Nebulizers and Atomizers

Nebulizers and atomizers used in aerosol research produce a polydisperse aerosol consisting of particles under 10 mm in diameter. Most nebulizers use compressed air for atomization, whereas some use ultrasonics. Many models of compressed-air nebulizers have been developed, but they basically use the principle of air blast atomization of liquids issuing through a small orifice. Impaction plates or baffles are used to remove the larger droplets. Mass median diameters normally range from 2 to 5 mm, with a compressed-air pressure of 20-30psig. A detailed discussion of nebulizers can be found in Raabe [5]. Most nebulizers or atomizers tend to have a small liquid reservoir and cannot be used for long duration unless the reservoir is refilled continuously.

A model that is of high stability and capable of long duration operation is that developed by Liu and Lee [17]. The principle of operation is illustrated in Fig. 2 for a commercially available version of the atomizer. Air is forced through an orifice to a small passage where the liquid is fed in. The liquid is atomized by the high-velocity airstream. The larger droplets impact on the wall of the passage and flow down to the excess liquid reservoir. The smaller droplets are carried up by the airflow. The droplet size in the aerosol stream that is produced can be controlled by varying the compressed-air pressure. Submicrometer particles or droplets can be produced by using solutions. The solution droplets are then dried by heating the aerosol and passing them through a diffusion dryer. In the case of water as the solvent, a simple but effective dryer can be made by using a bed of silica gel. Submicrometer-particle concentrations generated by this method range from 106 to 107cm"3. The size distributions have a nominal sg value near 2. Larger aerosol output volume can be obtained with multiple-nozzle generators.

The compressed-air pressure is controlled by a pressure regulator. A syringe pump can be used to regulate the liquid feed. The excess liquid is not recycled back to the liquid reservoir as in many other designs. Evaporation of the solvent occurs rapidly after atomization because dry air is normally used. This will concentrate the solution droplets produced, and recycling of the larger droplets back to the liquid reservoir will change the concentration of the solution and cause the particle size produced to increase with time. Improvements to the design of Liu and Lee have been made by Leong et al. [18]. The constriction at

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Aerosol Out Aerosol Out

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Figure 2 Schematic of an atomizer (TSI Model 3076).

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Figure 2 Schematic of an atomizer (TSI Model 3076).

the outlet of the atomizer was eliminated to prevent collection of droplets that may drip down. A low-pressure constant-liquid feed was used with a large reservoir for long-duration operation. Liquid feed control was achieved with a set pressure on the liquid reservoir and a flowmeter with a micrometer valve. Stable operation was obtained at a minimum liquid feed rate of 0.4 mL/min with very low liquid flow to the excess liquid reservoir.

The technique of nebulization of solutions to produce aerosols can also be used for suspensions. Insoluble or inert particles can be resuspended by nebulizing a suspension and by heating the aerosol to drive off the volatile liquid. A monodisperse aerosol of polystyrene particles can be generated with this method. To avoid having doublets or triplets of the primary particles, the suspension concentration should be less than that necessary to have less than one particle for the largest droplet produced. Otherwise, an impactor may be used to remove the larger particles. The nebulizers thus considered produce droplets of a few micrometers or less in diameter. Consequently, micrometer-size particles, such as bacteria and pollen, cannot be resuspended by this technique because they will be retained by the impaction plate in the nebulizer system. An atomizer capable of generating larger droplets is required. This is easily accomplished for the case of the atomizer shown in Fig. 2 by redesigning the atomizer so that the impaction wall is eliminated. If this is the case, the modal diameter of the droplets produced is approximately 10 mm at a pressure of 30 psig. The largest droplets are about 60 mm in diameter. The particle concentrations produced by atomizing suspensions can approach that of nebulizers for the case of submicrometer sizes, and the concentrations decrease substantially for larger particles because of the size distribution of the droplets produced. Higher concentrations of monodisperse particles in the micrometer-size range are more easily produced by other methods of generation, such as a fluidized-bed generator or a vibrating-orifice aerosol generator.

Ultrasonic nebulizers produce droplets in the micrometer-size range and are frequently used in aerosol drug therapy. Concentrations at 107 cm"3 are easily obtained for micrometer-size particles [19,20]. A simple ultrasonic nebulizer is shown in Fig. 3. Acoustic waves are generated in the liquid with a transducer. A coupling fluid is frequently used to prevent overheating of the transducer, which may occur when the liquid reservoir is empty. The frequency used is normally 100 kHz or higher. Disturbances on the surface of the liquid result, and droplets are formed. The airflow that is used can affect the size of the aerosol transported out of the nebulizer [5,20]. This occurs either through the decrease in coagulation of the droplets or the more efficient transport of larger droplets out of the nebulizer at higher flowrates [19,21].

Figure 3 A simple ultrasonic nebulizer.

The mechanism of droplet formation from ultrasonic excitation of the liquid depends on the frequency and intensity applied [21]. At an intensity of 10 W/cm2, capillary surface waves are formed from cavitation at frequencies less than 100 kHz. Cavitation can occur at lower-power intensities when dissolved gases are present. Droplets are thrown outward by the rupture of the cavitation bubbles or the wave crests. For such conditions, the droplet diameter can be related to the capillary wavelength [21,22].

W v where s is the surface tension, p[ is the density of the liquid, and f is the ultrasonic frequency. For a given intensity level, the amplitude of wave motion decreases at higher frequencies, and cavitation becomes more difficult. However, the acceleration on the liquid surface increases with the applied frequency. This force causes miniature fountains or geysers to be formed. At power intensities less than 30 W/cm2, droplets are thrown off intermittently by the geysers, at higher intensities, the droplets are produced continuously [23]. For this geyser mode of droplet formation, a reliable correlation of droplet size with applied power or frequency is not available, although many researchers have found that the droplet size tends to decrease with the frequency (see Ref. [21] and Ref. [22]). The previous equation for the cavitation mode probably can be used to estimate the droplet size, even though most commercially available ultrasonic nebulizers operate in the megahertz frequency range and droplets are produced by the geyser mode.

Although ultrasonic nebulizers produce substantially higher concentration of droplets than pneumatic-type nebulizers in the micrometer range, they have not found wide use in fundamental aerosol research. This is due largely to several inherent characteristics of the ultrasonic nebulizers. Given the modes of droplet formation and the physical phenomena, the parameters controlling droplet production have been shown to be the viscosity (static and dynamic), surface tension, and vapor pressure of the liquid [23,24]. Increased droplet production can be expected for liquids with high vapor pressure or low viscosity. The presence of dissolved gases will also contribute positively to droplet formation. Viscous liquids with low vapor pressure, such as oils, are difficult to nebulize. On the other hand, liquids with low surface tension, such as surfactants, are also difficult to nebulize, because of foaming. Most of the energy introduced into the liquid by the ultrasonic transducer goes into heating the liquid. Temperature increases of 10K have been observed for small commercial nebulizers [19]. For this temperature change, small changes in droplet size can be expected unless drastic change in the viscosity or surface tension occurs. In addition, increased vaporization of the liquid will occur. At high power levels, breakdown of organic molecules has been observed in addition to expected increases in temperature and vaporization rates (see Ref. [21]). For most aerosol studies, a low-volatility substance is preferred. Ultrasonic nebulization of a low-volatility liquid is difficult. The use of solutions avoids this difficulty, but, with time the increased vaporization of the solvent changes the concentration and, hence, the particle size generated.

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