Increasing the mixing rate of fluid droplets within modern lab-on-a-chip technology is being increasingly stressed by members of the biological and chemical communities that wish to study reaction kinetics and mechanisms. Faster mixing allows for a more complete analysis of a wider range of chemical and biological reactions, and due to this technologies ability to help better characterize these reactions, there is an increasingly large number of demands for chips that can fully mix droplets in an order of microseconds. High speed mixing like this is difficult to characterize using standard microscopy. This article looks at how to achieve high-speed droplet collisions using flow focusing geometry, and how these collisions are visualized using a novel laser-imaging system.
Microchannel droplet mixing is plagued with severe limitations due to the restricted microchannel lengths between elements within the chips, as well as strong surface tension forces involved and low Reynolds numbers that are found in all microchannel flow. These characteristics further reduce the rate of mixing, and thus the rate of diffusion, between the two colliding droplets. Previous attempts at mixing droplets in microchannels stemmed from two methods; hydrodynamic flow focusing, and chaotic advection mixing, this later approach leading to the advent of inertial-based droplet micromixer technology. Both of these techniques rely on rapidly decreasing the diffusion length scales between the two mixing droplets through the creation of striations. Generating these droplets while developing a method for quantifying high-speed diffusion mixing is the main focus of this article. For this study, liquid droplets are formed in a flow-focusing configuration using a high speed gaseous flow and collided at angled Y-junctions in the microchannel fabricated in PDMS. In order to determine the instantaneous rates of mixing within the internal droplet flow during the collision process, an experimental setup using epi-fluorescence microscopy in conjunction with high speed laser-induced fluorescence was developed. This setup is capable of tracking and recording fluorophore concentration within the droplets during the collision and coalescence process in multiple images. The droplets are distinguished using differing concentrations of fluorophore which correspond to differing levels of fluorescent intensity while the colliding droplets remain unmixed, and consistent levels of fluorescent intensity when the colliding droplets are fully mixed. These images are then processed using a statistical analysis software to determine both the diffusion length scale as well as the quasi-instantaneous rate of mixing at all times during the collision process.