Synthetic polymeric biodegradable nanoparticles were produced by micromixing combined with nanoprecipitation in a co-flow glass capillary device consisted of coaxial assembly of glass capillaries, fabricated by aligning a tapered-end round capillary inside a square capillary with 1 mm internal dimension. Micromixing of water and organic phase (1 wt% polylactide or polycaprolactone dissolved in tetrahydrofuran) was modelled using a commercial software package Comsol Multiphysics (TM) and experimentally investigated using dynamic light scattering, Nanoparticle Tracking Analysis (NTA) and in situ microscopic observation. The organic phase was injected through a nozzle with a diameter of 60 gm at the organic-to-aqueous flow-rate ratios ranging from 1.5 to 10. The locations at which the nanoparticles would form were determined by using the solubility criteria of the polymer and the concentration profiles found by numerical modelling. The convective flux of the polymer in the radial direction was 2-3 orders of magnitude higher than the diffusive flux of the polymer; hence responsible for mixing the streams. The convective flux near the orifice was 3-4 orders of magnitudes higher than at the end of the computational domain. A maximum convective flux of 0.115 kg m(-2) s(-1) was found for polycaprolactone at the cloud point for the lowest flow rate ratio investigated. The numerical results were consistent with the experimental observations in terms of flow patterns and mean particle size. Narrower particle size distributions and smaller mean particle sizes were obtained at the higher aqueous-to-organic flow-rate ratios. (C) 2015 Elsevier B.V. All rights reserved.