We experimentally study the formation and evolution of threads containing more viscous liquids surrounded by less viscous, immiscible liquids through hydrodynamic focusing in square microchannels. Over a large range of viscosities and interfacial tensions, five characteristic regimes of flow behavior are identified: threading, jetting, dripping, tubing, and displacement. We locate the boundaries between these regimes on a flow map based on the capillary number of each fluid. In the jetting and the dripping regimes, the droplet size is measured and related to fluid properties, flow parameters, and geometry. The critical thread length before jetting droplets and the critical length of a viscous tail before breakup in dripping are also examined. This study classifies and defines regimes of thread instabilities that can be used to produce supra- and subchannel size viscousdroplets in an elementary microfluidic geometry.
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