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M I C R O F L U I D I C S
DROPLETS INSIDE MORE DROPLETS Device makes uniform double emulsion in a single step
ANEW MICROFLUIDICS-BASED device made by physicist David A. Weitz and col
leagues 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 emul-
T INY BUBBLES Microfluidic device forms uniform droplets. Shown here are double emulsions consisting of a single silicone oil droplet inside a water droplet.
sions—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 Malaga 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 (unidirectionat) 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
Outer fluid Middle fluid
Collection tube Injection tube
E M U L S I F I E R A three-capillary device is used to make double emulsions. Three fluids move through the device as shown by the arrows.
droplet breakup," he says. "Ύοιι 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 tiiings/'^Xfeitz says. "That's important for things like drugs or other high-valued-added materials."
"vvfeitz 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"-CELIA HENRY
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C O U N T E R I N G T E R R O R I S M
UN Approves Nuclear Terrorism Treaty fter seven years of negotiations, the United Nations General Assembly has adopt
e d the International Treaty for the Suppression of Acts of Nuclear Terrorism, the first counterterrorism treaty to be approved by the UN since Sept. 11,2001.
The treaty—supported by the U.S. and Russia—opens for signature from Sept. 14, 2005, to Dec. 31, 2006. it enters into force once 22 nations have ratifierait.
In a prepared statement, State Department spokesman Richard Boucher said the treaty "will provide a legal basis for international cooperation in the investigation, prosecution, and extradition of those who commit terrorist acts involving radioactive material or a nuclear device." Treaty provisions make it a crime for any individual or
. group to possess or use radioactive material or a device with the intent to harm or kill another individual or to substantially damage the environment or property, especially a nuclear facility.
The treaty mandates that those nations ratifying the treaty enact national laws that include "appropriate penalties which take into account the grave nature" of such terrorist acts. In the aftermath of a terrorist act, the treaty also states that the radioactive material be handled in accordance with International Atomic Energy Agency safeguards.
According to Deputy U.S. Ambassador to the UN Stuart W. Holliday, the adoption of the treaty sends a "clear signal that the international community will not tolerate those who threaten or commit terrorist acts involving radioactive mate-Tial or nuclear devices."—LOIS EMBER
W W W . C E N - 0 N L I N E . O R G C&EN / APRIL 25, 2005 7
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Inner fluid
