Segmented flows of viscous threads in microchannels

The passage of microfluidic droplets in miscible thread-forming flows is shown to improve the mixing efficiency of low- A nd high-viscosity fluids at the small scale. Hydrodynamic interactions between recirculating flow patterns and viscous core-annular flows are experimentally investigated in square microchannels. The deformation of thick segmented flows injected in thin fluids is examined through the evolution of droplet size, spacing, and velocity along a square microchannel. Droplets also help in probing the various states of lubrication of the viscous central stream in the bulging, tubing, and threading regimes at various capillary and Péclet numbers. A range of intriguing flow phenomena is revealed using a dual approach based on thread and droplet mutual behaviors, including droplet breakup, formation of low-viscosity currents, thread splitting, viscous fingering, and viscous buckling instabilities. The thread-forming ability of miscible fluids having large viscosity contrasts is discussed in conjunction with mixing applications where a thinner or a viscosifier is continuously added to a viscosity-differing fluid in confined microsystems.