Applied Physics

Liquid Russian Dolls

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Science  17 Jul 2009:
Vol. 325, Issue 5938, pp. 247
DOI: 10.1126/science.325_247b
CREDIT: ABATE AND WEITZ, SMALL 5, 10.1002/SMLL.200900569 (2009)

Oil and vinegar–based salad dressing is a classic example of an emulsion, wherein the droplets of one fluid are trapped inside the bulk of another. Though techniques exist for making higher-order emulsions (with two or more nested droplet layers), they tend either to be inefficient or else to produce droplets that vary widely in size. Abate and Weitz used lithography to fabricate polydimethylsiloxane devices that can efficiently create uniform distributions of emulsion droplets with up to five nested layers. Single emulsions were prepared using pinned-jet flow focusing—injection of the inner fluid from two side ports into a central stream of outer fluid. To increase the number of emulsion layers, additional injection ports with alternating wettability were added along the length of the flow channel. In order to ensure synchronized droplet formation for triple and higher-order emulsions, the nozzles at each injection port were designed to be slightly narrower than the emulsion arriving from upstream; a new droplet thus formed at the injection port only upon perturbation by an incoming droplet. The droplets exhibited very narrow size dispersity at all orders, as illustrated by their hexagonal packing when confined in two dimensions.

small 5, 10.1002/smll.200900569 (2009).

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