Dynamics of Droplet Pinch-off: Stiff and Soft Emulsions

Red blood cells are responsible for the rheological behavior of blood, which is a complex non-Newtonian fluid. Patients with sickle cell anemia (SCA) have reduced deformability of their red blood cells, resulting in flow blockage through the blood vessels. It has been shown that the rigidity of red blood cells and the concentration of rigid cells significantly affect the shear viscosity of the blood. While extensional viscous effects influence the microcirculation, its influence on SCA has yet to be understood. Using a model fluid and a uniaxial extensional flow experimental set-up, we investigated the effect of particle rigidity on the extensional viscosity. We studied the pinch-off dynamics of a thread of a suspension consisting of a photocurable emulsion and found that emulsions transition from a yield stress to a viscoelastic response upon exposure to UV. Although the mixtures of soft emulsion droplets show self-similar thinning behavior similar to Newtonian fluids, the thinning of an emulsion thread follows an exponential decay upon droplet polymerization. The magnitude of the elongational viscosity and emulsion relaxation time, calculated from the time constant of exponential decay, depend on the degree of polymerization and concentration of rigid droplets in the emulsion.