Nonspherical microparticles have myriad applications in various fields, because they possess distinct advantages over their spherical counterparts as a result of their scattering, rheological, coagulation and other properties. A variety of particle shapes have been achieved by the seeded polymerization techniques. However, these methods involve complicated material formulations, limiting their utilities for practical applications. Recently, a few microfluidic systems have been reported for the in-situ synthesis of nonspherical polymer particles, such as ellipsoids, rods, disks, and truncated particles. Photolithography in a microstream of polymer precursors has also been proposed for the fabrication of particles with arbitrary 2D shapes. However, all these microfluidic approaches share several drawbacks, such as the risk of clogging in the channels, increased pressure loss, limited controllability in the 3D shape of the particles, and low production throughput.
To overcome the above-mentioned limitations, the author previously proposed the off-chip synthesis of shape-controlled polymer particles through the formation of biphasic droplets in a symmetric co-flow channel. Herein, we report the use of asymmetric microfluidic channels (i.e., T-junction and cross junction) for the off-chip synthesis of shape-controlled polymeric microparticles, which is more efficient in the parallelization of microchannels for the industrial-scale production.
Microchannels with rectangular cross section were fabricated on a synthetic silica substrate by the deep reactive ion etching (DRIE) technique. The microchannel consists of a upstream Y-shaped channel and a downstream T-junction/cross-junction. Around the T-junction/cross-junction, the channel widths were 100 Ám with a uniform depth of 100 Ám. An acrylate monomer (1, 6-hexanediol diacrylate, HDDA, 6.35 mPa s) was used as a polymerizable organic phase, and silicone oil (SO, 9.34 mPa s) or perfluorocarbon fluid (PFC, 1.6 mPa s) was used for the non-polymerizable organic phase. Aqueous solution of sodium dodecyl sulfate (SDS, 0.3 wt%) or polyvinyl alcohol (PVA, 2.0 wt%) was used as external aqueous phase. UV polymerization was continuously performed off the fluidic module.
A two-phase parallel stream was formed at the Y-junction when the two organic phases were infused separately into the two arms of the Y-junction. This two-phase stream then entered a shearing stream of the aqueous phase at the T-junction, resulting in the rapid formation of biphasic organic droplets by a shear-rupturing mechanism (~300 drops/s). The droplets were highly monodisperse in size, typically with a coefficient of variation (CV) around 2 %. When we used the combination of two Y-shaped channels and a cross-junction, biphasic droplets could be alternately formed at the two opposed inlet channels.
When SO was used as the non-polymerizable organic phase, the produced droplets formed Janus structure. The Janus droplets had a snowman-like appearance, with a curved internal interface separating the two organic phases. The satellite droplets (~20 Ám) also had similar Janus structure. By varying the ratio of the flow rates of the two organic phases, it was possible to adjust the volume ratio of the two organic phases in the Janus droplets. We consider that the formation of these Janus geometries was driven only by the minimization of the interfacial free energies among the three liquid phases, because similar results were obtained when we exchanged the positions of the two organic streams in the microfluidic channels. By subsequent UV polymerization, we could synthesize monodisperse nonspherical particles. The particle shape could be tuned by varying the flow-rate ratio of the two organic phases between 0.1-10.0.
On the other hand, when PFC was used as the non-polymerizable organic phase and the PVA aqueous solution as the continuous aqueous phase, HDDA droplets engulfing PFC were reproducibly formed, because of the strong hydrophobicity of PFC. The UV polymerization of the external HDDA produced macro-porous shells with a small pore on their surfaces.
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