Droplet generation and mixing in confined gaseous microflows

Fast mixing remains a major challenge in droplet-based microfluidics. The low Reynolds number operating regime typical of most microfluidic devices signify laminar and orderly flows that are devoid of any inertial characteristics. To increase mixing rates in droplet-based devices, a novel technique is presented that uses a high Reynolds number gaseous phase for droplet generation and transport and promotes mixing through binary droplet collisions at velocities near 1m/s. Control of multiple gas and liquid streams is provided by a newly constructed microfluidic test bed that affords the stringent flow stability required for generating liquid droplets in gaseous flows. The result is droplet production with size dispersion and generation frequencies not previously achievable. Limitations of existing mixing diagnostic methods have led to the development of a new measurement technique for measuring droplet collision mixing in confined microchannels. The technique employs single fluorophore laser-induced fluorescence, custom image processing, and meaningful statistical analysis for monitoring and quantifying mixing in high-speed droplet collisions. Mixing information is revealed through three governing statistics that that separate the roles of convective rearrangement and molecular diffusion during the mixing process. The end result is a viewing window into the rich dynamics of droplet collisions with spatial and temporal resolutions of 1μm and 25μs, respectively. Experimental results obtained across a decade vi of Reynolds and Peclet numbers reveal a direct link between droplet mixing time and the collision convective timescale. Increasing the collision velocity or reducing the collision length scale is the most direct method for increasing droplet mixing rates. These characteristics are complemented by detaching droplets under inertial conditions, where increasing the Reynolds number of the continuous gaseous phase generates and transports smaller droplets at faster rates. This work provides valuable insight into the emerging field of two-phase gas-liquid microfluidics and opens the door to fundamental research possibilities not offered by traditional oil-based architectures.