Viscous Wave Breaking and Ligament Formation in Microfluidic Systems.

Rapid layering of viscous materials in microsystems encompasses a range of hydrodynamic instabilities that facilitate mixing and emulsification processes of fluids having large differences in viscosity. We experimentally study the stability of high-viscosity stratifications made of miscible and immiscible fluid pairs in square microchannels and characterize the propagation dynamics of interfacial waves, including breaking and viscous ligament entrainment from wave crests at moderate Reynolds numbers. For large viscosity contrasts, parallel fluid streams adopt widely different velocities and provide a simple model system to probe the role of inflectional instabilities of stratified microflows in relation with classic inviscid-stability theory. We reveal novel viscous wave regimes and unravel dispersion relationships in the presence and absence of interfacial tension. Detailed examination of wave celerity shows the existence of optimal operation conditions for passively disturbing miscible fluid flows and continuously dispersing low-and high-viscosity fluids at the small scale.