Specific microfluidic flows with two-phase separation mixed solvent solutions, such as ternary water–hydrophilic/hydrophobic organic solvent, water–surfactant, water–ionic liquid, and fluorocarbon/hydrocarbon organic solvent mixtures, have been reported as a new type of multiphase flow. When homogeneous mixed solvent solutions are fed into a microspace, the solutions change from homogeneous to heterogeneous solutions featuring a liquid–liquid interface via phase transformation by changing the pressure and/or temperature. This type of flow is called “phase separation multiphase flow” (PS-MPF) and differs from conventional immiscible multiphase flows. PS-MPFs show droplet, slug, parallel, and annular flows under different conditions. In particular, the annular flow in a PS-MPF, which generates an inner and outer phase, is interesting and useful. This flow is called a “tube radial distribution flow” (TRDF). PS-MPFs were examined in detail primarily with a ternary water–hydrophilic/hydrophobic mixed solvent solution through fluorescence photographs, the construction of phase diagrams, and flow conditions to discuss their creation mechanism. TRDFs with various two-phase separation mixed solvent solutions were also constructed, and the formation and configuration of inner and outer phases in the TRDFs were then discussed through hydromechanics, including dimensionless numbers and viscous dissipation principles. The outline of the PS-MPF has been clarified from various viewpoints of the first research stage. Furthermore, new technologies were developed based on a TRDF in a PS-MPF; for example, a capillary chromatographic system, a microreactor, and unit microoperation systems referring to separation, extraction, and mixing on a microchip. These were well matched to the Lab-on-a-Chip concept.
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