Hypothesis Janus polymeric nanoparticles have attracted much attention for their wide range of applications in various fields due to the presence of two domains with different chemistries, compositions or functionalities. This type of particles is obtained by various conventional methods which require several steps (the particle elaboration and the surface functionalization), sometimes leading to the elaboration of emulsions with large and multimodal sizes distributions. Various physical and chemical parameters influence the particles size, morphology and stability. Experiments In this study, we propose to prepare polymeric Janus nanoparticles (JNPs) by a one-step method from a mixture of two different polymers and to compare several emulsification-evaporation processes (sonication, shear mixing and elongational-flow micromixing). Findings The possibility to obtain monomodal JNPs (diameter lower than 200 nm) with a hydrophobic domain (poly(lactic-co-glycolic acid), PLGA) and a hydrophilic charged domain (based on poly(styrene sulfonate), PSS) was demonstrated only with the elongational-flow micromixer.