(541d) Drop Production in Yield-Stress Fluids

We investigate the two-phase flows of a Newtonian fluid and a granular yield-stress fluid (YSF) using a co-flowing extrusion device. YSFs act like solids at rest and flow like liquids when stresses are applied. The capillary breakup of an immiscible Newtonian fluid inside a YSF can be eliminated completely when the yield stress of the YSF is sufficiently high. This contrasts with simple liquids, where the dispersed phase of co-flowing streams always breaks into spherical droplets. There is, however, and intermediate regime where the evolution of capillary breakup and shape relaxation of drops is arrested, resulting in droplets of non-spherical shapes. By carefully tuning the flow and shear parameters, we demonstrate precision control of dispersed drop shapes through simple physical parameters in co-flowing streams of a Newtonian fluid and a surrounding YSF.