Optical Technique Enhances Aerosol Studies
Michael D. Wheeler
WASHINGTON -- Understanding the nature of aerosols is vital to designing better nebulizers for chemical analysis and drug delivery, and to investigating fuel mixing in automotive and aerospace applications. Researchers now have a technique at their disposal that uses laser light to capture images of an aerosol field.
In the past, mechanical techniques such as cascade impaction and mechanical patternation dominated aerosol diagnostics, but these approaches are intrusive. Laser-based alternatives like phase-Doppler particle analysis probe only a single point in the aerosol, so users must take many measurements to obtain the spatial spray characteristics.
Optical patternation, however, enables researchers to assess the qualities of the entire aerosol structure quickly and noninvasively. Developed by a team from George Washington University in Washington and Aerometrics/TSI Inc. of St. Paul, Minn., the three-dimensional approach directs a laser beam through a cylindrical lens, a rectangular beam mask and a parabolic collimator, producing a 2-mm-thick light sheet. The light illuminates a slice of the aerosol, either perpendicular or parallel to the aerosol spray, and a pixel-intensified CCD array collects images of the scattered and fluorescent light from the sample. Image analysis determines the aerosol's spatial mass distributions and planar droplet size.
Pick a laser
In the initial tests, a 7-W argon-ion laser served as the illumination source for water doped with a fluorescent dye. In later tests, the researchers replaced the argon-ion laser with a 532-nm, frequency-doubled Nd:YAG. For automotive applications, a frequency-quadrupled Nd:YAG at 266 nm can excite native fluorescence in many fuels.
"The type of laser and wavelength used is not too important so long as it satisfies two primary criteria," explained Akbar Montaser, a professor at the university and member of the research team, which presented its results in the Oct. 15, 2000, issue of Analytical Chemistry. "One, the laser power must be high enough to provide high illumination across the area of interest in the aerosol. Two, the wavelength used must be able to excite fluorescence."
Montaser said the next step is to address issues in image processing and droplet size calibration and to map other aerosol properties.
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