Drops in drops hold practical promise

A team of Harvard researchers has developed a technique that allows the precise formation of double emulsions – droplets within droplets – that offers new ways to deliver drugs, nutrients, and other consumer and industrial products.
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.