atomizer ultrasonic nebulizers humidifiers misters particle generators geyser-type nebulizer (the sound interface between the water surface and the air forms a "geyser" of water above the surface. At the peak of this geyser the water bursts into micrometer sized particles).
Particle size is related to transducer frequency (f), fluid surface tension (T), and density (p).
dh = 0.73 * (T / (p * f2)) 1/3
For water, T=.0729 N/m, p=1000 kg/m3, and f=2.4 mHz. This gives a particle size of around 1.7 micrometers.