Spectroscopic Studies of Optically Tweezed Aerosol Droplets

J.P. Reid, L. Mitchem, J. Buajarern, J. Butler, K.H. Hanford, K.J. Knox, A. Laurain and H. Meresman

Aerosols play important roles in scientific fields as diverse as atmospheric chemistry and physics, drug delivery, combustion science and dusty plasma physics. The distribution of particles sizes, the bulk and surface chemical composition of the aerosol, and the mixing state of different chemical constituents are all important in determining the chemical and physical properties of an aerosol. Particle sizes can span 4 orders of magnitude (10 nm to 100 mm), a change in volume of 12 orders of magnitude, a challenging analytical problem to address. Laser techniques can yield important information on the size, composition, mixing state, refractive index and temperature of a single particle. When coupled with optical or electrostatic techniques for trapping and manipulating the particle, these techniques can allow the direct interrogation of the heterogeneous dynamics that lead to a change in particle size and composition. Studies on single particles also have the advantage of avoiding the unavoidable averaging that occurs when the dynamics of the entire aerosol ensemble are probed in conventional measurements.

The combination of optical tweezing with spontaneous and stimulated Raman scattering provides a unique approach for investigating the dynamics of aerosols, and also to probe the dynamics that occur at the gas-liquid or gas-solid interface. Although optical tweezers have been extensively used in the condensed phase to manipulate and study micron-sized objects, their application to the study of aerosol properties is a recent development [1]. Spontaneous Raman scattering can allow the identification of chemical components and the quantitative determination of composition [2]. Stimulated Raman scattering provides a signature of the size and refractive index of spherical droplets, allowing the determination of droplet radius with nanometre accuracy [3]. When it is also recognised that establishing multiple optical traps can allow the simultaneous control over multiple particles, unique measurements can be made on aerosol dynamics [4]. In this paper, we will review the underlying principles of aerosol optical tweezing, the information that can be gained from spontaneous and stimulated Raman scattering measurements, and some applications of this approach in characterising the heterogeneous dynamics of aerosols.

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