C&EN: TODAY'S HEADLINES - Change Your Partner, Do-Si-Do

PAMELA ZURER

Like square dancers following the caller’s directions, the little plastic pieces (shown, insets) regroup themselves in response to changes in their environment. These two-way jigsaw puzzles are among the many aspects of mesoscale self-assembly being explored by the group of chemistry professor George M. Whitesides at Harvard University [J. Am. Chem. Soc., 124, 14508 (2002)].

Each geometrical component—measuring just under 1 cm across—is made of the hydrophobic polymer poly(dimethylsiloxane) doped with alumina. Certain faces of each have been oxidized to make them hydrophilic. (Thick lines in the shapes shown in the insets represent hydrophobic faces; thin lines, hydrophilic.)

Depending on their surroundings, capillary interactions between the hydrophobic faces can be quite strong, while those between the hydrophilic are negligible, or vice versa. Thus, the assemblies shown on the left form at the interface between water and the dense hydrophobic liquid perfluorodecalin. But when the density of the aqueous phase is increased by adding sodium metatungstate, the components aggregate in the patterns shown at the right. The rearrangements are examples of geometric dissection, in which a shape is cut into pieces that can be rearranged into another regular shape.