Colloid Polym Sci (2007) 285:847-854DOI 10.1007/s00396-006-1625-1

ORIGINAL CONTRIBUTION

Synthesis of monodisperse polymer microspheresby photopolymerization of microdroplets

Zhiqiang Gao · Eric A. Grulke · Asit K. Ray

Received: 15 August 2006 / Accepted: 19 November 2006 / Published online: 9 March 2007© Springer-Verlag 2007

Abstract We have examined photopolymerization ofhighly monodisperse microdroplets of monomer so-lutions under UV-light radiation. Microdroplets weregenerated using a modified vibrating aerosol genera-tor, and the diameter of the droplets can be tunedto any size between 5 to 100 μm. Polymer particlesderived from the droplets were characterized by op-tical microscopy and SEM. The results show that thepolymer particles, under optimum conditions, can behighly spherical and monodisperse. The diameter andmorphology of resulting microspheres depend on thediameter of the monomer solution droplets, monomerconcentration, photopolymerization reaction tempera-ture, residence time, and droplet dispersion.

Keywords Photopolymerization ·Polymer microspheres ·Vibrating orifice aerosol generator

Introduction

In recent years, polymer microspheres are finding everincreasing applications in various fields, such as drugdelivery systems, biotechnology, optics, electronics andchemical technology. Several applications in emerg-ing technologies, such as controlled release drug and

Z. Gao · E. A. Grulke · A. K. Ray (B)Department of Chemical and Materials Engineering,University of Kentucky, Lexington, KY 40506-0045, USAe-mail: akray@engr.uky.edu

optical microsensors, require highly monodisperse mi-crospheres with very narrow size distributions. Polymermicrospheres can be produced by a number of tech-niques. Emulsion based polymerization methods [1–7]can produce monodisperse particles with moderatelynarrow size distributions, but have some inherent draw-backs. The final product may include some undesirablecomponents, such as surfactants or emulsifiers, andthe size of the particles cannot be tuned arbitrarily.Polymer particles of various sizes have been preparedby contacting polymer solution droplets with an antisol-vent that is completely miscible with the solvent for thepolymer but immiscible with the polymer [8–10]. Useof dense gases (e.g., CO2) as antisolvents yields parti-cles that are free of residues. This technique, however,produces polydisperse particles of irregular shapes. Thesedimentation polymerization technique [11–14], basedon partial polymerization of individual monomer so-lution droplets as they settle through an immiscibleliquid, has been successfully used to generate monodis-perse spherical particles of size greater than 1 mm. Re-cently, Berkland et al. [15] have developed a techniquebased on the spraying of monodisperse polymer solu-tion droplets into a solution where the droplets weresolidified by extraction and evaporation. The monodis-perse polymer solution droplets of controllable sizewere produced using a vibrating needle or an orificethrough which the solution was pumped, and the size ofthe droplets controls the final size of microspheres. Thistechnique can produce relatively monodisperse poly-mer microspheres ranging from a few to a few hundredmicrometers. The disadvantage of the technique is thatthe droplets transform very slowly to solid particles in aliquid phase where they must be dispersed, and finally,centrifuged and dried to obtain solid particles.

848 Colloid Polym Sci (2007) 285:847-854

Among the solution based particle generation tech-niques precipitation polymerization is the most ver-satile method for producing nearly monodispersesurfactant-free spherical particles with narrow size dis-tributions [16–23]. The particle size can be tuned fromsubmicron to a few micrometers by controlling theinitial composition of the reaction mixture and the poly-merization conditions. The precipitation polymeriza-tion technique, however, has a number of limitations.The largest size of the particles obtainable throughthis method is about 5 μm. In addition, low monomerconcentrations must be used to obtain monodisperse,unagglomerated particles, and only a fraction of thepolymerized species transforms to solid particles. Thesefactors result in a final product mixture that contains adilute concentration of particles. As with other solutionbased methods, after polymerization particles must beseparated by centrifugation or filtration, washed andthen dried under vacuum.

Polymer particles can also be produced throughaerosol based methods that involve transformation ofmonomer or polymer containing solution droplets tosolid particles. These methods avoid the use of sur-factants, and do not involve separation and drying ofparticles from liquid suspensions. The simplest aerosolbased method is the spray drying technique, wheredroplets generated from a solution containing poly-mer dissolved in a volatile solvent are dispersed ina gas, and evaporation of solvent from the dropletsleads to the formation of solid particles. The spraydrying method usually produces particles with broadsize distributions [24, 25], and the particles may notbe spherical. Monodisperse solid particles of variousmorphologies have also been generated by reactions ofdroplets dispersed in a gas with coreactants [26–31].

The photo or radiation polymerization is one ofthe viable methods for manufacturing polymer mi-crospheres. The radiation polymerization has been suc-cessfully used in liquid phase dispersions to producemicrospheres [32, 33]. The studies [34–37] on photoin-duced polymerization of single suspended monomerdroplets have demonstrated that the photopolymer-ization technique can be applied to aerosol systems.Esen et al. [38, 39] were the first to show thatmonodisperse polymer microspheres can be producedby photopolymerization of monodisperse monomer so-lution droplets. In their method, monodisperse solutiondroplets produced by a vibrating aerosol generatorwere dispersed in a nitrogen atmosphere and allowed toevaporate. Finally, these droplets were transformed tosolid spheres by photopolymerization induced by expo-sure to UV light. Their results show that monodispersepolymer microspheres of diameters ranging from 5 to

50 μm with a coefficient of variation (CV) of about 1%can be produced by the method.

In this paper, we examine photopolymerization ofmonodisperse monomer solution droplets, and the fac-tors that control the morphology of the final poly-mer particles. Highly monodisperse droplets containingmonomer dissolved in a volatile solvent were generatedby a modified vibrating orifice aerosol generator. Thepolymerization reaction in the droplets was initiatedby exposure to UV radiation. The droplet generatorpermits tuning of the droplet size to within 0.01% of thedesired value in the size range 5–100 μm. The effectsof the droplet size, monomer concentration, dispersionand reaction temperature on the final particles wereexamined.

Experimental

Materials

SOMOS 10220, commonly used in laser stereolithog-raphy for building three-dimensional parts, was pur-chased from DSM Desotech Inc., USA. It is a mixtureof multiacrylate monomers 45–50%, epoxies 15–25%,polyols 20–35%, and photoinitiators 0.2–5%. As sol-vents for SOMOS 10220, we have used ethanol that wasobtained from Aaper Alcohol and Chemical Co. Allchemicals were used as is, without further purification.

Preparation of microspheres by photopolymerization

A schematic diagram of the experimental system usedin this study is shown in Fig. 1. The system consistsof a droplet generator, a cylindrical quartz reactor of2.2 m length, irradiation sources and heating tapes. Asolution of monomer (i.e., SOMOS 10220) dissolvedin a highly volatile solvent (i.e., ethanol) was used toproduce highly monodisperse droplets through a mod-ified vibrating orifice aerosol generator (VOAG). Thedetails of the VOAG and its operation can be foundelsewhere [40–42]. Briefly, the solution in a reservoiris driven through a vibrating orifice under a constantgas pressure from a ballast tank, and the resulting cylin-drical liquid jet passing through the orifice breaks intomonodisperse droplets by the vibration of the orifice.The droplet diameter for a solution stream flowing at avolumetric rate of Q, through the orifice vibrating at afrequency f , is given by

Dd =(

6Qπ f

)1/3

(1)

Colloid Polym Sci (2007) 285:847-854 849

Fig. 1 Schematic of experimental system

The orifice is driven by a square wave with a frequencystability on the order of 1 part in 106, and the pressuredifference driving the solution through the orifice iskept within 1 part in 105. The VOAG system is highlystable; we have shown that the short term fluctuation inthe droplet size is about 1 part in 105, and the long termdrift in the size is about of 0.01 nm/min [41]. Equation 1shows that the droplet size depends on the solution flowrate and the frequency. The solution flow rate throughthe orifice depends on the gas pressure in the ballasttank and the orifice diameter, and for a given flow ratethe droplets can be tuned in size over a range of about20% by varying the orifice vibration frequency. Usingan appropriate combination of the orifice diameter,the ballast tank gas pressure, and the orifice vibrationfrequency highly monodisperse droplets of any specificsize can be produced in the diameter range 5–100 μm[40, 43].

Droplets for this study were generated from a so-lution of SOMOS 10220 dissolved in ethanol. Thedroplets were dispersed by nitrogen gas before intro-duction into the reaction chamber, and allowed to evap-orate before exposure to UV radiation that is producedby eight 48′′ long backlight fluorescent strip lamps.Each lamp provided 40 W of UV light at wavelengthλ = 355 nm. To prolong the residence time of thedroplets and to enhance solvent evaporation dry nitro-gen gas was allowed to flow countercurrently past thedroplets. The reaction chamber was heated to raise theactivity of radicals and the monomer, and the temper-

ature was controlled by heating tapes wrapped aroundthe wall of the chamber. The particles emerging out ofthe reaction chamber were deposited on a collector.

Particle characterization

The collected particles were characterized by the opti-cal microscopy and the scanning electron microscopy(SEM-Hitachi S-3200). The particles were examinedunder an optical microscope and sized with a calibratedstaged micrometer. For SEM, the particles were placedon an aluminum sample holder and coated under vac-uum with a thin layer of gold. More than 1,000 particleswere measured and counted for the size distributiondetermination from each of the optical and SEM micro-graphs. The number mean diameter and the coefficientof variation were calculated from the size of the par-ticles, and ImageJ program was used to determine thesize of microspheres from their SEM images.

Results and discussion

We carried out photopolymerization experiments un-der various conditions. We varied the monomer so-lution flow rate through the orifice by varying thepressure in the ballast tank, the vibration frequencyof the orifice, the orifice diameter, the monomer con-centration and the reaction temperature. Table 1 liststhe experimental parameters for five different experi-

850 Colloid Polym Sci (2007) 285:847-854

Table 1 Summary of typicalexperimental conditions andresults obtained from fiveseparate experiments

Experiment # 1 2 3 4 5

Orifice dia. (μm) 15 20 15 20 15Orifice frequency (kHz) 170 116 175 150 162Dispersion gas flow rate (ml/s) 50 35 27 25 33Reaction chamber temp. (◦C) 65 60 60 91 55Solution droplet dia., Dd (μm) 29.56 43.50 29.42 42.03 31.49SOMOS vol. fraction, vm 0.12 0.12 0.10 0.10 0.12Mean residence time (s) 186 128 205 108 164Polymer particle dia., Dp (μm) 14.7 21.9 13.8 - 16.0Micrograph Fig. no. 2a 2b 3 & 4 5a & 5b 5c & 5d

ments. The solution flow rate and the frequency dic-tate the diameter of droplets as given by Eq. 1, andTable 1 includes droplet diameters calculated fromEq. 1. The optical micrographs of polymer particlesproduced from Experiment # 1 and Experiment # 2 areshown in Fig. 2a and b. The polymer particles resultingfrom the photopolymerization of monomer dropletsin these experiments are spherical and monodisperse.Droplets in Experiment #2 were larger than Experi-ment # 1, but were generated from solutions containingthe same concentration of monomer. Table 1 showsthat the ratio of the diameters of the final polymerparticles from these two experiments is nearly the sameas the ratio of the initial diameters of the monomersolution droplets. Particles in Experiment # 3 wereproduced under the same experimental conditions asin Experiment #1, except for the monomer concen-tration. Use of the lower monomer concentration inthe droplets of the former experiment results in theproduction of smaller size polymer particles. Figure 3shows an optical micrograph of microspheres producedfrom Experiment #3 that were placed on a glass slide.The micrograph confirms that the microspheres aretransparent as reflected by the presence of opalescence.Figure 4 shows SEM micrographs of particles prepared

under the same experimental conditions. The sphericityand monodispersity of the polymer particles are evidentin the micrographs. In addition, the enlarged micro-graph of a particle in Fig. 4b shows that the surfaceof the particle is smooth. It should be noted that someof the particles in the micrograph in Fig. 4a appear asagglomerates, that is, two or more spherical particlesadhering to each other during the solidification process.The percentage of agglomerates depends on the dis-persion, and for dispersion gas flow rates used in theseexperiments we observed less than 5% agglomerates.

The size of microspheres is dictated by the size of theinitial monomer solution droplets, the concentration ofmonomer, the solvent evaporation rate and the poly-merization kinetics in the droplet. Another factor thatplays an important role on the size and the morphologyof the final solid particles is the reaction temperature.Higher temperatures lead to higher solvent evapora-tion rates as well as higher polymerization reactionrates. High temperature environments, however, causerapid shrinkage of a droplet due to solvent evaporation,and at the same time rapid solidification of the regionnear the surface by photopolymerization induces struc-tural stress in the particle that can result in the fractureof the particle. Figure 5a and b show examples of

Fig. 2 Optical micrographsof polymer particles preparedby photo-polymerization. (a)particles produced under theconditions of Experiment # 1listed in Table 1, (b) underthe conditions of Experiment# 2 listed in Table 1

Colloid Polym Sci (2007) 285:847-854 851

Fig. 3 Optical micrograph of polymer particles produced underthe conditions of Experiment # 3 listed in Table 1. The particleswere placed on a glass

fractured particles produced at a temperature of 91◦Cunder conditions of Experiment # 4 in Table 1. Differ-ent complexities, however, arise at lower temperatures.Figure 5c and d show particles produced at 55◦C underconditions of Experiment # 5 in Table 1. Slow poly-merization produces a gooey surface through whichsolvent molecules escape nonuniformly, and as a result,ripple structures (i.e., golf-ball like ) appear on thesurface of the resultant particles. In addition, prolongedsurface stickiness allows more particles to agglomerateas shown in Fig. 5c. There is a range of temperature,from about 60 to 90◦, that yields microspheres withsmooth surfaces as demonstrated by particles producedfrom Experiments #1 through #3.

For given experimental conditions the polymer mi-crosphere size can be directly controlled through thesize of the monomer solution droplets. As mentionedbefore, the monomer solution droplet size dependson the solution flow rate through the orifice and theorifice vibration frequency. The flow rate depends onthe orifice diameter and the gas pressure in the ballasttank. For a given size orifice flow rate is proportional

to the square root of the pressure drop driving thesolution through the orifice. For a given flow ratethe droplet size can be continuously tuned by varyingthe frequency. Figure 6 shows the effects of the orificefrequency and the pressure drop on the size of thepolymer particles produced using a 20 μm orifice. For agiven pressure drop (i.e., solution flow rate) the resultsshow that the particle size decrease as the frequencyincreases, and is approximately proportional to f −1/3,as expected from the relation for the monomer so-lution droplet diameter (i.e., Eq. 1). Furthermore, anincrease in the pressure drop increases the diameterof the monomer solution droplets generated at a givenfrequency, and as a result, the polymer particle sizeversus frequency curve in Fig. 6 shifts upward as thepressure drop increases.

We have reported results in Table 1 and Fig. 6 on thebasis of the mean polymer sphere diameter. We haveexamined more than 1,000 particles from each experi-ment to determine the size distribution, and the coeffi-cient of variations in the diameters for our experimentswere less than 1%. VOAG produces droplets whosediameters are contained within a coefficient variation ofabout 2 × 10−5. We have shown that VOAG produceddroplets, when traverse an identical path along theaxis of the chamber, maintain the same monodisper-sity along the path, and the size and composition ofthe droplets are uniquely related to the position (i.e.,distance traversed) [40–42].

Therefore, for given experimental conditions oneexpects the coefficient of variation in the size of themicrospheres to be the same as that of the solu-tion droplets. The results, however, show considerablyhigher variation in the microsphere size (i.e., ∼ 1%).This can be attributed, mainly, to the droplet dispersionused to prevent agglomeration among the particles. Asa result of the dispersion, various droplets traversedvarious paths, and the residence times of the dropletsvaried slightly with the paths. In addition, the solventevaporation rate was affected by the path (e.g., slower

Fig. 4 SEM micrographsof polymer microspheresprepared under conditionsof Experiment # 3 listed inTable 1. (a) particles with 600times magnification, (b)particles with 6,000 timesmagnification

852 Colloid Polym Sci (2007) 285:847-854

Fig. 5 SEM micrographsof polymer microspheresprepared at two differenttemperatures. (a) and (b)particles prepared underconditions of Experiment # 4listed in Table 1, (c) and (d)particles from Experiment # 5listed in Table 1

rate from the droplets near the chamber wall). Thephotopolymerization kinetics in a droplet was, in turn,influenced by the solvent evaporation, since the reac-tion rate depends on the monomer concentration andthe particle structure.

Highly cross-linked macrostructures result fromthe photopolymerization of multifunctional monomers(e.g., tetraethyleneglycoldiacrylate and trimethylol-

Fig. 6 Polymer microsphere size as a function of orifice vibrationfrequency for various pressure drops used to drive 12 vol %monomer solution through a 20 μm pinhole

propane ethoxylate triacrylate) present SOMOS 10220.The photopolymerization increases density by convert-ing double bonds to single bonds. As a result, thediameter of a polymer microsphere forming by pho-topolymerization of a solution droplet is expected tobe smaller than the diameter of the particle that wouldresult from the complete evaporation of the solventwithout any polymerization, that is,

Dp < v1/3m Dd (2)

where Dp is the diameter of the polymer microsphereresulting from the photopolymerization of a solutionmicrodroplet of diameter Dd, containing monomer ata volume fraction vm. The results in Table 1, how-ever, show that the diameters of the microspheresin all the experiments are slightly (i.e., about 1–3%)higher than the solvent free monomer droplet diameter,v

1/3m Dd. This suggests the microspheres produced in

this study are porous. The porosity of a microspheredepends the degree of polymerization of the monomermolecules in a droplet. We have examined the effectof the reaction temperature on the apparent density(i.e., porosity) of microspheres. Figure 7 shows resultsfrom experiments where identical monomer solutiondroplets exposed to the reaction temperature that wasvaried from 50 to 90◦C. Initially, the solution dropletswere of 42.3 μm diameter, and contained 12 vol %monomer. The results show that the diameter of poly-mer microspheres decreases (i.e., apparent density in-

Colloid Polym Sci (2007) 285:847-854 853

Fig. 7 Size of polymermicrospheres prepared atvarious temperatures fromidentical solution droplets,and the results are comparedwith percent conversionsof bulk samples byphotopolymerization.Initially, the solution dropletswere of 42.3 μm diameter,and contained 12 vol %monomer

creases) as the reaction temperature increases. Thehigher density results from the higher formation rate ofmacromolecules with cross-linked and network struc-tures, as manifested by the bulk photopolymerizationresults shown in Fig. 6. The bulk experiments wereconducted on monomer solution samples on trans-parent slides that were exposed to constant UV ra-diation for a period of 1 min. The percentage ofconversion at a given temperature was determinedfrom the polymer amount that remained undissolvedafter soaking the slide in ethanol. As expected, theresults show that the fractional conversion over 1 minperiod increases slowly as the temperature increasesfrom 20 to 40◦C, then rises sharply till about 60◦C,and finally, the polymerization reaction quenches asthe conversion approaches 100% at around 90◦C. Thediameter of polymer microspheres qualitatively followsthe results of bulk polymerization experiments, thatis, the diameter decreases sharply between 60 and80◦C, but at much slower rates outside this temperaturerange. The temperatures shifted to higher values thanthe bulk because of the differences involved in theresidence times and due to solvent evaporation.

Conclusions

We have presented a technique for generation of tai-lored microspheres of polymeric materials by in-situphotopolymerization of monomer solution droplets.Highly monodisperse droplets, with size fluctuationsof less than 0.01%, were generated by a vibratingorifice aerosol generator (VOAG), and dispersed inair to prevent agglomeration. The droplets were pho-topolymerized by exposing them to ultra-violet light.The droplet size can be tuned to any desired value

by controlling the volumetric flow rate of monomersolution through the orifice and the vibrating frequencyof the orifice. A number of factors play critical roleon the morphology of the final solid polymer particles.These factors include the monomer solution dropletdiameter, the monomer concentration, the surroundinggas temperature, and dispersion. In this study we havedemonstrated the aerosol based photopolymerizationusing a commercial monomer-photoinitiator mixture(i.e., SOMOS 10220). In principle, the technique ofthe present study can be used to generate polymermicrospheres from any monomer with the use of an ap-propriate initiator that may exist in the droplet phase orin the vapor phase. For example, droplets of monomersof p-tertiary butylstyrene (TBS), styrene, and divinyl-benzene (DVB) can be polymerized to microspheresusing a vapor initiator, trifluo-romethanesulfonic acid(TFSA) [28, 29]. On the other hand acrylate monomerdroplets can be photopolymerized using a dropletphase photoinitiator, Irgacure 369c [36]. One of therequirements, however, is that the polymerization ki-netics must be fast enough to produce solid particles inshort residence times involved in the reactor.

The results of our experiments suggest that tailoredmicrospheres (e.g., multicomponent, layered, and withnanoparticle inclusions) of highly reproducible size andphysical characteristics may be produced, and the mor-phology of the final solid polymer particles can con-trolled by the initial droplet size and composition aswell as through the reaction conditions. We are cur-rently investigating techniques for generation of suchmicrospheres.

Acknowledgements The authors gratefully acknowledge the fi-nancial support of the National Science Foundation (grant # CTS-0130778), and the Kentucky Science & Engineering Foundation(grant # KSEF-456-RDE-005).

854 Colloid Polym Sci (2007) 285:847-854

References

1. Tuin G, Peters ACI, Van Diemen AJG, Stein HN (1993) JColloid Interface Sci 158(2):508

2. Keville KM, Franses EI, Caruthers JM (1991) J Colloid Inter-face Sci 144(1):103

3. Omi S (1996) Colloids Surf A., Physicochem Eng Asp 109:974. Ugelstad J, Mork PC, Kaggerud KH, Ellingsen T, Berge

(1985) A Adv Colloid Interface Sci 13:1015. Leroux J-C, Allémann E, Doelker E, Gurny R (1995) Eur J

Pharm Biopharm 41:146. Arshady R (1991) J Control Release 17:17. Jonsson M, Nordin O, Malström E, Hammer C (2006)

Polymer 47:33158. Young TJ, Johnston KP, Mishima K, Tanaka H (1999) J

Pharm Sci 88:6409. Reverchon E, Della Porta G (2001) Pure Appl Chem 73:1293

10. Tu LS, Dehghani F, Forster NR (2002) Powder Technol126:134

11. Ruckenstein E, Hong L (1995) Polymer 36: 285712. Ruckenstein E, Sun Y (1996) J Appl Polym Sci 61:194913. Zhang H, Cooper AI (2002) Chem Mater 14:401714. Zhang H, Cooper AI (2005) Soft Matter 1:10715. Berkland C, Kim K, Pack DW (2001) J Control Release 73:5916. Li K, Stöver HDH (1993) J Polym Sci A Polym Chem 31:

325717. Li W-H, Stöver HDH (1998) J Polym Sci A Polym Chem 36:

154318. Li W-H, Stöver HDH (2004) Macromolecules 33:435419. Bai F, Yang X, Huang W (2004) Macromolecules 37:974620. Li S-H, Yang X-L, Huang W (2005) Chin J Polym Sci 37:974621. Bai F, Yang X, Li R, Huang B, Huang W (2006) Polymer

47:577522. Wang J, Cormack PAG, Sherrington DC, Khoshdel E (2003)

Angew Chem Ind Ed 42:5336

23. Yang S, Shim SE, Choe S (2005) J Polym Sci A Polym Chem43: 1309

24. Eerikäinen H, Kauppinen, EI, Kansikas (2004) J Pharm Res21:136

25. Chu M, Zhou L, Song X, Pan M, Zhang L, Sun Y, Zhu J, DingZ (2006) Nanotechnology 17:1791

26. Visca M, Matijevic E (1979) J Colloid Interface Sci 68:30827. Ingebrethsen BJ, Matijevic E (1980) J Aerosol Sci 11:27128. Partch RE, Matijevic E, Hodgson AW, Aiken BE (1983) J

Polym Sci A 21:961-967.29. Nakamura K, Partch R, Matijevic E (1984) J Colloid

Interface Sci 99:11830. Partch RE, Nakamura K, Wolfe EKJ, Matijevic E (1985) J

Colloid Interface Sci 105: 56031. Mayville FC, Partch RE, Matijevic E (1987) J Colloid

Interface Sci 120:13532. Lee M-G (2002) Polymer 43:430733. Dai Q, Wu D, Zhang Z, Ye Q (2003) Polymer 44(1):7334. Ward TL, Zhang SH, Allen T, Davis EJ (1987) J Colloid

Interface Sci 118:34335. Widmann JF, Davis EJ (1996) Colloid Polymer Sci 274(6):52536. Widmann JF, Aardahl CL, Johnson TJ, Davis EJ (1998)

J Colloid Interface Sci 199:19737. Sprynchak V, Esen C, Schweiger G (2003) Aerosol Sci

Technol 37:72438. Esen C, Schweiger G (1996) J Colloid Interface Sci 179:27639. Esen C, Kaiser T, Borchers MA, Schweiger G (1997) Colloid

Polym Sci 275:13140. Lin H-B, Eversole JD, Campillo AJ (1990) Rev Sci Instrum

61(3):101841. Devarkonda V, Ray AK, Kaiser T, Schweiger G (1998)

Aerosol Sci Technol 28:53142. Devarkonda V, Ray AK (2000) J Colloid Interface Sci

221:10443. Berglun RN, Liu BYH (1973) Environ Sci Technol 7:147