Quantification of Inertial Droplet Collision Mixing Rates in Confined Microchannel Flows Using Differential Fluorescence Measurements

Efficient mixing at the microscale remains a formidable engineering challenge. Recent advancement and proliferation of Lab-on-a-Chip (LOC) and Micro Total Analysis Systems (μTAS) has demanded accelerated development and demonstration of novel micromixers as successful mixing is critical to device performance. Passive techniques such as chaotic advection and shear thinning as well as active methods utilizing electric fields show great promise at meeting these requirements. A new droplet-based mixing technique currently being developed aims at improving micromixer rates passively by increasing the Reynolds number in the microchannel. High speed gaseous flows with Reynolds numbers from 1 to 300 are used to detach and transport discrete droplets to a collision zone where droplet interaction and subsequent mixing is achieved under highly inertial conditions. The design utilizes variants of the standard T-junction arrangement for both the detachment and collision process. A fluorescing and nonfluorescing droplet pair are brought into contact in a collision zone and allowed to interact with relative velocities in the 0.1 to 5m/s range. Mixing rates are quantified using an optical based measurement technique that examines temporal changes in droplet intensity as mixing progresses. Both the detachment and collision processes are captured using a high speed camera capable of frame rates in excess of 10MHz. Experimental results are obtained for different collision zone geometry arrangements and microchannel aspect ratios to assess mixing performance. A description and sufficient explanation of the optical measurement techniques used to quantify mixing rates is provided, including limitations and shortcomings of this simplified approach. Analytical models are developed to gain better understanding of the key physical mechanisms driving droplet mixing and experimental results are correlated against this order of magnitude model. Based on these results, recommendations are made for potential design improvements and issues are addressed concerning mixing using two-phase gas/liquids flows. INTRODUCTION Fast, efficient mixing at the microscale remains a challenge in the burgeoning field of microfluidics. The laminar flow regime representative of microchannel flows is not conducive to mixing, which is limited by molecular diffusion under these conditions. The turbulent flow regime exploited at the macroscale for fast mixing is difficult and impractical to induce in microfluidic devices due to conflicting length scales and low Reynolds numbers. The success of future LOC and μTAS is dependent upon achieving fast mixing rates. 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 the chemical and biological fields to achieve mixing rates that are on the order of microseconds or less, while still taking advantage of low sample volumes associated with the use of microfluidics. However, a number of techniques have been successfully implemented that significantly improve mixing rates for microflows. Such methods can be generally 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 femtoliter detection volumes [2] and milliseconds for nanoliter volumes [3]. Active mixers use means external to the device to promote mixing. Although not inclusive, such mixers use lasers [4], electric and or magnetic fields [5], or mechanical agitation [6]. Proceedings of the ASME 2010 3rd Joint US-European Fluids Engineering Summer Meeting and 8th International Conference on Nanochannels, Microchannels, and Minichannels FEDSM-ICNMM2010 August 1-5, 2010, Montreal, Canada