A microfluidic cross-flowing emulsion generator for producing biphasic droplets and anisotropically shaped polymer particles

We present a microfluidic cross-flowing system for producing biphasic emulsion droplets and non-spherical polymer microparticles. Microfluidic channels on a glass chip comprise a Y-shaped channel so as to form a two-phase organic stream of photocurable and non-curable phases, and a T-junction to generate phase-separated droplets in a cross-flowing aqueous stream. The biphasic droplets at equilibrium formed a Janus configuration (partial engulfing) or a core–shell configuration (complete engulfing) consistent with minimizing the interfacial free energies among the three liquid phases, according to the three spreading coefficients. When silicone oil was used as the non-curable phase, monodisperse Janus droplets were generated reproducibly in a one-step process; for e.g., the mean particle size was 119 μm with a coefficient of variation (CV) of 1.9%. Subsequent UV-initiated polymerization yielded monodisperse particles with controlled convex/concave structures, which were tunable through variation of the ratio of the flow rates between the two organic phases. In contrast, when perfluorocarbon fluid, which is more hydrophobic than silicone oil, was used as the non-curable phase, monodisperse core–shell droplets were generated in a two-step regime, leading to the synthesis of cross-linked polymeric shells with a pore on their surfaces. We also investigated how the asymmetric flow configuration influenced droplet formation at the T-junction.