Phase separation features of oil-water parallel flow at hydrophobic and hydrophilic micro-T-junctions

The phase separation features of typical oil-water parallel flow (stratified flow/annular flow) at both hydrophilic and hydrophobic micro-T-junctions are studied experimentally. Complete (100%) separation of oil and water is only observed at the hydrophilic T-junction as the inlet flow is stratified. The maximum separation efficiency of stratified flow at both hydrophilic and hydrophobic T-junctions is higher than that for annular flow. The phase separation efficiency of annular flow is below 50%. Of note, a comparative study illustrates that the phase separation features of stratified flow at macro- and micro-T-junctions are highly similar and are both attributed to the "channeling effect". At micro-T-junctions, the flow confinement induces the aqueous (water) phase to flow through a stable "water channel," while the oil phase flows through an "oil channel." The position of the channel interface depends on the hydrodynamic conditions at the two arms of the T-junctions. A simple model is developed to predict the critical conditions for the complete separation of the two fluids. This study shows that micro-T-junctions with uniform hydrophilic channels are potential candidates for onsite phase separators in micro-fluidic devices. (C) 2022 Elsevier Ltd. All rights reserved.