Congealing useful oddballs

Many microsystems manipulate tiny quantities of liquid to conduct chemical reactions or biomedical investigations. In a new twist on such technology, engineers at North Carolina State University in Raleigh have adapted a microfluidics device to fabricate tiny, solid balls of various patterns that could serve in applications ranging from drug delivery to electronic displays.

GOLDEN GAZE. Gold particles spontaneously clump together, irislike, at the center of this 1-millimeter-diameter polystyrene ball. Velev

Orlin D. Velev and his colleagues previously showed that they could induce minuscule liquid droplets to move, hover, or merge by placing them over a circuit board coated with a film of electrically insulating oil and by controlling the electric currents that flow there. In the January Nature Materials, the team reports taking a further step by loading the droplets—made of water, hydrocarbons, or polymers—with particles of polystyrene, gold, silica, and other manufacturing ingredients.

As expected, the particles congealed into tiny spheres smaller than poppy seeds. Surprisingly, however, the particles spontaneously arranged within each sphere into extraordinary patterns reminiscent of eyeballs and striped billiard balls, Velev says.

The Raleigh team is now studying fluid effects peculiar to minute liquid volumes. By controlling such effects and the droplet ingredients, Velev says, researchers might produce a wide range of ball patterns.

Among potential applications, the team says, are drug-delivery capsules, bar codes for tagging tissues in biomedical research, pixels for electronic paper, and components in electronic or light-based circuitry.

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