Vibrating Orifice Monodisperse Aerosol Generator

The vibrating-orifice monodisperse method of aerosol generation is based on the principle of the disintegration of a jet of liquid issuing from an orifice that is

Figure 5 An electrospray aerosol generator (TSI Model 3480).

driven by a periodic vibration of the appropriate frequency and amplitude. The phenomenon of the breakup of a liquid jet into uniform droplets was studied by Plateau [31] and Rayleigh [32,33]. Castleman [34] observed that a jet can be made to disintegrate into uniform droplets on the application of a vibration of a suitable frequency and amplitude. This principle of uniform droplet production has been used by several groups of researchers to produce droplet generators [35-40]. Uniform droplets can be produced by vibrating the liquid, the container holding the liquid, or the orifice. The droplet size is primarily controlled by the size of the orifice. Droplet diameters produced are nominally twice the orifice diameter. The name vibrating orifice is commonly used because of the success of the commercial version of the generator developed by Berglund and Liu [40].

An illustration of the design of the vibrating-orifice generator head is shown Fig. 6. Liquid is fed into the generator head and forced out of the orifice.

Figure 6 A vibrating-orifice aerosol generator (TSI Model 3450).

A syringe pump is usually used, and the liquid is filtered through a 0.5-mm, 13-mm-diameter filter in the commercial version of the generator manufactured by TSI, Inc. A drain tube is available for flushing the cavity upstream of the orifice. The piezoelectric ceramic is excited by a sinusoidal signal from a function generator. The vibration produced is transmitted to the orifice and breaks up the liquid jet into uniform droplets. The monodispersity of the jet can be verified by deflecting the jet of droplets with a jet of air that is a simple version of an inertial spectrometer [39]. Multiple deflected streams indicate that the droplets are not monodisperse. The jet of droplets passes through an aperture where dispersion air flows to disperse the jet to prevent coagulation of the droplets. Dilution air is supplied through a perforated plate around the base of a generator head. A radioactive bipolar ion source is frequently used to neutralize the charged particles produced, to decrease particle loss to the wall of the transport tubing. If a solution is used for the liquid, the dilution air aids in evaporating the solvent. The aerosol is usually heated, to ensure dry particles are produced. The aerosol produced by this method is very monodisperse, with sg < 1.05. The deviation from monodispersity is caused by the presence of a few percent of doublets formed from the coagulation of two droplets. The vibrating-orifice aerosol generator possesses an inherent advantage over other aerosol generators, in that the particle diameter, d, can be computed directly from the ratio of the liquid feedrate, Q, to the frequency of vibration, f, applied if the particle density, pP, is the same as that of the bulk material used:

where c is the mass concentration of the solute per unit volume of the solution. For the case of a pure liquid, c has the value of 1. Particle diameters that can be produced range from 0.5 to 50 mm for solutions. The production of larger particles is not practical because of the difficulties in evaporation and transport, high particle losses, and low concentrations at large orifice sizes. Smaller particles can be produced if a very pure solvent is used. The particle concentrations obtainable using a 10-mm orifice are about 103 cm"3. The particle concentration decreases monotonically with the orifice size because the optimum exciting frequency also decreases. For a given orifice, a stable monodisperse stream of droplets may be obtained at more than one frequency of vibration. Schneider and Hendricks [36] determined experimentally that uniform droplets can be produced if the wavelength of the disturbance (applied signal) is between 3.5 and 7 times the diameter of the liquid jet, which is approximately the diameter of the orifice. The optimal wavelength for the disturbance to be most unstable is given by Rayleigh as 4.508d [32,33]. Hence, several monodisperse particle sizes can be obtained with a given orifice, particularly for larger orifices. It becomes more difficult to obtain a stable stream at frequencies other than the fundamental for orifice sizes less than 50 mm.

In practice, most researchers use a 20-mm orifice and obtain stable operation of the generator for longer than 5 hr. Difficulties are encountered for orifices 10 mm or less, which are preferred for the generation of higher concentrations. For these orifice sizes, pressures over 20psig are required to maintain a stable jet. Conventional syringe pumps are inadequate, and special pumps are required when syringe feed is used. Many users have resorted to the economical method of pressure feed of the liquid using a liquid reservoir and a compressed gas tank (e.g., see Ref. [41]). Large liquid reservoirs can easily be used, in contrast to the limited capacity of syringes, but the feedrate has to be determined by weighing the output of the generator over a time interval. This is not a serious disadvantage if the orifice does not plug and the feedrate remains stable for a fixed pressure. Another alternative liquid feed method superior to the syringe pump is a high-pressure metering pump, which is commonly used in liquid chromatography [42]. The length of operation is not limited by the large refill reservoir that can be used.

Plugging of small orifices has been encountered by many users, even though the liquid was filtered. Special techniques have been developed by several users to overcome plugging of orifices to obtain stable operation of the generator for orifice sizes of 10 mm or lower. A point-of-use filtration technique was developed by Kreyling and Erbe [43] in which a filter was inserted immediately upstream of the orifice. Stable operation for several hours was achieved for a 5-mm orifice. Leong [41,44] developed a pressure-feed method using a 0.2-mm capsule filter with a 500-cm2 filtration area and a flushing procedure to prevent plugging. Stable operation for up to 6hr was achieved for 10-mm orifices and a few hours for an 8.7-mm orifice. Examination of the orifices, after the jet became unstable, by microscopy showed no plugging, and the reason for the instability was unclear. Orifices became unusable after an extended period, even though they appeared round and were not plugged.

The vibrating-orifice aerosol generator is commonly used to produce micrometer-size monodisperse aerosols. When solid particles are required, a solution of the substance is used, and the solvent has to be evaporated. Particles formed from the evaporation of solution droplets are not necessarily spherical and solid, as has been observed for submicrometer-size particles generated by nebulizers. Charlesworth and Marshall [45] examined the morphology of particles derived from solution droplets and found that different particle shapes and densities can arise, depending on the mode of evaporation. Leong [46,47] studied in detail the morphology of particles produced by a vibrating-orifice aerosol generator. The particle morphology is determined by the chemical properties (particularly the solubility) of the compound used and the conditions controlling the evaporation of the solvent. The variation in morphology was more diverse for compounds of lower solubility. An aerosol was generated that was monodisperse in mass but nonuniform in shape. A bimodal aerosol was also generated where the primary particles were hollow, with a small hole, and the secondary (in mass) component was derived from the fragment that left the hole. For most compounds, monodispersity in both mass and shape was obtained.

Particles derived from the evaporation of solution droplets are spheroidal. Shape (primarily in surface features), density, and size control of particles can be achieved by the appropriate selection of the compound, the concentration of the solution, the size of the droplet generated, and the conditions for the evaporation of the droplets. Fast evaporation rates tend to produce less solid and rough-surface particles, but this is tempered by the chemical properties of the compound. Smooth, spherical particles call for compounds with high solubility and slow evaporation rates. These requirements were used by Vanderpool and Rubow [48] to produce solid, smooth spheres of up to 70 mm in diameter. The different types of particles that can be produced from the evaporation of solution droplets include solid spheres with surfaces that are smooth, cracked, or wrinkled; hollow spheres, shells, and spheroidal particles that have a wrinkled surface like raisins; porous-type particles that are perforated with holes, and single crystals and particles composed of several crystals, which may be angular or spheroidal in shape.

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