Droplets inside More Droplets

MICROFLUIDICS

A new microfluidics-based device made by physicist David A. Weitz and colleagues at Harvard University and Unilever Corp. makes precisely controlled double emulsions in a single step (Science 2005, 308, 537).

In an emulsion, droplets of one liquid are dispersed in another immiscible liquid. Double emulsions--droplets inside droplets--could be useful for encapsulating products such as drugs, cosmetics, or food additives. Making double emulsions typically requires two separate emulsification steps, and the resulting droplet size and shape are difficult to control.

"We're trying to make a general device for encapsulation structures," Weitz says. "When you're making these structures, the control is all through the kinetics of how you mix things. That's what microfluidics lets you do really well."

Ignacio G. Loscertales, a mechanical engineering professor at the University of Mlaga in Spain, says the device "opens up a new and very promising technique" for producing uniform microscale emulsions.

The device consists of three glass capillaries through which the three fluids that will make up the double emulsion flow--an outer square tube and two cylindrical inner tubes lying in one axis. The inner fluid is pumped through a tapered injection capillary, and the middle fluid is pumped through the outer region, forming a coaxial (unidirectional) flow with the inner fluid at the capillary exit. The outer fluid is pumped through the outer region from the opposite direction. The three fluids are forced through the exit orifice formed by the other capillary, rupturing into droplets of inner fluid within droplets of middle fluid dispersed in the outer fluid. The geometry and relative flow rates can be used to adjust the size and number of inner droplets.

Takasi Nisisako, an engineer at the University of Tokyo who has made planar devices that generate double emulsions, believes that the advantage of Weitz's device is in its three-dimensional coaxial flow. "Neither the middle fluid nor the inner fluid touches the capillary wall during the droplet breakup," he says. "You don't have to care about the wettability of the solid surface, which has a significant effect on the drop formation in two-dimensional microfluidic techniques."

The inner fluid remains completely separate from the outer fluid the whole time. "Since the streams are completely separate, they're very efficient at encapsulating things," Weitz says. "That's important for things like drugs or other high-valued-added materials."

Weitz believes the device could be scaled up to produce hundreds of grams of material per day, but cautions that "it's never going to be something that could make enormous quantities--tons per day."