13700
2006, 110, 13700-13703 Published on Web 06/27/2006
A Strategy for Characterizing the Mixing State of Immiscible Aerosol Components and the Formation of Multiphase Aerosol Particles through Coagulation Laura Mitchem,† Jariya Buajarern,† Andrew D. Ward,‡ and Jonathan P. Reid*,† School of Chemistry, UniVersity of Bristol, Bristol, BS8 1TS, U.K., and Lasers for Science Facility, CCLRC, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. ReceiVed: May 10, 2006; In Final Form: June 13, 2006
We demonstrate that the coagulation of two aerosol droplets of different chemical composition can be studied directly through the unique combination of optical tweezers and Raman spectroscopy. Multiple optical traps can be established, allowing the manipulation of multiple aerosol droplets. Spontaneous Raman scattering allows the characterization of droplet composition and mixing state, permitting the phase segregation of immiscible components in multiphase aerosol to be investigated with spatial resolution. Stimulated Raman scattering allows the integrity of the droplet and uniformity of refractive index to be probed. The combination of these spectroscopic probes with optical tweezers is shown to yield unprecedented detail in studies of the coagulation of decane and water droplets.
Coagulation is a fundamental process influencing the size distributions and mixing states of aerosols. Many of the physical and chemical properties of aerosols, such as light scattering, the diffusion rates of particles, the partitioning of components between the gas and condensed phases, and the electrostatic charging of particles and their toxicity, are dependent on particle size and composition.1-3 Thus, studies of interparticle interactions and coagulation are important for a wide range of scientific disciplines, including combustion technology, atmospheric science, and epidemiology.1,2,4,5 Further, models of aerosol dynamics have been largely limited to studies of homogeneous particles that consist of a uniform mixture of different chemical constituents or of externally mixed components.5 However, phase segregation and internal mixing within an aerosol droplet are expected to occur in much the same way as in a macroscopic sample. Previously, it has not been possible to investigate directly and in real time the factors governing interparticle interactions and aerosol coagulation. It has also not been possible to investigate systematically the segregation of different chemical constituents and the formation and properties of multiphase aerosol. In this communication, we demonstrate that, by combining optical tweezers and spontaneous and stimulated Raman spectroscopy, the coagulation of two liquid aerosol droplets can be controlled and characterized, and the formation and properties of multiphase aerosol investigated. Optical tweezers have found widespread application in the condensed phase for the manipulation of micron-sized particles and studies of interparticle interactions.6 When trapped within a tight focal waist, a particle experiences a restoring force to an equilibrium position in the most intense part of the beam, arising from the gradient of the light field in the focal region.7 We have demonstrated that optical tweezers at a wavelength of 514.5 nm can capture indefinitely a single aqueous aerosol † ‡
University of Bristol. Rutherford Appleton Laboratory.
10.1021/jp062874z CCC: $33.50
Figure 1. Images and spectra of (a) the initially trapped water droplet of radius 4.352 µm; (b) the decane droplet in a second optical trap (right droplet), the water droplet has been translated to the left of the image, out of the field of collection of the Raman scattered light; (c) the multicomponent water/decane droplet following controlled coagulation.
droplet 4-14 µm in diameter doped with sodium chloride (∼0.1 M) from a dense aerosol flow.8 A Raman spectrum of the droplet can be recorded using a spectrograph/CCD with subsecond time resolution and the droplet imaged using conventional brightfield microscopy. The relative humidity in a surrounding cell is constantly monitored and is always within the range 80-90%. To study the coagulation of two liquid droplets, the trapping laser beam is split into two using a 50:50 beam splitter, forming two traps, and allowing two droplets to be controlled independently.9 © 2006 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 110, No. 28, 2006 13701
Figure 2. A sample of images of coagulated decane/water droplets and the spectra of multiphase droplets a (shown in black) and d (shown in green), compared with that for a pure water droplet (red). The similarity in shapes of the OH stretching bands confirms that the hydrogen-bonding network within the aqueous component is not altered by the presence of the decane component, and the two components are phase-separated.
The sequential trapping, manipulation, and coagulation of a decane droplet with a water droplet is illustrated in Figure 1. Initially, a single water droplet is loaded in one optical trap and a Raman fingerprint recorded. Two components are observed in the Raman scattering: an underlying broad spontaneous Raman band arising from the OH stretching vibration of water and stimulated Raman scattering (SRS) at three discrete wavelengths. The profile of the spontaneous Raman band centered at 3400 cm-1 can be used to assess the hydrogenbonding environment within the aqueous phase.10 In addition, the SRS provides a signature of droplet size.11 For a droplet with a size in the Mie scattering regime, spontaneous Raman scattering at discrete wavelengths commensurate with whispering gallery modes (WGMs) can become trapped within the droplet and circulate for nanoseconds. The circulating Raman scatter can reach a threshold intensity above which SRS is observed.11 The droplet size can be estimated with nanometer accuracy from comparison of the observed WGM wavelengths with Mie theory.8,11 The second optical trap is loaded with a decane droplet from an aerosol stream generated by a second nebulizer. By translating the water droplet (left droplet, Figure 1b) out of the field of view of the Raman collection and the decane droplet into the field of view, the Raman fingerprint of the decane droplet can be acquired. The spontaneous Raman band arising from the CH stretching vibrations of the decane is apparent, along with the superimposed SRS structure. The narrow width of the CH band precludes the appearance of progressions of multiple modes and prevents an accurate determination of size. By translating the water droplet toward the decane droplet, coagulation of the two droplets can be controlled (Figure 1c), forming a single droplet containing two immiscible phases. The resulting Raman spectrum shows the presence of both CH and OH stretching vibrations in the coagulated droplet, although no SRS structure is apparent. Two mechanisms can quench the SRS structure, and both are likely to be important in these studies. Deformation from sphericity can lower the lifetime of light circulating in a WGM, lowering the intensity of spontaneous Raman that circulates below the threshold intensity required for SRS to occur.11 Further, inhomogeneities in refractive index can disrupt
the circulation of WGMs, changing the cavity quality factors of the WGMs.12 The SRS signal arises from the outer shell of the droplet, propagating to a depth estimated to be 1 µm from the surface of a 4-µm-radius water droplet.11 The presence of decane/water interfaces within this outer shell leads to inhomogeneities in refractive index, degrading the cavity quality factors and quenching the SRS. Experimentally, a range of equilibrium configurations of the coagulated two-phase droplet are observed, some examples of which are shown in Figure 2. The equilibrium configuration can be established by considering the surface tensions of decane and water (23.4 and 72.8 mN m-1 at 20 °C) and their interfacial tension (52.9 mN m-1), assuming that interparticle forces, gravity, and fluid motion do not influence the equilibrium configuration.13 The spreading coefficient for decane on water is close to zero and dependent on the exact concentration of sodium chloride within the aqueous droplet. Thus, partial engulfing and complete engulfing are possible.13 In partial engulfing, the three surface/interfacial tensions form a closed Neumann triangle, and the equilibrium shape is governed by the relative sizes of the two droplets prior to coagulation.13 Complete engulfing of a water droplet within the host decane droplet is observed following the collision of a small water droplet with a large decane droplet (Figure 2d). The inhomogeneity in composition near the droplet surface also quenches the SRS signal for this droplet. Images showing partial engulfing are more complex to interpret, although clear phase segregation is observed (Figure 2b and c). By dosing a trapped decane droplet with aerosol from the water nebulizer, many inclusions of
