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The Air We Breathe

Just as the clearest pond water comes alive with tiny organisms when viewed under a microscope, the specialized equipment in the laboratory of Barbara Wyslouzil reveals that the air surrounding us is really an aerosol containing thousands of particles per cubic centimeter.

For the last six years, Wyslouzil, associate professor of chemical engineering, has focused on the finest of these particles, called nanodroplets because they are typically less than 100 nanometers in diameter. These droplets can impair human health, change the chemistry of the atmosphere and alter our perception of air quality, yet little is known, from a molecular perspective, about how they form when fossil fuels are burned or through incineration and other industrial and natural processes.

Wyslouzil heads three aerosol science research projects funded by the National Science Foundation and another funded by the Petroleum Research Fund. As a leading figure in this emerging field, she has been recognized by the NSF with a Faculty Early Career Development (CAREER) award and by WPI with the 2001 Trustees' Award for Outstanding Research and Creative Scholarship.

A primary focus of her research is the formation and structure of multicomponent nanodroplets. She hopes to learn how conditions in the gas phase affect the rate at which these droplets form. She is also interested in knowing whether the droplets contain regions with distinctly different compositions, since the way a droplet interacts with its environment depends on which molecules lie at its surface.

In her laboratory in Olin Hall, Wyslouzil and her team of undergraduate and postdoctoral students produce aerosols using a supersonic nozzle, then study them with conventional methods, including light-scattering. Once a year, they pack their equipment in a 15-foot truck and drive to the Center for Neutron Research at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. Over the course of four to five days, they use a highly sophisticated piece of equipment called a Small Angle Neutron Scattering (SANS) instrument. Because the wavelength of the neutrons is smaller than the size of the droplets, the neutron-scattering patterns can provide information about both the size and the internal structure of the droplets that can't be derived with other methods. The NIST campaigns are grueling, Wyslouzil says, because the experiments run 24 hours a day.

The most recent trip to NIST, in June, "was the most rewarding yet and produced exceptionally good results," she says. "For the first time, we were able to observe that some nanodroplets really are segregated and consist of a water-rich core with an alcohol-rich surface layer. It was our third attempt to get a 'signal' from this type of droplet and this time the spectra looked right!"

With this information, Wyslouzil and her team can complete a more quantitative analysis of the results--for example, determining the exact thickness of a nanodroplet's outer layer.

Having pioneered the use of SANS to investigate the properties of atmospheric nano-droplets, Wyslouzil says she and her group are keen on extending her work into other areas of aerosol science.

To develop a method for probing the structure of atmospheric nanodroplets, Wyslouzil created test droplets by spraying a mixture of water or heavy water (D20) and d-butanol through a supersonic nozzle. The curves to the left show that the droplets were expected to have a water-rich core and an alcohol-rich shell. The curves to the right show actual results obtained with small-angle neutron scattering. The results demonstrate the usefulness of this technique, the only one yet developed that can probe the microstructure of nanodroplets.

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