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A strategy is developed for characterizing the properties and microphysical structure of a mixed phase aerosol droplet consisting of an aqueous sodium chloride host and an insoluble organic inclusion using aerosol optical tweezers and Raman spectroscopy. In particular, palmitic acid (PA), solvated in ethanol and nebulized to produce a flow of aerosol, is coalesced with an optically trapped sodium chloride droplet. Once the ethanol solvent has evaporated, a hydrated crystalline organic phase is formed. Two distinct classes of mixed phase structure are observed. The observation of a PA aggregate formed immediately on coalescence at the surface of the host droplet is consistent with the hydrophobicity of the organic component; this is the dominant structure observed. However, PA inclusions are also observed within the droplet bulk following the evaporation of the ethanol solvent. This may be attributable to the interplay of the optical forces exerted on the inclusion and the lowering of the surface tension of the aqueous droplet by the presence of a monolayer of PA, or through direct incorporation of a PA inclusion by impaction. In this latter class of structure, an autocorrelation analysis of the Raman signatures suggests that the inclusion diffuses throughout the entire droplet volume. Finally, the coupling of nonresonant and resonant heating of the droplet, arising from motion of the PA inclusion within the droplet, is contrasted with observations of the lowering of equilibrium droplet size that accompanies the presence of a weakly absorbing inclusion, a polystyrene bead, within the droplet core. [ABSTRACT FROM AUTHOR]

The application of optical tweezers (a single-beam gradient force optical trap) to the manipulation and characterisation of aerosol particles is discussed in this tutorial review. Optical tweezers allow not only the indefinite control over a single droplet, but control over arrays of particles. Typical particle sizes span the 1–10 µm diameter range. When coupled with spectroscopic techniques for probing evolving particle size (with nanometre accuracy), composition, phase and mixing state, detailed investigations of the thermodynamic properties of aerosol, the kinetics of particle transformation, and the nature of interparticle forces and coagulation can be undertaken. [ABSTRACT FROM AUTHOR]

The mass and heat transfer dynamics of evaporating multicomponent alcohol/water droplets have been probed experimentally by examining changes in the near surface droplet composition and average droplet temperature using cavity-enhanced Raman scattering (CERS) and laser-induced fluorescence (LIF). The CERS technique provides a sensitive measure of the concentration of the volatile alcohol component in the outer shell of the droplet, due to the exponential relationship between CERS intensity and species concentration. Such volatile droplets, which are probed on a millisecond time scale, evaporate nonisothermally, resulting in both temperature and concentration gradients, as confirmed by comparisons between experimental measurements and quasi-steady state model calculations. An excellent agreement between the experimental evaporation trends and quasi-steady state model predictions is observed. An unexpectedly slow evaporation rate is observed for the evaporation of 1-propanol from a multicomponent droplet when compared to the model; possible explanations for this observation are discussed. In addition, the propagation depth of the CERS signal, and, therefore, the region of the droplet from which compositional measurements are made, can be estimated. Such measurements, when considered in conjunction with quasi-steady state theory, can allow droplet temperature gradients to be measured and vapor pressures and activity coefficients of components within the droplet to be determined. [ABSTRACT FROM AUTHOR]

High-sensitivity optical absorption measurements on individual sub-picoliter aqueous droplets are reported using aerosol optical tweezers to simultaneously manipulate and characterize a sample droplet and a control droplet for comparison. It is demonstrated that the detection sensitivity to trace analytes is set by the weak absorption by the solvent, water, and that absorbances less than 5 × 10 −7can be measured over pathlengths of less than 10 μm. The potential applications of this approach to analyze aerosol particle composition and to perform trace analysis are discussed. [ABSTRACT FROM AUTHOR]

Aerosol optical tweezers coupled with Raman spectroscopy can allow the detailed investigation of aerosol dynamics. We describe here measurements of the evolving size, composition, and phase of single aqueous aerosol droplets containing the surfactant sodium dodecyl sulfate and the inorganic salt sodium chloride. Not only can the evolving wet particle size be probed with nanometer accuracy, but we show that the transition to a metastable microgel particle can be followed, demonstrating that optical tweezers can be used to manipulate both spherical and non-spherical aerosol particles. Further, through the simultaneous manipulation and characterization of two aerosol droplets of different composition in two parallel optical traps, the phase behavior of a surfactant-doped particle and a surfactant-free droplet can be compared directly in situ. We also illustrate that the manipulation of two microgel particles can allow studies of the coagulation and interaction of two solid particles. Finally, we demonstrate that such parallel measurements can permit highly accurate comparative measurements of the evolving wet particle size of a surfactant-doped droplet with a surfactant-free droplet. [ABSTRACT FROM AUTHOR]

We demonstrate that nonlinear Raman spectroscopy coupled with aerosol optical tweezers can be used to probe the evolving phase partitioning in mixed organic/inorganic/aqueous aerosol droplets that adopt a core−shell structure in which the aqueous phase is coated in an organic layer. Specifically, we demonstrate that the characteristic fingerprint of wavelengths at which stimulated Raman scattering is observed can be used to assess the phase behavior of multiphase decane/aqueous sodium chloride droplets. Decane is observed to form a layer on the surface of the core aqueous droplet, and from the spectroscopic signature the aqueous core size can be determined with nanometer accuracy and the thickness of the decane layer with an accuracy of ±8 nm. Further, the presence of the organic layer is observed to reduce the rate at which water evaporates from the core of the droplet with an increasing rate of evaporation observed with diminishing layer thickness. [ABSTRACT FROM AUTHOR]

The partitioning of an immiscible and volatile organic component between the gas and aqueous condensed phases of an aerosol is investigated using optical tweezers. Specifically, the phase segregation of immiscible decane and aqueous components within a single liquid aerosol droplet is characterized by brightfield microscopy and by spontaneous and stimulated Raman scattering. The internally mixed phases are observed to adopt equilibrium geometries that are consistent with predictions based on surface energies and interfacial tensions and the volume fractions of the two immiscible phases. In the limit of low organic volume fraction, the stimulated Raman scattering signature is consistent with the formation of a thin film or lens of the organic component on the surface of an aqueous droplet. By comparing the nonlinear spectroscopic signature with Mie scattering predictions for a core−shell structure, the thickness of the organic layer can be estimated with nanometer accuracy. Time-dependent measurements allow the evolving partitioning of the volatile organic component between the condensed and vapor phases to be investigated. [ABSTRACT FROM AUTHOR]

The vapor pressures of two dicarboxylic acids, malonic acid and glutaric acid, are determined by the measurement of the evaporation rate of the dicarboxylic acids from single levitated particles. Two laboratory methods were used to isolate single particles, an electrodynamic balance and optical tweezers (glutaric acid only). The declining sizes of individual aerosol particles over time were followed using elastic Mie scattering or cavity enhanced Raman scattering. Experiments were conducted over the temperature range of 280−304 K and a range of relative humidities. The subcooled liquid vapor pressures of malonic and glutaric acid at 298.15 K were found to be 6.7−1.2.6× 10−4and 11.2−4.7.6× 10−4Pa, respectively, and the standard enthalpies of vaporization were respectively 141.9 ± 19.9 and 100.8 ± 23.9 kJ mol−1. The vapor pressures of both glutaric acid and malonic acid in single particles composed of mixed inorganic/organic composition were found to be independent of salt concentration within the uncertainty of the measurements. Results are compared with previous laboratory determinations and theoretical predictions. [ABSTRACT FROM AUTHOR]

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were carried out on the 1-propanol−water (abbreviated as 1PA−W) mixtures over the entire 1-propanol molar fraction range at 298 K. The two bands at ∼1053 and ∼1068 cm−1, assigned to the vibrational modes of the gauche (vC−C−C−O−G) and the trans (vC−C−C−O−T) conformational isomers, respectively, which both include C−O and C−C stretching motions, were used to monitor the structural changes of the mixtures. When the water to 1-propanol molar ratio (WPR) is smaller than 0.2, the absorbance ratio of the two bands (AvC−C−C−O−G/AvC−C−C−O−T) remains constant at 1.42, characteristic of the existence of the 1-propanol aggregate chains, hydrogen-bonded by the O−H groups of 1-propanol in gauche conformations. When increasing the WPR from 0.2 to 20, there is an abrupt decrease in the absorbance ratio (AvC−C−C−O−G/AvC−C−C−O−T) from 1.42 to 1.01, corresponding to penetration of water molecules into the gauche-aggregate chains. The penetrated water molecules disrupt the 1PA chains and transform these gauche-aggregate 1PA chains to trans-aggregate chains, which are 1PA dimers of trans-conformation. The structural change induces complicated spectroscopic changes, including the red shifts of the series of bands 1016, 1053, and 1098 cm−1and blue shifts of the bands 2877, 2937, and 2961 cm−1. With further increase of WPR up to 100, the absorbance ratio of AvC−C−C−O−G/AvC−C−C−O−Tincreases from 0.98 to 1.07, indicating a transformation of partial 1PA dimers to single molecules with gauche-conformation in the water hydrogen-bonding network. Together with results from quantum calculations at the B3L YP/6-31G (d, p) level, and two-dimensional infrared correlation and excess spectroscopy analysis, the structural evolution of water and 1PA molecules in 1PA−W mixtures has been inferred. [ABSTRACT FROM AUTHOR]

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis. [ABSTRACT FROM AUTHOR]

Aerosol optical tweezers are used to simultaneously characterize and compare the hygroscopic properties of two aerosol droplets, one containing inorganic and organic solutes and the second, referred to as the control droplet, containing a single inorganic salt. The inorganic solute is either sodium chloride or ammonium sulfate and the organic component is glutaric acid. The time variation in the size of each droplet (3−7 μm in radius) is recorded with 1 s time resolution and with nanometre accuracy. The size of the control droplet is used to estimate the relative humidity with an accuracy of better than ±0.09%. Thus, the Köhler curve of the multicomponent inorganic/organic droplet, which characterizes the variation in equilibrium droplet size with relative humidity, can be determined directly. The measurements presented here focus on high relative humidities, above 97%, in the limit of dilute solutes. The experimental data are compared with theoretical treatments that, while ignoring the interactions between the inorganic and organic components, are based upon accurate representations of the activity-concentration relationships of aqueous solutions of the individual salts. The organic component is treated by a parametrized fit to experimental data or by the UNIFAC model and the water activity of the equilibrium solution droplet is calculated using the approach suggested by Clegg, Seinfeld and Brimblecombe or the Zdanovskii−Stokes−Robinson approximation. It is shown that such an experimental strategy, comparing directly droplets of different composition, enables highly accurate measurements of the hygroscopic properties, allowing the theoretical treatments to be rigorously tested. Typical deviations of the experimental measurements from theoretical predictions are shown to be around 1% in equilibrium size, comparable to the variation between the theoretical frameworks considered. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]