Microfluidic devices have been successfully used in forming functional colloidal structures such as droplets, microcapsules and Janus particles with various applications in laundry and household care, medicine, diagnostics, cosmetics, foods, catalysis and paints.^1,2 To date, most of the colloidal structures developed in microfluidic devices are based on flow focusing or fluid instabilities that require a water immiscible organic phase, which oftentimes limits applications in the food and medicinal sectors. In this study, we describe methodologies to create a host of structures such as polymer-electrolyte aggregates, electrolyte droplets, droplet networks and nanoparticle assembled capsules (NACs), using environmental benign aqueous reactants under controlled flow conditions in microfluidic channels. Anticipated applications of these structures are as drug delivery agents, advanced separation devices in membranes and as encapsulating agents.

Our work involves the reaction of a linear cationic polymer {poly(allylamine hydrochloride) or PAH} with an electrolyte solution (trisodium citrate) in a microfluidic channel consisting of three fluid streams (dimensions: 50 µm Χ 50 µm, width Χ height) that merges into a single stream (dimensions: 150 µm Χ 50 µm, width Χ height). The fluorescently labeled polymer is flowed through the central channel while electrolyte is flowed through the outer channels. Using fluorescence microscopy, we first observe the formation of polymer-electrolyte aggregates at the interface of the reactant streams after which, these aggregates further cross-link into a polymer network. Subsequently, emergence of electrolyte droplets within the polymer network occurs. These droplets continue to grow in the absence of a flow field, resulting in a polymer matrix containing electrolyte droplets.

We have found that droplet formation depends on parameters such as polymer concentration, molar ratios of electrolyte to polymer and volumetric flow rates of reactant streams. A threshold molar ratio of electrolyte to polymer is required to crosslink the aggregates to form polymer networks. Droplets are formed as the electrolyte diffuses into the polymer network which is saturated of reaction sites. Droplets were seen to increase in size via a necking and coalescence mechanism In an alternate reaction flow scheme, NACs were synthesized when aggregates of PAH and trisodium citrate, formed by vortex mixing outside the channel, were drawn through the central channel of the microfluidic and reacted with streams of negatively charged silica nanoparticles (13-15 nm) drawn through outer channels. Formation of non-spherical NACs are observed for the first time in contrast to spherical NACs formed in earlier bulk synthesis methods.³

Our study demonstrates the effect of microfluidic flow for novel colloidal structure formation. Droplet formation was rendered feasible under laminar flow conditions with interfacial mixing of reactants that otherwise resulted in polymer-electrolyte aggregates when prepared using beaker synthesis under vortex mixing. Elongation of droplets under shear indicated that preformed polymer-electrolyte aggregates can be deformed in flow fields, thus giving rise to non spherical NACs.

References:

1. Xu et al. Angewandte Chemie-International Edition 2005, 44, (25), 3799-3799

2. Shepherd et al., Langmuir 2006, 22, 8618-8622.

3. Rana et al., Adv. Mater. 2005, 17, 1145-1150.