Optical Aerosol Characterization
Although it weighs literally nothing, light packs a punch, and it can push particles around. A trio of researchers from Technical University of Munich in Germany exploited this fact to devise a light-based method to characterize the optical properties of aerosols. The technique can distinguish between cases where all the aerosol particles are the same and those where many types of particles are present.
The trajectory of particles affected by light is shown. Small aerosol particles flow through a cell, where they transit an infrared laser beam. The force generated by the light alters the flight of the particles, enabling characterization and separation. Images courtesy of Christoph Haisch, Technical University of Munich.
The method also can segregate particles based on their color or other optical properties, a capability that had been lacking, noted Christoph Haisch, team member and head of the applied laser spectroscopy laboratory at Technical University. Haisch developed the new technique working with fellow researchers Carsten Kykal and Reinhard Niessner.
Haisch and Niessner had been using strong laser sources for diesel soot emission monitoring for a long time when the question arose as to what was happening to the particles in the beam. Such soot is one example of an aerosol, a term that applies to any particles (solid, liquid or a mixture) suspended in and moving with a gas. Aerosols can come from a variety of sources and can have important climate and health effects.
In this flow cell, an aerosol enters one port and exits the other. An infrared laser transits the cell, altering the flight path of the aerosol particles according to their size, color and other optical properties. Illumination from a green laser can be seen in the cell, providing the light needed to track the movement of individual particles.
As for the question about what happens to particles in a laser beam, it is known that they migrate when illuminated by an intense beam of light. In fact, the mechanisms behind this photophoresis have been known for almost a century. The phenomenon is caused by either a direct momentum transfer because of the particles’ interaction with photons or by an indirect force from the particle-absorbing light. The resulting heat is picked up by the surrounding gas, and because heating happens only on one side, the particle experiences a net force.
According to the group’s calculations, indirect photophoresis is stronger than direct photophoresis, even for weakly absorbing particles. Because of this, aerosol particles move away from a source but in the same direction as an illuminating laser. This movement forms the basis of the team’s particle characterization and separation technique.
The three researchers put the photophoretic effect to work in an experimental setup that consisted of a flow cell with silica windows on the ends and at the midsection. The aerosol entered through a port at one end of the cell, flowed through the cell and exited a port at the other end, producing a steady stream of particles.
To create photophoresis, the researchers used a custom-made fiber-coupled diode laser system operating at 806 nm, sending the beam at a 90° angle to the aerosol flow. An Nd:YAG laser beam at 532 nm illuminated particles in the cell so that their individual movements could be tracked. The researchers measured their movement using optics and an Allied Vision Technology CCD camera. Once they had the raw motion data for single particles, they subtracted the effect of gas flow. The work was published in the March 1 issue of Analytical Chemistry.
To test the setup, they injected aerosols of varying compositions. They used white latex spheres with diameters from 0.18 to 4.13 μm in diameter, as well as 1-μm diameter fluorescent particles of white, yellow and red. Because of the near-infrared wavelength of the excitation beam, the particles didn’t fluoresce; only their color affected movement.
The investigators found that the photophoretic velocity showed a correlation with excitation laser power and successfully tracked particle size. There was also a velocity difference resulting from particle color, enabling the characterization and separation of particles according to optical properties. That ability could be useful, Haisch noted, adding, “I would see most benefits from color and maybe thermal property measurements, since there are no other methods doing that.”
According to the researchers’ estimates, their setup could determine particle size from microns to about 100 nm. A more powerful laser and a modified flow system could improve that resolution at least tenfold, they noted. Other modifications could lead to separation systems, whereby, based on their photophoretic susceptibility, incoming aerosol particles could be divided into two streams.
Contact: Christoph Haisch, Technical University of Munich, Germany; e-mail: firstname.lastname@example.org.
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