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Polymer-Electrolyte Aggregation and Nanoparticle Assembled Capsules In Microfluidic Channels

Nan Du1, Gautam C. Kini2, Michael Wong2, and Sibani Lisa Biswal2. (1) Department of Chemical and Biomolecular Engineering, Rice University, MS 362, 6100 Main Street, Houston, TX 77005, (2) Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005

Microfluidics enables precise manipulation of fluids at small length scales, and presents opportunities to control and apply emulsification processes to problems in colloidal science. We report the use of microfluidic devices which have been fabricated to generate stable monodisperse emulsions that can be used for nanoscale synthesis of nanoparticle assembled capsules (NACs). Within these devices, pressurized immiscible fluids are combined at a junction of two or more microchannels, combining crossflow and viscoelastic shear to rapidly generate droplets at regular periodicity. The pioneering work in this field has demonstrated that spatial and temporal instabilities in immiscible fluid systems at small length scales may lead to the controlled formation of dynamic, metastable droplet patterns (add reference).

Our device starts from generating the droplets of a linear cationic polymer {poly(allylaminehydrochloride) or PAH} in a microfluidic device consisting of three fluid streams merging into a single stream at a junction with a flow focusing geometry. The PAH tagged, with a fluorphore (Rhodamine), is flowed through the central channel and oil or air through the side channels. Using fluorescence microscopy, we observe the formation of polymer droplet, whose size depends on parameters such as volumetric flow rates and polymer concentration. Then, an electrolyte (trisodium citrate) solution is introduced through an additional microfluidic channel further downstream. We are able to get positively charged polymer-electrolyte aggregates after the injection of trisodium citrate and the ratio of aggregates is determined by the molar ratios of electrolyte to polymer.. Finally, negatively charged silica nanoparticles are introduced and reacted with the aggregates of PAH and trisodium citrate. Formation of non-spherical NACS is observed, in contrast to spherical NACs in earlier bulk synthesis methods.

Our study demonstrates that the great benefits by integrating microfluidics into traditional chemistry. First, all related operations are in a closed system, preventing from the unwanted contamination when open to the environment. Second, we can exactly control every step in the process by changing the flow rates and the concentration of solution into the system, and thus we are able to obtain various products. Third, by combining the NACs synthesis in microfluidics, we can tune the shapes of the capsules.