Experimental Investigation of Inertial Mixing in Droplets

Achieving the fast mixing requirements posed by the chemical, biological, and life science community for confined microchannel flows remains an engineering challenge. The viscous and surface tension forces that dominate conventional micro-flows undermine fast, efficient mixing. By increasing the collisional velocity of reagent droplets, inertia can be exploited to increase mixing rates. This paper experimentally investigates inertial droplet mixing in micro flows. A high speed, gaseous flow is used to detach, transport, and collide droplets of nanoliter-size volumes in standard T and Yjunction microchannel geometries. Mixing rates are quantified using differential fluorescent optical diagnostics. Measured droplet mixing times are compared to the characteristic time scales for mass and viscous diffusion and bulk convection. Results show that mixing times are decreased as the droplet inertia is increased, indicating the potential benefit of inertiadriven mixing. INTRODUCTION The turbulent flow regime exploited at the macroscale to promote mixing is difficult and impractical to achieve in microfluidic devices due to conflicting length scales, dominance of surface forces, and the resulting low Reynolds numbers characteristic of conventional micro flows. The success of the next generation of Lab-On-a-Chip (LOC) and Micro Total Analysis Systems (μTAS) is dependent upon achieving fast mixing rates using practical and highly adaptable techniques. Scientific understanding of chemical reaction mechanisms requires that molecular mixing be faster than the reaction kinetics under investigation. Proteomics is another emerging application for microfluidics that also requires substantial improvement in fluid mixing rates to accurately examine biological assays. As such, there is currently a push from these industries to achieve mixing rates that are on the order of microseconds for detectable sample volumes. A number of techniques have been successfully demonstrated that significantly improve mixing rates for micro-scale flows. Such methods can be categorized as passive and active techniques. A formal review of such mixer technologies is presented by Nguyen [1]. Passive micromixers utilize microchannel geometry (chaotic advection) and stream thinning (hydrodynamic focusing) to improve mixing rates. State of the art devices in this category have achieved mixing times on the order of micro-seconds for femto-liter volumes [2] and milliseconds for nanoliter volumes [3]. Active mixers use external flow disturbances to promote mixing. Although not inclusive, such mixers use lasers [4], electric and or magnetic fields [5], or mechanical agitation [6]. Mixing times in the range of milliseconds to seconds have been achieved using active methods. Given the wide range of mixing techniques currently employed, no one method completely satisfies all metrics required to meet the current demands of the next generation of μTAS and LOC devices. These metrics include high mixing rates, increased detection sensitivity for downstream components, high throughput, fluid compatibility, and straightforward integration and implementation. Because mixing time is proportional to the square of the characteristic length scale, reducing the length for mass diffusion significantly increases the mixing rates. A physical reduction in the system length scales, however, results in detectionlimited volumes and decreased throughput. Instead, the most effective micromixers reduce the length scale for diffusion by inducing a flow patterns that continually stretch, fold, and swirl the scalar field (concentration, temperature, etc.). This process exponentially reduces the effective length over which mass diffusion must occur. The challenge is inducing these flow patterns quickly and controllably. An alternative to existing mixing technologies that potentially satisfies the metrics outlined above is an inertial-based droplet micromixer, as shown in Figure 1. This system promotes mixing by utilizing the inertia of two approaching droplets. Proceedings of the ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels ICNMM2011 June 19-22, 2011, Edmonton, Alberta, CANADA