Spheres, tears, and spears: Regulating the perimeter and circularity of millimeter-sized alginate hydrogel beads

Generating hydrogel beads pertains to many engineering applications. We examined two alginate-based fluids at three concentrations of alginate, $c_{\text{AG}}$ . We used the "Map of Misery" to determine which material property (viscosity, elasticity, and inertia) drives droplet formation. Next, we measured the perimeter and circularity of hydrogel beads as a function of nozzle radius and velocity to generate an operating space that identifies the parameters necessary to form spherical, monodisperse millimeter-sized hydrogel beads (spheres) for all $c_{\text{AG}}$ . Further, we observe interactions between $c_{\text{AG}}$ and Ca that resulted in an increase of the slope between perimeter and Capillary number (Ca), and tear and spear shapes of hydrogel beads. Lastly, we corroborated the value of the jet velocity used for the dimensionless groups with high-speed imaging and computational data of the interfacial flow maintained during drop formation. Computations were performed using the open-source computational fluid dynamics software, OpenFOAM.