Atomizers for Droplet Aerosol Generation

Liquid droplet aerosols ("sprays" for coarser types, and "mists" for finer types) are generated to make airborne either solid particles (after drying) or liquid droplets. The liquid may have a solute or may have particles suspended in it that become singlets or multiplets. Examples of two different "two-fluid" (gas and liquid) atomizers are shown below. It requires energy to break up the liquid, energy supplied by the pressure loss in the atomizing gas. Surface tension (units of force per length, such as dyne/cm, or energy per area, such as dyne-cm/cm²) reflects the cohesion of the liquid. The smaller the droplets, the more total surface area and thus the greater the amount of energy needed to produce the aerosol. Similarly, the lower the surface tension (perhaps due to surfactants, such as detergents or alcohols), the finer the droplets for the same energy input.

"Thumbnail" figure (click to expand) of Collison atomizer, drawing from Green and Lane (1964).

"Thumbnail" figure (click to expand) of DeVilbiss No. 40 nebulizer, drawing from Mercer et al. (1968) in Dennis (1976).

In inhalation experiments, it is usually desired to maximize the respirable fraction of the aerosol, generally by making the droplets finer. Users may get more respirable mass and less loss in transport from generator to chamber by reducing drop size, accelerating evaporation. Besides heating the air to accelerate evaporation and lowering solids content to lower the dried volume, one could get smaller drops by adjusting operating variables as indicated below. [One can test this with a respirable (cyclone) sampler.]

Lefebvre (1989) noted that the surface median diameter (SMD) is often used for characterizing sprays. [Half the aerosol surface area will be in particles smaller than the SMD.] Empirically, it has been found that one can model as

SMD = (constant)*(variable)^exponent

The following information comes from Lefebvre's

Table 6.8 Empirical Power - Law Dependencies for SMD

Variable Exponent Comments {Ours]

air velocity	-1.0	Proportional to square root of pressure.
air density	-0.3	Awkward to adjust
liquid density	-0.5	Awkward to adjust
surface tension	-0.5	Use surfactant? Add alcohol?
air/liquid mass	----	Larger ratio is better up to 10
jet orifice size	-0.0	Virtually no effect

It is clear, also, that higher - viscosity liquids produce larger droplets, other things being equal.

REFERENCES

R. Dennis, Ed., Handbook on Aerosols, USERDA, Washington, DC, 1976.

H. L. Green and W.R. Lane, Particulate Clouds: Dusts, Smokes, and Mists, Second Edition, Spon Ltd., London, 1964.

A. H. Lefebvre, Atomization and Sprays, Taylor & Francis, Bristol, PA, 1989.

By: Douglas W. Cooper, PhD