A New Monodisperse Droplet Generator and its Applications

Monodisperse droplet standards are valuable for calibrating spray diagnostics instrumentation and for investigating the effect of drop diameter on the spray phenomena, such as droplet combustion and droplet-film interaction. There is a growing interest in finer monodisperse droplets. To address this need, a new instrument for generating a wide range of monodisperse droplets is introduced, which is an advancement of the technology underlying Vibrating Orifice Aerosol Generator or VOAG. In a VOAG, monodisperse droplets are generated by using uniform ultrasonic perturbations for breaking a liquid jet, which is created by forcing the liquid out of a small orifice. This approach is suitable for generating relatively large drops (e.g. greater than 50 micron), but difficult for producing finer droplets, as smaller orifices are easily clogged or damaged. This problem is solved in a newly-developed instrument, referred to as Flow-focusing Aerosol Generator (FMAG), in which liquid is released from a nozzle with a large internal diameter (100 micron in the present realization). Liquid jet is subsequently attenuated using acceleration of the surrounding co-flowing air. A final jet diameter as small as 8 micron is realized. As in VOAG, ultrasonic perturbations are used in FMAG to break up the attenuated liquid jet into uniformly-sized droplets. In order to demonstrate the use of FMAG as a calibration standard, response curves of a phase Doppler interferometer using 15 to 65 micron monodisperse droplets are presented. Also, results of our preliminary experiments on spraying biological material are included. Activity of lipase enzyme after nebulization with FMAG and a conventional nebulizer was measured. Activity of the sample sprayed by FMAG was about twice that of the conventional nebulizer. Enzyme activity of FMAG-sprayed sample dropped only by 6% when ultrasonic perturbation was turned on. FMAG is shown to be promising for spraying liquids containing large and fragile biological molecules. * Corresponding author: [email protected] Introduction Monodisperse droplets of known diameters are extensively used for calibration of droplet sizing instruments and for research studies investigating the role of drop size in a variety of processes, such as liquid fuel combustion, spray coating and spray drying. Monodisperse droplet generator developed by Berglund and Liu [1], commonly known as Vibrating Orifice Aerosol Generator (VOAG), has been a main research tool for this purpose for over 40 years. In a VOAG, a liquid jet is formed by forcing the liquid out of an orifice, while ultrasonic perturbation is used to break up the jet into uniform droplets. Drop diameter is typically twice the orifice diameter. This approach has been highly successful for generating relatively large drops (e.g. greater than 50 micron), but it is difficult to implement for generating fine droplets (such as droplets smaller than 25 micron), as smaller orifices are easily clogged or otherwise damaged due to the high liquid pressure needed to maintain the liquid flow. This problem is solved in the present device, referred to as Flow-focusing Aerosol Generator (FMAG), which does not require fine orifices for generating fine liquid jets [2,3]. Instead, FMAG utilizes flow-focusing to reduce the diameter of a relatively large liquid jet to the desired size, as illustrated in Figure 1. Liquid is released from a nozzle with a fairly large internal diameter (100 micron in the present realization). It is subsequently attenuated using flow-focusing air (‘FF Air’ in Figure 1). Using this approach, a final jet diameter as small as 8 micron is realized. Figure 1. Liquid jet attenuation mechanism of FMAG. The attenuated jet in FMAG is broken into uniform droplets using ultrasonic perturbation in a manner similar to VOAG. Use of FF Air has two main advantages: (1) FMAG generates fine droplets without using narrow orifices that are prone to damage and clogging, (2) Unlike VOAG, FMAG does not require replacing orifices to cover a wide droplet size range. As elaborated in the next section, three operating parameters are adjusted to generate a wide range of monodisperse droplets—without any change in the hardware configuration. Current version of FMAG generates monodisperse droplet diameters of 15 to 150 microns using a single nozzle. FMAG System Complete FMAG system is shown schematically in Figure 2. A syringe pump is used to supply liquid to the droplet generator. Ultrasonic perturbations are induced by a piezoelectric crystal, whose frequency is controlled from the front panel of the instrument. The instrument has a compressed air inlet, where pressure is maintained at 15 psi. An electronic pressure controller is used to control the pressure of FF Air, which is typically less than 2 psi. Figure 2. Schematic layout of FMAG. Figure 2 shows certain additional components in gray for generating solid aerosols starting with a liquid solution of the solid material. These components allow supply of dilution air at the rate of 5-25 liters/minute, which disperses and dries the droplets. The solid particles generated in this way usually carry an electric charge, which may result in electrostatic precipitation of the particles. FMAG allows neutralization of the particles using a bipolar corona source [4]. Bipolar corona is preferred over a radioactive source for aerosol neutralization, as the latter requires additional measures to meet safety and regulatory requirements. Operation of FMAG In order to generate monodisperse droplets using FMAG, user needs to set three parameters that are highlighted in Tables 1 and 2. These tables pertain to the experimentally determined operating parameters for methanol and ultrapure water. Attenuation of the liquid jet is determined by the combination of the liquid flow rate (Q) and the pressure of flow-focusing air (FF Δp). Strongest attenuation is obtained with a low value of Q and high value of FF Δp. Attenuation also depends on the liquid properties, especially the surface tension. Higher FF Δp is needed to attenuate the jet of a liquid with a higher surface tension. Q (ml/h) FF Δp (psi) Dj (μm) ƒ (kHz) Dd (μm) 1.5 0.9-1.0 8.6 120-230 15-20 2 0.9-1.0 10.0 110-140 20-23 4 0.9-1.0 14.1 110-130 25-28 8 0.9-1.0 19.9 80-125 33-38 12 0.7-0.8 26.2 45-70 45-52 18 0.7-0.8 32.1 40-65 53-62 24 0.7-0.8 37.1 35-66 58-72 30 0.6-0.7 45.4 20-40 73-92 40 0.6-0.7 52.4 20-35 85-102 50 0.6-0.7 58.6 20-30 91-110 60 0.6-0.7 64.1 20-30 102-116 70 0.6-0.7 69.3 10-25 114-150 Table 1. Operational parameters for methanol Q (ml/h) FF Δp (psi) Dj (μm) ƒ (kHz) Dd (μm) 2 1.8-1.9 9.2 120-200 17-21 4 1.8-1.9 13.1 70-170 23-31 8 1.8-1.9 18.5 70-160 30-40 12 1.4-1.5 24.1 50-95 41-50 18 1.4-1.5 29.5 50-80 51-65 24 1.4-1.5 34.1 40-75 56-69 30 1.1-1.2 39.6 25-50 68-86 40 1.1-1.2 45.8 25-45 77-94 50 1.1-1.2 51.2 20-40 87-109 60 1.1-1.2 56.1 15-40 92-128 70 1.1-1.2 60.6 10-35 102-150 Table 2. Operational parameters for ultrapure water Tables 1 & 2 show the experimentally determined values of the final jet diameter Dj before the jet breaks down into droplets. These values are verified by dry particle measurements as well as direct photographic recording [2]. The jet diameter determines the liquid velocity ( ) and hence, the distance traversed in each period (1/f) of ultrasonic perturbation. For generating monodisperse droplets, this distance should generally be within the wavelength range of jet hydrodynamic instability,