DRY DISPERSION OF POWDERS

Although powdered materials are sometimes dispersed into a liquid and then aerosolized by atomization (or by other methods of droplet formation), they are generally dispersed from a dry state. Alternatives are listed by Hinds (1999) in his book, Aerosol Technology: 

The following represent some of the available technologies. The products are listed by model (or Type), particle size output (µm), mass or quantity output, usual operating air flow (Lpm) and expected mass concentration (g/m³).Like automobile highway versus city mileage, yours may vary.

Pharmacon (modified Wright Dust Feeder with a packed dust cylinder, carbide scrapers, plumbed for inhalation exposure use): 0.2-10µm; 10-40Lpm; 0.012-12(g/m³). Specially configured units are available through CH Technologies.  See http://www.toxics.com for more details. 

Palas RGB 1000 (packed cylinder with brush): 0.1-100µm; 5-85Lpm; 0.1-200g/m³. Available from Palas GmbH, Karlsruhe, Germany. 

TSI 3400 (fluidized bed); 0.5-40µm; 5-20Lpm; 0.13-4g/m³. Available from TSI, Inc., St. Paul, MN, 55164. 

TSI 3433 (turntable with venturi aspirator); 1-50µm; 12-21Lpm; 0.0003-0.04g/m³. Available from TSI, Inc. 

We add to this list the Vilnius Aerosol Generator (VAG) which was developed by Arvydas Juozaitis. The device consists of a chamber above a vibrating membrane and a regulated air flow control. This system is coupled with a light-scattering photometer and feedback control computer
that allows a variety of exposure concentrations to be generated. Depending on the powder type, 2-3 µm at 12-15 Lpm with concentrations from < 0.01 to > 2 g/m³ are possible for limited periods of time. A refillable unit is being designed. Units of this type are available on a limited basis
through CH Technologies at http://www.toxics.com. 

Unless properly done, packing a cylinder to form a cake which is then scraped off with a blade or a brush may induce unwanted agglomeration of the material. Such effects are dependent on the relative humidity. In any case, only clean dry air should be used for dry dust dispersal. 

Depending on the resistivity of the powder and the humidity of the dispersion air, particle charging may be a problem in terms of impaired reproducibility of concentration and deposition characteristics. The generators that rely on a packed sample or on a fluidized bed seem likely to be most susceptible to these problems which can be ameliorated with charge neutralizers. 

Constancy of aerosol output concentration is generally a concern. The feedback approach of the Vilnius Aerosol Generator produces a more constant output concentration than do generators lacking such feedback control. However, the Pharmacon and similar Wright Dust Feed-type units
have a long history of successful use at constant rates of powder generation (Robert Gussman, personal communication). 

Other pages at the http://www.inhalation.net site discuss powders and the sources of them for aerosol generation. The generation of liquid aerosols (sprays, mists) is a subject for a future review. 

To this I (RJJ) add a brief note. For your information, Dr. David Bernstein of Geneva, Switzerland has developed a brush type fiber generator that he and his colleagues at RCC and elsewhere have used for fiber studies. This device is commercially available from CH Technologies (USA) Inc. 

By: Douglas W. Cooper, PhD with added comments by Rudolph J. Jaeger, PhD, DABT