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 non-fluorescing 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.