CAREER:Collective hydrodynamics of confined drops in microfluidic parking networks

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1150836PI: VanapalliCurrent high-throughput screening (HTS) methods use robotic actuators for dispensing and diluting fluids in microliter-scale multi-well plates. This approach requires significant investment and imposes constraints on reducing the fluid volumes due to evaporation. Microfluidic arrays of immobilized drops could emerge as an inexpensive and powerful alternative to multi-well plate screening. However, the technical challenge in generating these static drop arrays (SDAs) is to develop a means to (i) array drops of tunable volume and (ii) vary the reagent concentration from drop to drop in the array. Despite recent progress, current microfluidic devices are incapable of manipulating individual drop concentration and varying the volume in the static array. The field is ripe for breakthroughs and if the challenge is met the benefits are enormous ? low cost; reduced fluid volumes; capability to monitor many reactions in drops simultaneously; and ability to further manipulate drops as the position is indexed in the array.To address this challenge, the PI proposes to investigate the dynamics of trains of confined drops and/or long plugs in a special class of fluidic networks called microfluidic parking networks (MPNs). MPNs typically consist of a repeated sequence of loops, with each loop containing a fluidic trap to park (i.e. immobilize) drops. Preliminary exploration in just a small region of the control parameter space yielded a series of unanticipated and astonishing behaviors driven by collective hydrodynamic resistive interactions in the network. Sub-classes of collective behavior involving drop parking, break-up, and coalescence led to the generation of SDAs with tuneable volumes as well as with variation in reagent concentration from drop-to-drop. To harness the full potential and autonomous control offered by the preliminary observations, the PI proposes a comprehensive investigation of the collective hydrodynamics of drops in MPNs, than is currently available. The investigation will focus on (i) coordinated experimental and modeling efforts to predict the spatiotemporal dynamics of drops in MPNs that drive many of the collective behaviors we observed. New tools involving drop-on-demand generators will be integrated into MPNs, to vary the control parameters to map the full phase space of collective dynamics (ii) providing a complete picture of drop/plug break-up in MPNs, by characterizing the fragmentation dynamics to control the size of drop relative to trap. This data when combined with a novel strategy to measure pressure variations during break-up will enable rigorous confrontation of existing models of drop break-up at bifurcations; and (iii) controlling the coalescence and material exchange between parked and moving drops, by probing the factors that regulate film drainage, and passive tracer transport and mixing when drops fuse. The proposed fundamental investigations will enable the development of inexpensive SDAs with sophisticated capabilities. This work is poised to deliver a true nanoliter-scale analog of the multi-well plate with the enormous simplification that dilutions and mixing are carried out passively, thus solving a long-standing challenge. Replacing a room full of robots with these penny-sized devices will tremendously benefit HTS methods in biology and material science. The CAREER project will also provide interdisciplinary training for students in the cutting-edge areas of microfluidics, multiphase flows, microfabrication and nonlinear dynamics. Graphic modules of droplet traffic and parking will be developed for active learning, where students will design their own network topologies and conduct original analysis. These modules will be integrated into an elective course on microfluidics and the core courses. Outreach activities will be developed for middle school girls on the theme "Bubbles on Chips" in which students will mold devices containing a street map of Lubbock and study bubble traffic to identify the exits that bubbles choose.