ABSTRACT
The jet nebulizer is a system in which a high-velocity gas flow is directed over a tube that is immersed in a water reservoir (Figure 10.8). The expansion of the driver gas decreases the pressure over the tube, which draws the formulation into the gas stream. The high shear rate in the jet stream then nebulizes it. The hydrodynamic nebulizer uses a system that prepares a film of water for aerosol formation by flowing it over a hollow sphere. A small orifice in the sphere expels gas at supersonic velocity. This high-velocity gas ruptures the thin film of water and produces a continuous dispersion of fine, liquid particles. A gas cylinder or compressor supplies the gas pressure. The ultrasonic nebulizer consists of a piezo-electric crystal which produces high frequency sound waves in the liquid in the nebulizing unit. The surface waves produce small droplets (Faraday crispations) which are conducted away by an airstream for inhalation. All these devices produce relatively broad droplet size distributions in which a large fraction of coarse droplets are present. Consequently most use some sort of baffle system in the airstream; coarse droplets impact on this and are returned to the reservoir for re-nebulization, while the smaller particles avoid the baffle and are passed to the patient.
