a generator for homogeneous

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A Generator for Homogeneous

Liquid AerosolsM. Suresh

a, R. A. Mackay

a& C. Acquista

b

a

Department of Chemistry, Drexel University, Philadelphia,PA, 19104b

Department of Physics, Drexel University, Philadelphia,

PA, 19104

Published online: 05 Jun 2007.

To cite this article: M. Suresh , R. A. Mackay & C. Acquista (1982): A Generator forHomogeneous Liquid Aerosols, Aerosol Science and Technology, 1:4, 441-447

To link to this article: http://dx.doi.org/10.1080/02786828208958607

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A Generator for Homogeneous Liquid AerosolsM . Suresh, R. A . M a c k a y ,Department of Chemistryand C . AcquistaDepartment of Ph ysics, Drexel University, Philadelphia, PA 19104

A generator of the evaporation-condensation type for the microscope. The generator is capable of maintaining aproduction of monod isperse aerosols has been designed stable output of aerosol over a long period of time. Theand tested. Dibutyl phthalate aerosols in the submicron effects of the flow rate of the carrier gas, the nucleirange and of narrow size distribution were generated concentration, the temperature of the furnace used forwith excellen t reproduc ibility when nucleated with NaCI. ma king the NaCl nuclei, and the heating temperature onThe particle size was determined from light scattering the aerosol output are reported.data using the polariza tion ratio metho d, and with a light

INTRODUCTION

Since the work of Sinclair and La Mer (1949), anumber of generators have been reported for thelaboratory preparation of relatively monodis-perse liquid aerosols with radii in the 0.1-1-pmrange. The generators function by the evapora-tion of a high-boiling point liquid, followed bycondensation on foreign nuclei (1966).A varietyof modifications designed to improve the sta-bility and reproducibility of the Sinclair-LaMer generator have been reported (Nicolaon etal., 1970; Nicolaon et al., 1971; Davis andNico laon, 1971). These modified generators useexternally produced salt nuclei and are quiteversatile in terms of size range and numberdensity. However, they d o require long warm-uptimes and rather large volumes of liquid, whichmust either be recirculated or wicked. This canbe inconvenient if corrosive or reactive fluids areinvolved. The Rapaport-W einstock generator(Rapaport and W einstock, 1955) employs anebulizer to generate a plydisperse aerosol,which subsequently enters an evaporation-condensation column. Impurities in the liquidwere used as nuclei. A similar generator wasrecently reported by M ulholland and Liu (1980).

Here, anthracene is added to a solution of thehigh-boiling-point liquid in a volatile solvent toact as a source of nuclei. A syringe and pum p isused to feed the so lution into the atomizer. Wehave designed a similar generator, which em-ploys externally produced salt nuclei, and reporthere a systematic study of its operating charac-teristics using dibutyl ph thalate (DB P) aerosols.

EXPERIMENT

Aerosol GeneratorA schematic of the appara tus is shown in F igure1. Helium carrier gas produces NaCl nuclei inthe furnace (B), and this gas stream is then usedto atomize the liquid in the nebulizer (E). Theconcentration of nuclei can be controlled byboth the flow rate and the length of tubingbetween the furnace and nebulizer. The syringepum p is adjusted to m aintain a constant level ofliquid in the nebulizer, thus providing a constantoutput rate. The total amount of liquid in thenebulizer is about 10 ml, and a syringe of anyconvenient size may be employed (e.g., 10-100ml). In this study, pure liquid DBP was em-

Aerosol Science andTechnology 1:441-l47 (1982)@ 1982Elsevier SciencePublishing Co., Inc.

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Suresh, Mackay and Acquista

FIGURE 1. Schem atic of aerosol generator, show inggas supply (A), furnace (B), boat containing salt (C),flow meter (D), nebulizer (E), syringe and pump (F),entrance chamber (G) , heating tape wrapped evap-orator (H), voltage control ( I ) , air-cooled chirnney.(J),water-cooled condenser (K), and aerosol outlet (L).

ployed. Typically, 10 ml of liquid will provideabout 2 hr of continuous operation. The poly-disperse aerosol entering the heated zone (H ) isvaporized , and pa rt of this vapor is recondensedon the salt nuclei in the air- and water-cooledchimney sections (J and K). The homogeneous(monod isperse) aerosol emerges from the outlet(L). If the furnace and evaporator are at theirrespective operating temperatures, the colors ofhigher-order Tyndall spectra (HOTS) are ob-served within a few minutes of turning on thecarrier gas supply (maintained at 20 psi gauge).The output is stable within 10 min.The entrance chamber (G) is 2 cm insidediameter (i.d.) and 14 cm long. The tu be from thenebulizer enters about midpoint. This tubeconnects by means of 24/40 standard taper tothe evaporator (H) and air-cooled chimney (J),which is a single tube of 1.5 cm i.d. an d 35 cmlong. A length of 25 cm wrapped with heatingtape is the evaporator section (H). The water-cooled condenser (K) is 20 cm long with a 1.2 cmi.d. and connects to (J) by m eans of a 24/40 joint.The ou tlet (L) is a standard taper tha t necksdown within 3 cm to a 0.5-cm i.d. tube fromwhich the aerosol emerges.

Light Scattering MeasurementsA Brice-Phoenix series 2000 light scatteringphotometer was employed for measuring thescattered light intensity. The aerosol was madeto flow through a cell similar to that describedby M atijevic et al. (1964).Th e only mod ificationwas the addition of a sheathing gas (nitrogen)flow of 2.1 literlmin around the inlet tube toimprove the collimation and stability of theaeroso l stream. Th e stan dard slit in front of thephotomultiplier was replaced by a circularaperture of 1mm diameter in order to m inimizethe solid angle subtended by the pho tomultip-lier. The interna l surfaces of the cell were paintedblack to reduce reflections and stray light.Signals corresponding to the intensities of thevertically and horizontally polarized com pon-ents of the scattered light were measured at 20intervals between 50" and 130". The 436- and546-nm lines of the mercury lamp were em-ployed for all of the measurements. Abackground correction, taken with no aerosolflowing, was applied to the scattering measure-ments. After a set of background and sampleintensities was obtained as a function of angle,the set was repeated to ensure constancy of theaerosol concentration and flow pattern. Nopolarization of the mercury light source wasdetected at any scattering angle.

Microscope MeasurementsThe aerosol was diluted by a factor of about 8500and allowed to impinge on a cleaned glassmicro-scope slide from a tube of 0.7 crn i.d The tube washeld perpendicular to the slide, and 2-3 cm aboveit. Th e slide was examined under a Reichert LightMicroscope Mod el number 324882 at a magni-fication of 1300x , a photograph of which isshown in Figure 2. The drops are somewhatdistorted from collision with the slide. However,measuring both the major and minor axis ofeach drop yields a mean radius (T) of 0.65 pm,which may be compared with a modal radius(rm)of 0.63 pm from the light scattering measure-ments. The contact angle of the DBP on the

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A G enerator for Homogeneous Liquid Aerosols

FIGURE 2. Dibutyl phthalate droplets on a glassslide (se e the text). The reference mark is 10p m .

slides employed was greater than 9CP, and themicroscope was focused to read the actual d ro pdiameter. Therefore, the measured radius isexpected to be slightly greater than the actualradius of the airborne aerosol drop (Liu et al.,1980).'Mass ConcentrationsTh e mass concentrations reported here were allobtained by allowing the aerosol stream to passthrough 0.5-pm Milipore filters, 1 in. in d iame-ter, in teflon ho lders. The aerosol was collectedfor 15 min, and the filter assemblies wereweighed on a five place mettler balance, modelH18. The nebulizer rate was ob tained by weigh-

' ~ n quation for microscopic drop size determination for contactangles (0)


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Suresh , Mackay and Acquista

function a, from absolute he ights and widths ofthe peaks.Th e intensity measurements were carried outat two different wavelengths, 0.5463 pm (green)and 0.4367 pm (blue). Polarization ratios weremeasu red for scattering angles SO'< O< 1300 forapproximately 50 different DBP aerosols, usingthe previously described generator under dif-ferent co nditions.T he mod al radius of each aerosol sample wasobtained by comparing the positions andheights of the m easured peaks a gainst monodis-perse (a, =0) calculations of the theoreticalratio. F or each aerosol sample both the blue andgreen measurem ents individually matched onlyone monodisperse radius, and this radius wasthe same for both wavelengths. The radius wasdetermined by m atching (quan titatively) theabsolute ang ular po sition of all peaks, as well a s(qualita tively) the relative peak heights. Asexpected, absolute peak heights and peak w idthsfor these monodisperse calculations differedsignificantly rom ou r observed values. Attem ptsto estimate the w idth a, of the actual polydisp er-sion were less successful. The theoretical poly-disperse ratio curve (computed by forming aweighted average over the m onodisperse I, andI, curves separately) was substantially differentfrom ou r observations. By varying a, and r , wewere unable to match both the widths andheights of the peaks in our polarization ratiomeasu rements. We attrib ute this diserepancy toseveral experimental problems, including scat-tering cell design, sensitivity, and angu lar resolu -

tion, and are currently redesigning the photo-meter to overcome these problems. We con-firmed our values of r , by measuring aerosoldroplets deposited on a glass slide under amicroscope. After m easuring ab out a hundreddroplets, we found that the corrected modalradius measu red microscopically agreed exactlywith the on e inferred from the polarization ra tiomeasurements [see the experimental methodssection and Liu et al. (1982)l.The mean radius Fis expected to be slightly larger than r , ap-proaching it as a, approaches zero.Estimates of the upper bound on a,, based onmicroscopic measurements and on observationsof the higher order Tyndall spectrum suggestthat a,


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A Generator for Homogeneous Liquid Aerosols

FIGURE 4. Polarization ratio p versus scatteringangle 0 for a DBP aerosol using X = 436 mm. Theopen circles are experimental points (right-handordinate) and the solid line is the theoretical curve foroo= 0.03 and r , = 0.35 1 pm left-hand ordinate).

about 2-3 cm above the heating tape. Most ofthe remaining 25% represented by this vapor islost by condensing on the walls of the coolingchimney, and only abou t 1% emerges as hom o-geneous aerosol. Th e evaporation temperaturedoes not apear to be too im portant as long as itis above some critical value. For example, at aflow rate and furnace temperature of 3.3literlmin and 59032, respectively, essentially th esame polarization ratio curves were obtained forevaporator temperatures of 100,125, and 150C.However, at 75OC, appreciable polydispersitywas evident.A number of experiments were carried outunder a fixed set of conditio ns in order t o test thestability and reproducib ility of the generator. Bystability is mean t the uniformity of the p articleradius a s measured o ver a given period o f steadyopera tion of the generator, usually from severalhou rs u p to a day. Th e stability is illustrated bythe results in Table 1, obtain ed over a period of 3hr. These have been selected from am ong scoresof similar results as being typical of the perform -.ance of the generator. T he mean radius is 0.628pm with a relative standard deviation of 0.4%.By reproducibility is meant uniformity of par-ticle size under the sam e operatin g conditions ondifferent days or on the same day, with anintervening period when the generator waseither shut down or operated under differentconditions. The data in Table 2, taken over afour-month period, have a mean of 0.628 pmand a relative standard deviation of 1.5%.

4 0 60 8 0 100 120 1400.5

ANGLE IN DEGREE (8)

TABLE 1. Aerosol Generator Stability Undercontinuous Operation.Run Time (rnin) Radius (prn)

OThe furnace temperature was 5 9 W , evaporator temperature125C. and He flo w rate 3.3 literlmin. The radii were determined fmmlight scattering measurernens by the polarization ratio method at twowavelengths.

TABLE 2. Aerosol Generator Reproducibi1it)PRun Time Radius ( p m )5 30 min 0.6209 3 h 0.630

16 6 h 0.61525 1 0 h 0.63526 0.65034 2 days 0.61935 0.62848 8 days 0.63849 0.63060 20 days 0.64061 0.63072 2 months 0.61073 0.61985 4 months 0.63486 0.628

0 Same conditions as in Table I .

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446 Sure sh, Mackay and Acquista

The effect of flow rate on radius numberconcentration is shown in Figures 5 and 6. Theradius decreases with inc reasii~g ven tempera-ture and increasing flow rate, owing to anincrease in nuclei concen tration and possibly abit less mass transfer in the evap ora tor an d a bitmore condensation loss in the chimney. Thistype of behavior has been observed in othergenerators (Kerker, 1972). Since the numberFIGURE 5. Radius as a function of flow rate atvarious furnace temperatures; 590C (open, 725C(closed), and 790C (half-filled).

FLOW RATE (I/min)

FIGURE 6. Number concentration as a function offlow rate at various furnace temp erature; 590C (open,725C (closed), and 790C (half-filled).

concentration increases linearly with flow rateand the mass concentration does not varygreatly (1.5-2.5 x glliter), the nuclei con-centration is roughly proportional to the flowrate.That the particle radius in this generator doesin fact depend on the external nuclei concentra-tion is shown by the data in Table 3. Varyinglengths of Tygon tubing were inserted betweenthe furnace and the nebulizer to remove thenuclei by diffusion to the wall thus decreasingtheir concentration. As expected, the dropletradius increases.

It should be mentioned that we have alsosuccessfully generated monodisperse aerosols ofsulfuric acid and chlorosulfonic acid in thisgenerator. All the components were of glass, andthe only change made was the replacement ofTygon with flexible Teflon tubing.

SUMMARYWe have tested a liquid aerosol generator of theevaporation-condensation type. This generatoris easy to construct, rugged, and fairly inexpens-ive. Hom ogeneous aeroso ls of DBP in the 0.35-0.65-pm radius range at number densities of 3-8 x lo5cm -3 have been generated. This outputTABLE 3. Effect of Decreasing Nuclei Concentra-(Increasing T ubing Length) on P article Radiusa

- -Radius of particle5 Run Length (rn) (m)- 1 0 . 0 - 120 2.00 0.504z 121 2.00 0.500

0 122 2.00 0.506123 2.00 0.501

ZW 130 7.50 0.547131 7.50 0.545132 7.50 0.540135 15.00 0.630136 15.00 0.636137 15.00 0.628140 30.00 0.704141 30.00 0.710142 30.00 0.707

2.0 4.0 6.0 8.0 10.0 "The furnace temperature was 725C. evaporator temperatureFLOW RATE (I/min) 125C. and the tlowrate 3.3 literlmin.

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A Genera to r fo r Hom ogeneou s L iqu id Aeroso ls 447

is both stable and reproducible, and the genera-tor can be used with corrosive or reactiveliquids.

The authors wish to acknow ledge the support of the Chem-ical System Laboratory, Aberdeen Proving Ground, undercontract DAAK 11-80-C-0051.

REFERENCESDavis, E. J., and Nicolaon, G . (1971). J. ColloidInterfaceSci. 37:76&-778.Fuchs. N. A ., and Sutugin, A . G. ( 1966).Aerosol Science(C . N. Davis, ed. ), New York, Ch. 1, p. I.Kerker, M. (1969). The Scattering of Light and OtherElectromagneticRadiation, Acade mic Press, New York.Kerker, M. (1972). J . ColloidInt . Sci . , 39:2-24.

Liu, B. Y. H., Pui, D. Y. H., and Wang, Xran-Quing.(1982). Atmospheric Environment 16:563-567.Matijevic, E., Kitani, S . , and Kerker, M . (1964). J . ColloidInterface Sci. l9:223-237.Mulholland, G. W ., and Liu, B. Y. H. (1980). J . Research

N . B . S . 85:223-238.G. Nicolaon et al. (1970). J . Col loid Interface Sci. 34:534-544.G . Nicolaon et al. (1971). J. Colloid InterfaceSci. 35:490-501.Rapaport, E., and Weinstock, S. E. (1955). Experientia.11:363-364.Shahrian, S., Samiento, A. N., a ndG mdr ich, F. C. (1972).J . Colloid Interface Sci. 39:305-377.Sinclair, D., and LaMer, V . K. (1949). Chem. Rev. (44:245-267.Van de Hulst, H. C. (1957). Light Scattering by SmallParticles, Wiley, New York.Received 1 January 1982; accepted 12 July 1982

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a generator for homogeneous

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