Gas-Liquid Droplet Microfluidics

The possibility of replacing the highly viscous continuous phase oil for a less viscous gaseous phase offers opportunities for higher flow rates, reduced pumping power, and increased droplet inertia for high speed mixing applications. Liquid droplet generation in a gaseous microflow, however, is often characterized as unwieldy, difficult, and less forgiving compared to aqueous droplet generation in silicon oils. Creating droplets in common microchannel geometries, such as T-junctions and flow focusing arrangements, is undoubtedly possible but the metamorphosis of topology, such as spherical droplets, asymmetrical slugs, trailing pears, and liquid threads is significantly different than liquid-liquid flows. This presentation addresses the fundamental operation of liquid droplet generation in a confined gaseous microflow. Droplet volume, shape, and generation rates are experimentally characterized for common and not-so common microchannel geometries. Transitions from slugs to pools to films for T-junctions and dripping to jetting to threading for flow focusing are identified. High speed images are used to quantify the discrete phase characteristics and qualify the generation and detachment process.