A polyethylene glycol/citrate mixed solution was fed into a single channel of a Y-type micro-channel on a microchip as an aqueous two-phase system. A phase separation multi-phase flow with a liquid-liquid interface was generated due to a phase transformation. An annular flow, one of the flow types in the phase separation multi-phase flow, was observed through bright-field microscopy. The flow consisted of citrate-rich inner and polyethylene glycol-rich outer phases. We attempted to separate and collect the two phases in the single channel into two separate Y-type channels. When the pressure losses for the separated channels were not very different, we observed symmetric flow in the Y-type channel. When the pressure losses were quite different, the polyethylene glycol-rich phase with higher viscosity was selectively distributed to the separated channel with lower pressure loss. Thus, the polyethylene glycol-rich phase was successfully and intentionally collected from the chosen Y-type channel via the creation of annular flow in the single channel.
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