Microfluidic assembly of Janus-like dimer capsules.

We describe the microfluidic assembly of soft dimer capsules by the fusion of individual capsules with distinct properties. Microscale aqueous droplets bearing the biopolymer chitosan are generated in situ within a chip and, as they travel downsteam, pairs of droplets are made to undergo controlled cross-linking and coalescence (due to a channel expansion) to form stable dimers. These dimers are very much like Janus particles: the size, shape, and functionality of each individual lobe within the dimer can be precisely controlled. Dimers with one lobe much shorter than the other resemble a bowling pin in their overall morphology, while dimers with nearly equal-sized lobes are akin to a snowman. To illustrate the diverse functionalities possible, we have prepared dimers wherein one lobe encapsulates paramagnetic Fe2O3 nanoparticles. The resulting dimers undergo controlled rotation in an external rotating magnetic field, much like a magnetic stir bar. The overall approach described here is simple and versatile: it can be easily adapted in numerous ways to produce soft structures with designed properties.