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A single hydrodynamic loop was designed to trap a drop dilutor to generate concentration gradients in MDAs. Being coalesced with the bypassing drops, the stationary drop gets serially diluted while in a converse direction, it injects a decreasing amount (same volume but less concentrated) of chemical to the moving drops. The engineered fluidic loop is comprised of a bypassing and a trapping channel with a flow resistance ratio of Rt/Rb=1.56, which guides the coalescence of the moving droplets with the static drop under a total flow rate of 50 µL/hr. The flow channel is 200-µm wide.
Biomicrofluidics, 2014, 8, 034118
The static drop acts like a reagent injector, could inject a drug in varying concentrations to cells in droplets for cell-based drug screening. A recording of the serial dilution can be seen in a video on the Movie page.
I found this technique only worked well at low flow rates to reproduce reliable coalescences between the droplets. Nevertheless, 24 drops can be generated in a minute with varying concentrations. A more reliable electro-coalescencing scheme at higher flow rates has been realized in Dr. Vanapalli's Lab by integrating liquid electrodes nearby the trap, as detailed in a recent publication titled "Electrocoalescence based serial dilution of microfluidic droplets".
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