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-1. 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 per 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.