Droplet formation and entrainment in liquid-gas microfluidic systems

Abstract

Controlled generation of droplets in microfluidic networks is a promising tool for numerous applications in biochemistry and material sciences. Droplet-based microfluidic systems are typically utilized for creating dispersions of a liquid or gas within a second continuous liquid phase, such as creating uniform liquid emulsions or gas bubbles (foams). In this work, we have studied some of the fundamental aspects of liquid droplet formation within a continuous gas flow for creating uniform aerosols inside microfluidic channels. We have experimentally investigated interactions of liquid water within a high-speed air flow inside confined flow-focusing microchannels in which we identify six distinct flow regimes: Co-flowing, Threading, Plugging, Dripping, Multi-Satellite Formation, and Jetting. These flow regimes and their transitions are plotted and characterized based on the Weber number (We) of the two phases. Generation frequency, morphology, and monodispersity of the droplets are characterized in more detail for the Dripping regime and scaling laws are provided to better elucidate the mechanism of droplet formation for this system. Results of this work establish a relationship between generation frequency and the product of the liquid and gas flow rates. However, droplet morphology (length and width) is exclusively dependent on the air flow rate. Finally, we demonstrate the production of monodisperse droplets (d<100μm and σ/d<0.05) at kHz formation rates in liquid-gas microfluidic systems. The results of this work provide practical and useful guidelines for precise oil-free delivery of ultra-small volumes of fluid which can be integrated in Lab-on-a-Chip (LOC) and micro-Total-Analysis Systems (μ TAS) for a variety of applications in biochemical research and material synthesis.