This thesis aims to develop an instrumented and automated microfluidic platform dedicated to studying third-phase demixion in liquid-liquid extraction processes, applied to the challenge of securing diluent supply for the process implemented at the La Hague plant. As the diluent is essential to the plant's proper hydrodynamic operation, yet its supply has historically been fragile (production shutdowns, non-conformities, non-renewal of supplier contracts), this platform will enable rapid and automated characterization of alternative diluents with respect to third-phase formation in the TBP/diluent solvent system heavily loaded with actinides (U(VI), Pu(IV)). The device will ensure controlled contact between the aqueous and organic phases while automatically adjusting their respective compositions, with continuous in situ monitoring of actinide concentration along the flow channel via UV-Vis spectroscopy, as well as detection of third-phase onset, notably through light extinction methods particularly suited to the characterization of colloidal suspensions. This instrumental approach will be complemented by online SAXS measurements, enabling investigation of the local molecular arrangement of TBP and diluent in the biphasic domain for modeling purposes. The development of this tool will first draw on a literature review of the conditions governing third-phase formation and the mechanisms of binodal/spinodal decomposition in liquid-liquid systems. At least three candidate diluents will be evaluated on this platform following an experimental plan varying temperature, pH, and actinide composition, with tests conducted successively on surrogates, U alone, Pu alone, and finally a U+Pu mixture.