HARVARD GAZETTE ARCHIVES

David Weitz led the team that devised a technique using two tiny tubes that produces precisely sized fluid droplets wrapped inside a second fluid and suspended in a third. (Staff photo Justin Ide/Harvard News Office)

The team, led by Gordon McKay Professor of Applied Physics and Professor of Physics David Weitz, devised a technique using two tiny tubes that produces precisely-sized fluid droplets wrapped inside a second fluid and suspended in a third.

The advance has a wide potential range of applications in the pharmaceutical, food, cosmetic, and chemical industries. The double emulsions are potentially useful in any application where a product needs to be shielded from its surroundings for a period of time before being delivered.

Cosmetic creams are one example, Weitz said, where the active ingredient needs to be shielded from the carrier cream until it is spread on a person's skin. Drug delivery is another example, where a particular drug needs to be protected until it reaches the target area for delivery.

Any consumer who has eaten a salad is already familiar with emulsions. Two liquids that don't mix, such as oil and vinegar, form emulsions. A shaken bottle of salad dressing, with oil droplets floating in vinegar, illustrates a single emulsion. A double emulsion, such as Weitz and colleagues created, involves a third liquid surrounding the oil droplet and insulating it from the vinegar.

Weitz said that double emulsions are already widely used in industry, but current techniques have no way to control the size of the droplets involved. That means there's no reliable way to regulate the delivery of whatever cargo the drops contain, which limits the technique's use in applications that require exact quantities.

"There's just no way of doing it in a precise fashion," Weitz said.

Weitz and his colleagues' technique, described in the April 22 issue of the journal Science, uses two tiny tubes, with a smaller one inserted in the other, to form the droplets. Working with Weitz were lead author Andrew Utada, a graduate student at the Division of Applied Sciences (DEAS), former postdoctoral fellows Darren Link and E. Lorenceau; DEAS Professor H.A. Stone; and P.D. Kaplan of the Unilever Skin Global Innovation Center.

Utada, a doctoral student, said the project took about a year and a half, with work being completed in August 2004. He said the work inspired him, not just because of the physics involved and the potential applications, but also because of the physical beauty of the droplets themselves.

"I think they're so beautiful, these structures," Utada said.

Developing the technique required a shift in thinking about the problem, Weitz said. While today's techniques to form double-emulsion droplets create many droplets at once, Weitz's technique forms them one by one.

Weitz and colleagues demonstrated several variations in their technique that can prove useful in different applications. In one case, they used a liquid polymer for the outer shell fluid and then hardened it using ultraviolet light after double droplets formed, creating tiny hard capsules protecting the inner fluid.

Weitz said his work benefits from the strong microfluidics program at the Division of Engineering and Applied Sciences and in the Department of Chemistry and Chemical Biology, which involves several researchers. His lab is continuing to examine the applications of microfluidics.

"We're continuing all kinds of things with our microfluidics work in what is a very good group effort," Weitz said.

alvin_powell@harvard.edu

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