Measurements of mass and heat transfer at a liquid water surface during condensation or evaporation of a subnanometer thickness layer of water.

A novel approach for exploring the molecular dynamics during condensation or evaporation at a liquid water surface is reported at pressures between 2 and 100 kPa. By introducing or removing a heating laser illuminating an optically tweezed aqueous aerosol droplet, the temperature of the droplet can be controlled with sub-mK accuracy and the change in size to reequilibrate with the surroundings monitored with subnanometer accuracy. The time constant for equilibration is shown to provide important insight into the coupling of heat and mass transfer during condensation or evaporation.

The influence of resonant absorption and heating on the equilibrium size of aqueous-solute aerosol droplets.

The time-dependent evolution in the equilibrium size of an optically trapped aqueous sodium chloride droplet (>2 microm radius) within an environment of varying relative humidity (RH) is shown to depend on both the depression in vapour pressure due to the presence of the solute and the elevation in temperature due to optical absorption. In particular, the level of optical absorption is highly dependent on the size of the droplet relative to the wavelength of the absorbed light. Thus, as the droplet size tunes into a Mie resonance at the trapping laser wavelength, the increased level of optical absorption leads to an elevation in droplet temperature. This increase in resonant heating can balance a continual increase in RH, leading to only marginal growth in droplet size and change in solute concentration. Once the RH is sufficiently high that the resonance condition can be surpassed, the droplet cools instantaneously and the solute concentration again dominates in determining the vapour pressure, with a rapid increase in size and a decrease in solute concentration returning the droplet to equilibrium with the gas phase RH. Thus, a growing droplet is observed to pass through periods of apparent size stability followed by instantaneous growth, consistent with the variation in absorption efficiency with droplet size. This provides a clear example of the coupling between the optical and physical properties of an aerosol and their influence on the equilibrium state.

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets.

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.

Comparative thermodynamic studies of aqueous glutaric acid, ammonium sulfate and sodium chloride aerosol at high humidity.

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 microm 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 Kohler 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.

Optical manipulation and characterisation of aerosol particles using a single-beam gradient force optical trap.

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 microm 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.

In situ comparative measurements of the properties of aerosol droplets of different chemical composition.

Aerosol optical tweezers can be used to manipulate multiple aerosol particles simultaneously. When coupled with spontaneous and stimulated Raman scattering, the composition, size and phase partitioning of different chemical components within a liquid droplet can be investigated. In combination, these two techniques suggest the possibility of a new strategy for characterising the thermodynamic behaviour of aerosols and the kinetics of mass transfer between the gas and condensed phases. We demonstrate here that two droplets can be characterised simultaneously, examining specifically the variation in wet particle size with relative humidity, recording the changes in size with nanometre accuracy. In a further demonstration, we use the size of a sodium chloride droplet to determine the relative humidity of the gas phase, allowing the variation in hygroscopicity of a second aqueous glutaric acid/sodium chloride droplet to be studied. We suggest that such a comparative approach can provide new insights into aerosol dynamics.

Manipulation and characterization of aqueous sodium dodecyl sulfate/sodium chloride aerosol particles.

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.

Characterizing the formation of organic layers on the surface of inorganic/aqueous aerosols by Raman spectroscopy.

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.

Characterizing multiphase organic/inorganic/aqueous aerosol droplets.

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.

Controlling and characterizing the coagulation of liquid aerosol droplets.

We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of aerosols. Liquid aerosol droplets of 2-14 mum in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demonstrate that the coagulation of different aerosol components can be compared and the mixing state of the final coagulated droplet can be investigated. In particular, we compare the outcome of the coagulation of an aqueous sodium chloride aerosol droplet with a second aqueous droplet, with an ethanol droplet or with a decane droplet.

A strategy for characterizing the mixing state of immiscible aerosol components and the formation of multiphase aerosol particles through coagulation.

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.

Spectroscopy of growing and evaporating water droplets: exploring the variation in equilibrium droplet size with relative humidity.

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of Köhler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.

Laser probing of single-aerosol droplet dynamics.

Aerosols play a critical role in a diverse range of scientific disciplines. To characterize and quantify their role, it is essential that the fundamental details of mass and heat transfer between the aerosol particle and surrounding medium, the properties of multiphase droplets, and the coagulation of aerosol droplets be more fully explored. Elastic and inelastic light scattering can provide information on particle size, composition, morphology, and temperature. In addition, spatial inhomogeneities in composition and temperature can be probed. We review how such techniques can be used to develop an understanding of the chemical and physical dynamics of a single aerosol droplet.