Langmuir 1985,1, 365-367 IOOOP ,
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lated is n. n is calculated by data fitting the experimental J/Qabscurve to the theoretical curve. We have shown here that photophoresis can be used to experimentally obtain the complex index of refraction. A limitation on obtaining the complex index of refraction is that for high values of k the real part cannot be determined due to the insensitivity of J/Qabsto n (Figure 6 ) . But this case would only occur for metals and alternative methods exists for the evaluating the complex index of refraction for metals.
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Figure 7. Light intensity (MW/m2) needed to heat a particle to 1000 K as a function of a and k; X = 10.64 X 10") n = 1.3, k = 0.01) 0.1, 1, 10.
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Calculations of the photophoretic force on micron-sized particles in the continuum regime were performed. From these calculations, it was shown that a ratio of the asymmetry factor divided by the absorption efficiency?JIBab, can provide a sensitive measure of the complex index of refraction. Further, this ratio can be obtained theoretically and experimentally by the measurement of the photophoretic force, particle size, and temperature. significantly, the measurement of J/Qahedoes not depend on a direct measurement of the incident radiation intensity. The results indicated that two distinct regimes, depending on the normalized particle size, a,exist. For the large a regime, it is found that J/Qab is insensitive to the real part of the complex index of refraction, n. It is, however, strongly dependent on the imaginary part, k. Also a weak dependence on a is found. J/Qahecan be used for determining k in the high a regime. In the small a regime, J/Qahedepends strongly on a, n, and k . By measuring J / Q a bas a function of a and the use of the value of k obtained from the large a regime, n can be determined. For large k , J/Qabsis insensitive to n.
Acknowledgment. This work was partially funded by Exxon Research and Engineering Corp. W.M.G. acknowledges UROP at M.I.T. for its support. E.B.-Z. expresses his gratitude to the Bantrell Foundation for their support.
Characterization of Single Levitated Droplets by Raman Spectroscopy Richard E. Preston, Thomas R. Lettieri,* and Hratch G. Semerjian National Bureau of Standards, Gaithersburg, Maryland 20899 Received December 28, 1984 The results of a preliminary investigation into the use of Raman spectroscopy for the chemical characterization of single aerosol dropleta are reported. The diodyl phthalate droplets, 10 to 35 pm in diameter, were suspended by the radiation pressure of an argon ion laser beam. Initial experiments used a photomultiplier-based system which collected a 400 cm-l wide spectrum in about l/z h. This system was adequate for monitoring quasi-static droplet processes, but for faster processes an optical multichannel analyzer-based system was used to collect spectra in about 1s. Droplet spectra from both instruments showed sharp features not present in bulk liquid spectra. Investigations of chemical-composition changes in single aerosol droplets have usually relied upon indirect measurements, such as droplet mass and size, to infer the chemical state of the drop1et.l It would be advantageous in many studies to have a technique that more directly measures the droplet composition. A promising candida& (1)Rubel, G. 0. J. Colloid Interface Sei. 1981, 81, 188. Ray, A. K.; Davis, E. J. Chem. Eng. Commun. 1980,6,61. Richardson, C. B.; Spann, J. F. J . Aerosol Sci. 1984, 15, 1.
0743-7463/85/2401-0365$01.50/0
technique is Raman spectroscopy,which has been used for the chemical analysis of micrometer-sized particles resting on substrates.2 However, studies of liquid aerosol chemistry should ideally be performed in situ without the in(2) See, for example: Dhamelincourt, P.; Wallart, F.; Leclercq, M.; NGuyen, A. T.; Landon, D. 0. Anal. Chem. l979,51,414A. Etz, E. S. Scanning Electron Microac. 1979, 67. Rosasco, G. J. In "Advances in Infrared and Raman Spectroscopy"; Clark, R. J. H., Heater, R. E., Eds.; Heyden: London, 1980; Vol. 7, p 223.
0 1985 American Chemical Society
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Figure 2. Time-resolved Raman spectra of a growing droplet of dioctyl phthalate. Arrows indicate positions of sharp features which shifted to longer wavelengths with increasing droplet size.
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Figure 1. Raman spectra over the C-H stretching frequency region for three dioctyl phthalate droplets of various sizes. Bottom spectrum is from bulk liquid. terfering effects of a substrate. By using a stable levitation technique, compositional changes in a single droplet suspended in a well-controlled atmosphere should be measurable with Raman spectroscopy. This paper presents initial Raman spectroscopic measurements of single-component liquid droplets suspended in air by optical levitation. A recent study showed that similar Raman spectra could be obtained from optically levitated glass microsphere^.^ In the present experiments, a single droplet sprayed from an atomizer was trapped and stably levitated in an optical cell by the radiation pressure of a focused, 500-mW argon ion laser beam;4 the same beam also served as the Raman excitation source. The laser power incident on a l0-pm diameter droplet was estimated to be about 3 to 5 mW; the droplet size was determined by measuring the Mie fringe spacing at 90° and then applying a model based on the interference of two point sou~ces.~ Scattered light at 90° was collected by an f l l . 0 lens and focused into a double monochromator equipped with holographic gratings. Slit settings were adjusted so that the spectral resolution was typically 1-3 cm-'. In this system, a cooled photomultiplier tube with photon-counting electronics measured the intensity of the inelastically scattered light as the monochromator stepped sequentially in Raman frequency. Raman spectra taken with this system are shown in Figure 1. The spectra are from three droplets and the bulk liquid of dioctyl phthalate (DOP) over the C-H stretch region. In addition to the characteristic C-H vibrational band, the droplet spectra show sharp, reproducible features not seen in the spectrum of the bulk liquid. (3) Thurn, R.; Kiefer, W. A p p l . Spectrosc. 1984, 38, 78. (4) Ashkin, A.; Dziedzic, J. M. Appl. Phys. Lett. 1971, 19, 283. (5) Lettieri, T. R.; Jenkins, W. D.; Swyt, D. A. Appl. Opt. 1981, 20, 2799.
As the figure indicates, the sharp features are narrower and more closely spaced in frequency for the larger droplets. It was also observed that, qualitatively, the intensities of the Raman bands increased monotonically with droplet size, although the amplitudes of the sharp features had a more complicated dependence on droplet size. In the experiments described above, each of the spectra were taken over periods ranging from 20 to 30 min. While these time scales are acceptable for chemical analysis of quasi-static systems, they are not appropriate for the study of transient processes; the investigation of faster processes requires a measurement technique that can follow changes in droplet spectra in real time. For this reason, Raman spectra were also obtained using a different spectrometer which employed an optical multichannel analyzer (OMA) for simultaneous collection of the entire spectrum. The spectral resolution of the OMA system was estimated to be 4-6 cm-'. The utility of the OMA system for studying transient phenomena is demonstrated in Figure 2, which shows Raman spectra of a DOP droplet taken at three different times. The spectra, each integrated for about 1 s, again exhibit the sharp features which can now be clearly seen to shift to higher Raman scattered frequency with time. Note that during the experiment, the entire spectral window of 600 cm-' was continuously observable so that, in principle, chemical composition changes occurring over periods of about 1 s can be monitored. The sharp features in the droplet Raman spectra may have the same origin as the resonances observed previously in the fluorescence spectra of microspheres.6 Theoretical models of inelastic light scattering from dielectric microspheres' predict a coupling of the fluorescence and Raman emission to the electromagnetic modes of the sphere. Since the modes, or resonances, depend only upon the size parameter (7rD/X)for a fiied refractive index, the wavelength at which a particular resonance occurs will vary with changes in droplet size. The shift to longer wavelengths of the sharp features in Figure 2 suggests that the DOP (6) Chang, R. K.; Owen, J. F.; Barber, P. W.; Messinger, B. J.; Benner, R. E. J. Raman Spectrosc. 1981,10,178. Benner, R. E.; Barber, P. W.; Owen, 3. F.; Chang, R. K. Phys. Rev. Lett. 1980, 44, 476. Hill, S. C.; Benner, R. E.; Rushforth, C. K.; Conwell, P. R. Appl. Opt. 1984,23,1680. Tzeng, H . M.; Wall, K. F.; Long, M. G.;Chang, R. K. Opt. Lett. 1984,9,
273. (7) Roaaeco, G. J.; Bennett, H. S. J. Opt. SOC. Am. 1978, 68, 1242. McNulty, P. J.; Chew, H. W.; Kerker, M. In "Aerosol Microphysics I"; Marlow, W. H., Ed.; Springer-Verlag: Berlin, 1980; p 89.
Langmuir 1985,1, 367-372 droplet was slowly growing with time. To investigate this possibility, elastic scattering spectra were taken by focusing a beam of white light on to the DOP droplet. The elastic scattering resonances were also observed to shift to longer wavelengths with time, which is further evidence for droplet growth. For spectroscopic purposes, the sharp Raman features would seem to complicate measurements of Raman peak height and integrated area. A preliminary experiment has shown, however, that the features can effectively be integrated out by decreasing the instrumental resolution of the OMA system. With this decreased resolution the ratios of several Raman peak areas were identical for spectra taken of silicone oil droplets and bulk liquid silicone oil.
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Thus, by smoothing the spectra, either optically or numerically, the shape distortions caused by the features can be eliminated, yielding more quantifiable spectra. Future studies will use an electrodynamic balance,s rather than optical levitation, so that droplets can be suspended in a flowing gas stream. This will permit compositional studies of adsorption and evaporation processes, liquid-solid phase changes, and surface reactions on a droplet in a chemically reacting flow. Registry No. DOP,117-84-0. (8) Davis, E. J. Aerosol Sci. Technol. 1983, 2, 121.
Refractive Index and Evaporation Rate of Individual Smoke Dropletst George W. Mulholland,* Raymond L. McKenzie, Egon Marx, and Robert A. Fletcher National Bureau of Standards, Gaithersburg, Maryland 20899 Received December 28, 1984. In Final Form: February 26, 1985 The size and refractive index of individual smoke droplets have been measured by using a single-particle light-scattering instrument. Droplets of a given size were selected by use of a mobility classifier and then electrostatically charged. The measured scattered intensities were fitted to curves calculated from Mie theory to find the best values of the refractive indices. The refractive index of smoke from a moderately large, steady-state smolder reactor is found to be much less variable than that of cigarette smoke. Relative humidity is found to have a significant effect on the evaporation rate of the smoke droplets.
Introduction Smoke generated during smoldering combustion is a complicated aerosol system. The individual droplets consist of many organic compounds plus some water. The droplet sizes range from about 0.03 up to 10 pm. Furthermore, the chemical composition may vary from droplet to droplet as a result of different thermal chemical environments where the droplets nucleate and grow. This chemical compositional variation could in turn lead to a refractive index variation. The smoke rising from the source is dynamically unstable. The high concentration of smoke droplets is conducive to droplet growth through coagulation. Droplet sue can also change through either condensation or evaporation of water and organic species depending on the ambient temperature and environment of the smoke as it moves from the source to fill the space. Single-particle measurements provide the opportunity of characterizing the homogeneity of the smoke and the growth dynamics of the smoke droplet system. The measurement of particle size and refractive index of a single droplet is possible by using a light-scattering instrument coupled with an electrostatic levitator. M a n and Presented at the symposium on 'The Chemical Physics of Aerocolloidal Particles", 188th National Meeting of the American Chemical Society, Philadelphia, FA, Aug 26-31, 1984.
Mulholland' have demonstrated that for 1-pm polystyrene spheres the size and refractive index can be determined to an accuracy of 1.2% by using a single-particle instrumenL2 McRae3 has applied this technique to the determination of the refractive index of individual cigarette smoke particles. In this study we determine the refractive index of smoke droplets generated from a smolder reactor designed to simulate a realistic smoldering fire. We are able to select the particle size of the droplets entering the scattering cell by using a mobility classifier and thus find particle-to-particle variability in the refractive index for fixed particle size as well as detect any size-dependent trend in the refractive index. The aspect of smoke dynamics that can be studied by single-particle measurements is the dependence of the evaporation rate of smoke droplets on humidity. Chang and Davis4 and Davis and Ray5 have demonstrated that the evaporation rate of low vapor pressure liquids (diethyl hexylphthalate and dibutyl sebacate) can be accurately determined by single-particle measurements. It is very difficult to measure the evaporation rate accurately over (1)Marx, E.;Mulholland, G. W. NBS J . Res. 1983, 88, 321. (2) Phillips, D. Q.; Wyatt, P. J.; Bergman, R. M. J. Colloid Interface Sci. 1970, 34, 159. (3) McRae, D. D. J. Colloid Interface Sci. 1982, 87, 117. (4) Chang, R.; Davis, E. J. J. Colloid Interface Sci. 1976, 54, 352. (5) Davis, E. J.; Ray, A. K. J. Aerosol Sci. 1978, 9,411.
This article not subject to U.S. Copyright. Published 1985 by the American Chemical Society
