Spherical particles conjugated to molecules with affinity for, or able to interact with, an analyte are one of the most convenient tools for parallel multiplex analysis in research and diagnosis.[1] Identification of the different analytes is based on the signal associated to the marker particles, whose size or color is specific. For high-throughput applications, dye-labeled fluorescent microspheres have emerged as a very important alternative to traditional microarrays. [2] They present the possibility of multiplex color detection and are expected to be more flexible in target selection, faster in binding, and less expensive in production, using very small sample volumes with a three-dimensional (3D) configuration. Another advantage of fluorescent microparticle arrays is the possibility of using flow cytometry as a powerful, well-established, fast, sensitive, and accurate detection technique of biomolecular interaction, [3] particularly relevant in very recent cancer-diagnosis findings. The functionality of microbead-based arrays relies heavily on the properties of the microspheres, e.g., their size range, stability, uniformity, and ability to retain the fluorescent dye. Despite all of the efforts aimed at the preparation of labeled, functionalized polymeric beads, [4] a considerable challenge still remains, namely, how to develop and optimize simple methods for the preparation of huge quantities of fluorescently encoded microparticles for demanding applications (e.g., > 10 12 particles per day and device) with a uniform shape and surface, homogeneous size distribution, and controlled fluorescence properties.Numerous strategies and processes have been developed to produce polymeric microparticles; the determinant step being the drop formation, which fixes the size distribution of the resulting microparticles. Depending on the physical properties of the fluids, different techniques or mechanisms are used to produce monodisperse drops. The first straightforward strategy is the formation of a single drop at a time, as in dripping processes, [5] emulsification membranes, [6] or microfluidic emulsification. [5,7] Notwithstanding the very uniform microparticles obtained with these techniques, the drop-production rate is very low and the drop diameter scales with the diameter of the capillary or the pore, which makes it difficult to produce microparticles of a few micrometers or less. The second strategy is the formation of numerous drops at a time, as in mixing or stirring processes, [8] with scarce size predictability and homogeneity, or jet-disintegration techniques, in which a wide range of drop sizes is obtained, with different distributions depending on the Reynolds and Weber numbers of the jet. In particular, in laminar-jet disintegration or Rayleigh breakup, [9] the jet breaks up into uniform droplets due to capillary instability. Nevertheless, under real conditions, formation of satellite drops between the main drops, drop coalescence, and the presence of natural disturbances on the jet surface induce wid...
