a refractive index suited for PDMS (polydymethylsiloxane) micro- and milli-channels. In addition, these blood analogues can be modified in order to obtain a larger range of refractive indices from 1.38 to 1.43 to match the refractive index of several materials other than PDMS.