Viscous waves 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 viscous waves arising at the interfaces between viscous stratifications made of miscible and immiscible fluid pairs with large viscosity ratios in microchannels and systematically investigate the effects of fluid handling, flow rates, viscosity ratios, diffusion coefficients and interfacial tension between model fluid pairs. We demonstrate that key features of interfacial viscous waves, including emission frequency, propagating celerity, wavelength and amplitude can be readily described by functional relationships to delineate effects of inertia, viscosity and interfacial tension. 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 stratifications.