Molding many-faced particles

Scalable production of micron- and nanometer-sized particles with controlled sizes, shapes, and compositions is of interest for a host of applications. Toward that end, in 2005 Joseph DeSimone and colleagues (University of North Carolina, Chapel Hill) introduced PRINT—particle replication in nonwetting templates—a technique for making micro- and nanoparticles in molds. The researchers fill an array of molds with an organic monomer liquid, use UV light to cure the liquid into a polymeric solid, extract the polymer particles using an adhesive film, and free them by dissolving the adhesive. In addition to pursuing various biomedical applications, DeSimone and colleagues have used PRINT to produce Janus particles: Hydrophobic on one side and hydrophilic on the other, the particles are interesting for the self-assembled structures they form (see the Quick Study by Steve Granick, Shan Jiang, and Qian Chen in PHYSICS TODAY, July 2009, page 68 ). The researchers fill the molds with a dilute solution of a hydrophilic monomer, evaporate off the solvent, and top off the molds with a hydrophobic monomer. Now, they’ve exploited capillary forces in rod-shaped molds to produce a wider range of multiphase particles. As shown in the figure, when the molds are filled with a hydrophilic solution and the solvent evaporated, the remaining liquid is drawn to the molds’ ends; topping off with a hydrophobic monomer gives symmetric triblock particles. The researchers also created asymmetric triblock and diblock rods by spinning the partially filled molds in a centrifuge to drive some or all of the hydrophilic liquid to one end. (J.-Y. Wang et al., J. Am. Chem. Soc., in press, doi:10.1021/ja2066187.)