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

Abstract

Generating hydrogel beads pertains to many engineering applications. We examined two alginate-based fluids at three concentrations of alginate, cAG . 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 (<i>spheres</i>) for all cAG . Further, we observe interactions between cAG and Ca that resulted in an increase of the slope between perimeter and Capillary number (Ca), and <i>tear</i> and <i>spear</i> 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.