The manipulation of highly viscous materials at the microscale is a key challenge for implementing lab on chips with the ability to manage a variety of complex and reactive fluids. We describe methods for producing and controlling high-viscosity fluid threads flowing in sheath of less viscous fluids, i.e., viscous core-annular flows, in microchannels. The self-lubrication property of multi-fluid flows having large viscosity contrasts offers a promising means for manipulating interfaces between “thick” and “thin” fluids and for reducing the hydraulic resistance in micro- and nanofluidic devices. In particular, we focus on the flow behavior of threads as they traverse diverging-converging slit microfluidic chambers. The alteration of convective time-scales using extensional microgeometries permits the manipulation of complex phenomena such as viscous buckling, wetting, and coalescence. We examine the interrelation between these phenomena that are useful for passively enhancing mixing between miscible fluids and for initiating continuous emulsification processes between immiscible fluids having widely disparate viscosities.

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