Passive droplet generation in aqueous two-phase systems with a variable-width microchannel

Passive droplet generation for an aqueous two-phase system (ATPS) was performed with a fracture-based variable microchannel. A jet of dextran-rich phase (DEX) in a polyethylene-glycol (PEG)-rich phase was created by focused flow. The width of the inlet channel could be varied over the range 1–10 μm via mechanical strain, which extended the range of operational back pressure. This enabled the spontaneous formation of DEX droplets with an ultralow surface tension of 12 μN/m. The production of DEX droplets were examined with regard to driving pressure, flow rate, DEX/PEG concentration. The droplet properties are analyzed in terms of production rate (2–20 droplets/s), droplet diameter (10–100 μm), and diameter variance (5%–20%). Controlling the inlet-channel width with other operating conditions widened the range of droplet properties. This simple and robust method significantly strengthened droplet-generation in microfluidics, especially for ATPS of low solute concentrations relevant to live cells.