From droplets to waves: interfacial instabilities of viscosity-stratified flows in microchannels*

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 dispersed droplet and separated viscous wave flow regimes arising from viscosity-stratified microflows made of fluid pairs with large viscosity ratios and systematically investigate the effects of control parameters such as flow rate, viscosity ratio, and interfacial tension between model fluid pairs. We demonstrate that key features of periodic droplets and interfacial viscous waves, including emission frequency, propagating celerity, and wavelength, can be readily described by functional relationships, which delineate effects of inertial, capillary and viscous forces. We also shed light on wave crest breaking process, which produces viscous ligaments that continuously transport thick material into the fast co-flowing low-viscosity stream. Finally, we examine the transition from droplet to wave regime to provide a comprehensive scenario of interfacial instabilities in microfluidic viscosity-stratified flows.