Fundamental research and application of the specific fluidic behavior of mixed solvents in a microspace

The author herein reviews a specific microfluidic behavior exhibited by mixed-solvent solutions in a microspace, coined as the tube radial distribution phenomenon (TRDP). The specific fluidic behavior was observed in the following solution systems: ternary water-hydrophilic/hydrophobic organic solvents, water-surfactant, water-ionic liquid, and fluorous/organic solvents. When the mixed homogeneous solutions were delivered into a microspace under certain conditions, the solvent molecules were radially distributed in the microspace, generating inner and outer phases with a kinetic liquid-liquid interface. The TRDP was fundamentally evaluated by fluorescence microscopy, phase diagrams construction, and the elution behaviors of solutes in a capillary tube. A TRDP-based capillary chromatography, referred to as tube radial distribution chromatography (TRDC), where the outer phase serves as a pseudo-stationary phase under laminar flow conditions, has been developed as one of the applications of TRDP. We have also investigated TRDP-based extraction, chemical reaction, and mixing processes, coined as tube radial distribution extraction (TRDE), tube radial distribution reaction (TRDR), and tube radial distribution mixing (TRDM), respectively. The concept and experimental findings regarding TRDP, TRDC, TRDE, TRDR, and TRDM are described in this review.