A New Monodisperse Droplet Generator and its Applications

Transcription

1 ILASS Americas 28th Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May 2016 A New Monodisperse Droplet Generator and its Applications H. Duan, F. Romay, Z. Syedain, B. Y. H. Liu and A. Naqwi * MSP Corporation 5910 Rice Creek Parkway, Suite 300 Shoreview, MN 55126, USA Abstract 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: anaqwi@mspcorp.com

2 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

3 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 FF Δp D j (ml/h) (psi) (µm) ƒ (khz) D d (µm) Table 1. Operational parameters for methanol Q FF Δp D j (ml/h) (psi) (µm) ƒ (khz) D d (µm) Table 2. Operational parameters for ultrapure water Tables 1 & 2 show the experimentally determined values of the final jet diameter D j 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, given by [5], i.e. (1) Ultrasonic frequencies f in Tables 1 & 2 correspond to this range. Once the condition of monodisperse jet breakup is established, calculation of droplet diameter is straightforward. Volume of each droplet corresponds to the liquid volume released in each cycle of the ultrasonic frequency, i.e.. (2) Hence, the user of FMAG can determine the droplet diameter from the known liquid flow rate and the ultrasonic frequency, as long as the operating conditions enable monodisperse breakup. Tables 1 and 2 provide sets of operating conditions to cover 10:1 size range for methanol and water. The authors are preparing similar operating parameter tables for other solvents and liquids. Operating conditions for generating monodisperse droplets can also be determined by visual examination of the liquid jet (a magnifier is provided in FMAG for this purpose), as monodisperse sprays have minimal dispersion. Further, a well-calibrated droplet sizing instrument can be used to establish monodisperse operating conditions of FMAG for a new liquid. Experimental Results: PDI Calibration Phase Doppler Interferometer (PDI), also known as Phase Doppler Particle Analyzer (PDPA) or Phase Doppler Anemometer (PDA), is an advanced laser instrument for simultaneous measurement of size and velocity of spherical particles, especially, spray droplets [6,7]. Transfer function of phase Doppler depends on the wavelength of the laser, geometry of the system, light scattering mode and particle refractive index. These parameters are often known with high precision, so the instrument does not need calibration. However, situations may arise when a calibration may be desirable. These include uncertainty about the optical properties of the particles or the inner geometry of the PDI receiver. FMAG allows for the first time to cover a wide range (10:1) of monodisperse drop diameters without changing orifices in order to measure the PDI transfer function. Figure 3 shows results of recent measurements of phase shift versus droplet diameter for a PDI system (Artium Technologies, Inc.) using monodisperse water droplets (16 63 µm in diameter) generated by a single nozzle of FMAG. To cover the same size range, VOAG requires changing orifices several times. Geometric standard deviation of the droplets in Figure 3 was 1.01 for diameters greater than 40 µm, and <1.035 for all the droplet diameters. Measurements were taken with water droplets at off-axis angles of 30 and 60 degrees. Under these conditions, the transfer function of PDI is indeed linear and agrees with the theoretical

4 estimate based on refraction mechanism of scattering within 4%. Figure 3. Phase shift versus droplet diameter relationship of a PDI system. Experimental Results: Biological Spray Large biological molecules, as well as cells and microorganisms often need to be sprayed in order to study their behavior in aerosol form or to formulate them as powders. Conventional spraying processes involving strong mechanical shear or impact can damage the fragile structure of the biological material. Flow focusing, leading to liquid breakup, is shown to cause minimal damage to E-Coli bacteria [8]. FMAG is valuable for laboratory studies in which microorganisms need to be aerosolized, e.g. rodent inhalation studies involving airborne pathogens. Another area of interest is pre-formulation studies of spray drying processes for enzymes and probiotics in food and pharmaceutical industries. One of the methods in current practice involves droplet generation using a device based on ink-jet principle (or VOAG principle), which typically results in 200 µm drops that are allowed to free-fall in a 30 m tall column in order to simulate the spray-drying process [9]. FMAG enables a much more compact setup for such pre-formulation studies. One of the enzymes of interest for such studies is lipase [9], used extensively in food industry. Preliminary results of lipase activity after spraying with FMAG to generate 15 µm drops are given in Figure 4. For this study, lipase enzyme (Sigma Cat# L3126) was dissolved in water at 5 mg/ml concentration ( units/mg). The liquid was filtered with 0.45 µm filter and injected into FMAG at 2 ml/hr flow rate. Aerosols were collected for 10 minutes. For FMAG, two settings were used: one with ultrasonic frequency transducer turned on and the other with the transducer turned off. Enzyme solution was also aerosolized using a conventional nebulizer Aeroneb and collected for analysis. All 3 samples were analyzed using lipase enzyme activity kit (Sigma Cat# MAK046). The lipase activity was measured in units of enzyme generating 1.0 µmoles of glycerol from triglycerides per minute. Figure 4 shows enzyme activity for 3 tests performed. The enzyme activity of FMAG-sprayed liquid with frequency transducer on was 97% higher than Aeroneb. With the transducer off, it was even higher at 110% of Aeroneb. The ultrasonic perturbation lowered the enzyme activity only by 6%. In this preliminary test, there was not adequate amount of liquid available to accurately measure the activity prior to spray. Nonetheless, enzyme activity appears to have dropped significantly by the conventional nebulizer and may have not be affected much by FMAG. Enzyme Activity ( mole Glycerol breakdown/min) Transducer on off Liq Aeroneb FMAG Figure 4. Comparison of enzyme activity after nebulization with FMAG and a conventional nebulizer Conclusions A new monodisperse droplet generator, FMAG, is introduced for producing droplets of µm in diameter. Droplet generation is accomplished by creating a 100 µm diameter liquid jet, which is attenuated to as small as 8 µm diameter liquid filament using co-flowing air (also called flowfocusing air). Finally, liquid jet is broken into monodisperse droplets using ultrasonic perturbations. FMAG is valuable in calibrating droplet size measuring instrumentation as well as conducting laboratory studies requiring known droplet standards. Further, this droplet generator has a gentle mechanism of breaking up liquid into droplets and is suitable for studies involving large and fragile molecules and biological materials.

5 References 1. Berglund, R.N., and Liu B.Y.H. Generation of monodisperse aerosol standards. Environmental Science & Technology 7.2: (1973). 2. Duan, H., Romay, F.J., Li, C., Naqwi, A., Deng, W., and Liu, B.Y. Generation of Monodisperse Aerosols by Combining Aerodynamic Flow- Focusing and Mechanical Perturbation. Aerosol Science and Technology, 50:1-9 (2016). 3. Duan, H., Romay, F.J., Naqwi, A.A. and Liu, B.Y.H., Method and apparatus for generating monodisperse aerosols, US Patent Application # US 14/699,451 (2015). 4. Romay, F.J., Liu, B.Y.H. and Pui, D.Y.H., A sonic jet corona ionizer for electrostatic discharge and aerosol neutralization, Aerosol Science and Technology, 20:31-41 (1994). 5. Schneider, J.M., and Hendricks C.D. Source of Uniform-Sized Liquid Droplets, Rev. Sci. Instrum. 35: (1964). 6. Bachalo, W.D. and Houser, M.J., Phase/Doppler spray analyzer for simultaneous measurements of drop size and velocity distributions, Optical Engineering 23(5): (1984). 7. Buchhave, K. and Floegel, H.H., Simultaneous measurement of droplet size and velocity in nozzle sprays, Proc. Intl. Symposium on Applications of Laser Anemometry to Fluid Mechanics, 2nd, Lisbon, Portugal, July 2-5 (1984). 8. Thomas, R.J., Webber, D., Sellors, W. et al. Characterization and Deposition of Respirable Large- and Small-Particle Bioaerosols, Applied & Environ. Microbiology, 74: (2008). 9. Schutyser, M.A.I., Perdana, J., Boom, R.M., Single droplet for optimal spray drying of enzymes and probiotics, Trends in Food Sci. & Tech. 27:73-82 (2012).