Home

Personnel

Publications

Newsletters

Glossary

Bibliography

Calendar

Services

Products

Instructions,
Descriptions,
Manuals

Links

DRYING LIQUID AEROSOLS

Aerosols for inhalation studies can be generated from gas - phase reactions, fragmentation of dust cakes or dispersal of powders, and atomization of liquids, our focus here.

Except in an atmosphere that is super - saturated with its vapor, a droplet starts to evaporate as soon as it is formed. The process is not simple. As it evaporates, the droplet cools, losing heat energy to supply the latent heat of vaporization. Cooling slows evaporation, by lowering the vapor pressure, and evaporation may increase the concentration of solutes, also lowering the vapor pressure and slowing evaporation. 

After a transient initial period, the drop diameter can be approximated using the following equation

D² = Do² - k t

in which D² is the square of the droplet diameter; Do is the initial droplet diameter; t is time; k is a rate constant that depends on the properties of the liquid and the environment. A fuller treatment is available in the text by W. C. Hinds, Aerosol Technology, 2nd Edn., Wiley, New York, 1999.

For a pure water droplet of diameter D in an atmosphere of <= 50% relative humidity (RH), Hinds shows that the droplet lifetimes are

t' = (0.001s)(D/1µm)².

A droplet 10µm in diameter dries in 0.1s under these conditions. 

As the humidity approaches 100%, other factors come into play, producing a lifetime nearly proportional to the volume, so that a water droplet 1um in diameter takes about 1s to dry and a droplet 10um in diameter takes about 1000s.

An example of the significance of this is in the atomization of material to be used in inhalation studies, for which CH Technologies supplies associated equipment (link). In his Table 21.1, Hinds lists many different nebulizers along with the droplet size distributions associated with certain conditions. For example, the Collison atomizer working with air at about 20psig (2.3atm absolute) produces an initial mass median diameter (MMD) near 3µm and a geometric standard deviation (GSD) near 3, an easily - remembered combination. Thus, half the mass is in droplets 3µm or larger, which would take about 30s or more to dry at 100%RH.

Losses due to settling and impaction during transport will depend primarily on the aerodynamic relaxation time, proportional to the density of the droplet and its diameter squared. It is much easier for the droplets to be lost to these mechanisms than for the solute residues to which they dry to be lost. Thus, aerosols generated for such inhalation studies need to be dried rapidly and given adequate residence time for drying before they are subjected to significant accelerations, due to gravity or change of direction.